Taxonomic revision and palaeobiogeographic affinities of Berriasian–Valanginian oysters from the Vaca Muerta and Mulichinco formations, southern Mendoza, Neuquén Basin, Argentina

Journal Pre-proof Taxonomic revision and palaeobiogeographic affinities of Berriasian–Valanginian oysters from the Vaca Muerta and Mulichinco formations, southern Mendoza, Neuquén Basin, Argentina Agustina G. Toscano, Darío G. Lazo PII:

S0195-6671(19)30447-1

DOI:

https://doi.org/10.1016/j.cretres.2019.104358

Reference:

YCRES 104358

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Cretaceous Research

Received Date: 19 October 2019 Revised Date:

9 December 2019

Accepted Date: 14 December 2019

Please cite this article as: Toscano, A.G., Lazo,, D.G., Taxonomic revision and palaeobiogeographic affinities of Berriasian–Valanginian oysters from the Vaca Muerta and Mulichinco formations, southern Mendoza, Neuquén Basin, Argentina, Cretaceous Research, https://doi.org/10.1016/ j.cretres.2019.104358. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

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Taxonomic revision and palaeobiogeographic affinities of Berriasian–Valanginian oysters from the Vaca

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Muerta and Mulichinco formations, southern Mendoza, Neuquén Basin, Argentina

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Agustina G. Toscanoa and Darío G. Lazoa

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: Instituto de Estudios Andinos Don Pablo Groeber (IDEAN), Universidad de Buenos Aires, Facultad de Ciencias

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Exactas y Naturales, Departamento de Ciencias Geológicas, CONICET, Pabellón 2 Ciudad Universitaria,

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C1428EGA, Ciudad Autónoma de Buenos Aires, Argentina. Email: [email protected] (corresponding

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author); [email protected].

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Abstract

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Oysters are very abundant in marine Jurassic–Cretaceous deposits across the world, which has led to multiple

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taxonomic studies since the nineteenth century. In contrast, South American Jurassic–Cretaceous oysters have

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been largely neglected in the literature. Here, focus is on Berriasian–Valanginian oysters from previously

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undocumented benthic faunas of southern Mendoza (Neuquén Basin, Argentina). Four species belonging to

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three genera of the Family Gryphaeidae, Subfamily Exogirinae are described, including their shell

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microstructure characterization, and their taxonomic status is discussed. Of the studied genera, Aetostreon

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and Ceratostreon have been widely recorded in the Neuquén Basin previously, whereas the genus Nanogyra is

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recorded for the first time in Argentina. Aetostreon is represented by A. subsinuatum and A. latissimum, and

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Ceratostreon and Nanogyra by C. hilli and N. (N.) brevisulcata n. sp. The oldest worldwide records of A.

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subsinuatum (Berriasian) and C. hilli (Valanginian) are documented here. The paleobiogeographic distribution

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of these species suggests a Tethyan distribution for A. subsinuatum and A. latissimum and a Trans-Temperate-

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Pacific distribution for C. hilli, while N. (N.) brevisulcata is endemic to the basin. The three genera appear as

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isolated records and different types of oyster mass occurrences (OMOs), classified according to its lateral

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extension, geometry, maximum thickness, and stratigraphic relevance. The different hierarchy of the OMOs

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could possibly represent different scales of palaeoenvironmental and/or stratigraphic changes, from basin-

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wide to local level, and could provide tools to identify variations in palaeoenvironmental parameters such as

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sedimentation rate, siliciclastic and nutrient input and/or salinity variations.

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Keywords: Early Cretaceous; Bivalvia; Ostreoidea; Aetostreon; Ceratostreon; Nanogyra

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1. Introduction

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Oysters are a major component of fossil assemblages of marine Mesozoic deposits, usually occurring as highly

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abundant monotypic associations with potential value in sedimentology, palaeoecology and stratigraphy

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(Flatt, 1976; Seilacher et al., 1985; Andrews and Walton, 1990; Dhondt and Dieni, 1988). This has elicited many

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taxonomic studies that date from as early as the eighteen century (Bourguet, 1742), primarily concerning

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European species (Defrance, 1821; Sowerby, 1822; Goldfuss, 1834; Leymerie, 1842; Coquand, 1869; Bayle,

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1878; among many other classic works) and, to a lesser degree, Asiatic, African and North American ones (Nyst

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and Galeotti, 1840; Roemer, 1852; Cragin, 1893; Renngarten, 1926; Nagao, 1934; Stanton, 1947; Alencáster de

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la Cserna, 1956; Prozorovskii et al., 1961; Bogdanova, 1980; Malchus, 1990; Aqrabawi, 1993; Cooper, 1995;

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Scott, 2007; Kiessling et al., 2011). Due to their long taxonomic history, oysters have a complicated

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nomenclatural record, resulting in numerous synonyms, poorly preserved types and many arbitrary taxonomic

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conclusions (Stenzel, 1971). Given this scenario, taxonomic revisions are critical as a first step for further

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research, including palaeoecological and paleoenvironmental studies. This is especially important for the

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Jurassic–Cretaceous periods of South America, where oysters are ubiquitous, yet taxonomic studies are scarce

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in comparison with other bivalve groups (e.g., d’Orbigny, 1842; Behrendsen, 1891;Gerhardt, 1897; Dietrich,

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1938; Cox, 1954; Gutiérrez Palma, 1972; Damborenea et al., 1979; Guzmán,1985; Aberhan, 1994; Casadío,

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1998; Seeling and Bengston, 1999; Rubilar and Lazo, 2009).

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Particularly, in the Jurassic–Cretaceous strata of the Neuquén Basin, oysters are abundant and persistent

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throughout the sedimentary successions, either as soft-bottom recliners (Lazo, 2007), hard substrate

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encrusters (Luci et al., 2019) or as laterally extensive oyster mass occurrences generated by the aggregation of

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thousands of specimens (OMOs, Toscano et al., 2018). Several genera have been recorded from the

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Tithonian–Hauterivian interval, i.e. Aetostreon Bayle, 1878; Ceratostreon Bayle, 1878; Deltoideum Rollier, 1917

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and Liostrea Douvillé, 1904 (Weaver, 1931; Damborenea et al., 1979; Rubilar et al., 2000). Ambigostrea

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Malchus, 1990; Amphidonte Fischer de Waldheim, 1829; Cubitostrea Sacco, 1897; Gyrostrea Mirkamalov,

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1963; Pycnodonte Fischer de Waldheim, 1835 and Turkostrea Vialov, 1936, have been recorded in the

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Maastrichtian (Casadío, 1998; Griffin et al., 2005; Aguirre-Urreta et al., 2011).

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Here, focus is on a largely ignored oyster fauna occurring towards the top of the Vaca Muerta Formation and

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in the basal half of the overlying Mulichinco Formation, in the southern region of Mendoza Province,

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Argentina (Sierra de la Cara Cura and Puesto Sierra de Reyes localities, Fig. 1). The area was first studied by

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Groeber (1933), who worked in the region between 1914 and 1922 (Lazo et al., 2017a), noticed significant

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“oyster banks” and identified them as belonging to the genus ‘Exogyra’ (Groeber, 1933). However, no further

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taxonomic or stratigraphic studies were carried out in the area.

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The studied localities are characterized by the recurrent presence of oyster mass occurrences (OMOs)

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throughout the sedimentary column, which are characterized by their near monospecific composition and

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remarkable oyster abundance, along with a continuous record of ammonoids. Therefore, the objectives of this

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contribution are as follows: (1) to provide a thorough taxonomic revision, including both morphological and

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shell microstructural descriptions of the oysters forming these OMOs, (2) to date the studied oyster taxa, using

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the associated ammonoid fauna and local and standard ammonoid zonations, and 3) analyze the

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palaeobiogeographic distribution of the studied oyster taxa.

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2. Geological setting and mode of occurrence

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The Neuquén Basin is situated at the Andes foothill of west-central Argentina, extending between 32° and

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40°S. It encompasses a near continuous Upper Triassic to lower Paleogene sedimentary record consisting

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mainly of marine and continental siliciclastic deposits, carbonates, and evaporites (Howell et al., 2005). The

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Mendoza Group of Kimmeridgian to late Hauterivian age, represents primarily marine deposits that attain a

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thickness of up to 3000 m of sedimentary rocks (Aguirre-Urreta and Rawson, 1997). This group comprises in

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ascending order the Tordillo, Vaca Muerta, Mulichinco and Agrio formations and their lateral equivalents. The

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Tordillo Formation is not considered here since it corresponds to continental deposits.

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Particularly, the Vaca Muerta Formation, originally described by Weaver (1931), is a thick marine unit of early

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Tithonian to early Valanginian age according to a well-established ammonoid zonation (Aguirre-Urreta et al.,

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2007). It consists mainly of bituminous black shales and thin marlstones, on which its importance as oil source

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rock is based (Leanza et al., 2011). This unit has been interpreted as a marine carbonate ramp deposited under

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overall-quiet water conditions, with occasional turbidite and tempestite intercalations (Spalletti et al., 2008).

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Its macrofossil record consists chiefly of ammonites and marine reptiles (Fernández and De la Fuente, 1988;

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Aguirre-Urreta and Rawson, 1999; among others). The topmost part of the unit is regarded as representing a

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slightly shallower environment, between a distal mid-ramp to proximal outer ramp setting, influenced by

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storms (Spalletti et al. 2000; Kietzmann et al., 2008). There, the molluscan fauna becomes more diverse in

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contrast to the typical monotonous ammonite fauna of older strata (Doyle et al., 2005; Leanza et al., 2006;

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Kietzmann and Palma, 2009; Luci and Lazo, 2012).

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The Mulichinco Formation overlies the Vaca Muerta Formation. It too was also originally described by Weaver

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(1931) and represents a low frequency lowstand wedge of early to late Valanginian age according to a well-

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established ammonoid zonation (Aguirre-Urreta et al., 2007). This unit is very variable both laterally and

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vertically, consisting mainly of sandstones and secondarily of shales and siltstones, which have been

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interpreted as deposits of fluvial braidplains to outer-shelf settings (Schwarz and Howell, 2005). Its fossil

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record consists of a diverse invertebrate macrofauna (Aguirre Urreta et al., 2011) and both marine and

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continental vertebrates (Lazo and Cichowolski, 2003; Coria et al., 2010).

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The boundary between the Mulichinco and Vaca Muerta formations is given by the Intra-Valanginian

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Discontinuity, which is expressed either as an abrupt change in lithology, with coarse fluvial facies overlying

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offshore marine deposits, or as a firmground with Glossifungites ichnofacies, both result of the erosional event

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that took place when the sea level fell and subsequently rose again (Schwarz et al., 2011; Schwarz and Buatois,

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2012; Figs. 2-3). However, this unconformity is not always identifiable and in some areas of the basin the

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contact between these units is transitional.

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In the Sierra de la Cara Cura and Puesto Sierra de Reyes localities (Fig. 1), the studied outcrops correspond to

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the top of the Vaca Muerta Formation and the basal half of the Mulichinco Formation. Here, the Vaca Muerta

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Formation is represented by marlstones intercalated with mudstones, wackestones and floatstones. The

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Mulichinco Formation is represented mainly by dark green shales intercalated with siltstones and at the top by

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thick floatstones (Figs. 3-4). The dark green shales correspond to the lower member of the Mulichinco

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Formation, whereas the thick floatstones correspond to its middle member, which is laterally equivalent to

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the upper member of the Chachao Formation, a unit that outcrops towards the central region of the Mendoza

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province in the Malargüe Anticline area (Legarreta and Kozlowski, 1981; Schwarz et al., 2013, Figs. 2-4). The

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boundary between both units is clearly visible by the presence of a laterally extensive firmground with

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Glossifungites ichnofacies that is interpreted as the Intra-Valanginian Discontinuity (Fig. 3-4). Besides oysters,

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the macrofauna is composed of gastropods, infaunal and epibyssate bivalves, serpulids, corals, echinoids, and

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brachiopods (Lazo et al., 2017b).

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The studied oyster taxa are associated with massive to parallel laminated marlstones forming tabular

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floatstones of variable packing density of shells, from dispersed to loosely-packed, and variable in thickness

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(0.2 to 2 m). A distal mid to proximal outer ramp setting is interpreted based on lithology, sedimentary

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structures and taphonomic signatures (Kietzmann et al. 2008, 2014; Schwarz et al. 2011; Schwarz and Buatois,

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2012). These oyster-dominated palaeocommunities correspond to the vertical accretion of thin

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autoparabiostromes and autobiostromes, sensu Kershaw (1994), showing some degree of reworking by distal

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storm flows and bioturbation. Oysters dominate in abundance over other macrobenthic elements, which are

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found isolated, including serpulids (Rotularia sp., Filograna sp.), trigoniids (Virgotrigonia sp.), pectinids,

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ramose corals and echinoids (Pygurus sp.).

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3. Material and Methods

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This paper is based on 1451 specimens collected bed-by-bed from the Vaca Muerta and Mulichinco formations

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at Sierra de la Cara Cura and Puesto Sierra de Reyes localities, Neuquén Basin (Fig. 1). All specimens are

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housed in the Palaeoinvertebrate Collection (MCNAM-PI) of the Museo de Ciencias Naturales y Antropológicas

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“Juan Cornelio Moyano”, Mendoza city, Argentina. Each catalogue number refers to a sample of specimens

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from a given stratigraphic position and locality. Suffix numbers indicate the number of a given specimen within

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that catalogue number.

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For taxonomic identification, comprehensive comparisons with similar species were carried out, including both

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relevant literature as well as digitized material available online. The systematic classification of major groups

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and ligament area classification follow Malchus (1990). The terminology referring to the paradontal apparatus

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of the hinge follows Cooper (1995). The terminology of the shell microstructure was adopted from Carter et al.

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(2012). Measurements were made with a 0,01 mm precision digital caliper. Height (H) and length (L) of the

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shells were measured and the relation between them (H/L) was calculated to further characterize each

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species. Mean and standard deviation are informed. Local and standard ammonoid zonations and ages refers

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to Aguirre-Urreta et al. (2007) and Reboulet et al. (2014).

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As stated above, taxonomic studies of fossil oysters present many difficulties due to their great morphological

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plasticity that initially led to a trend of nominating new species for every slightly different morphology

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described (see e.g. Leymerie, 1840), which eventually generated multiple synonymies (Stenzel, 1971).

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Subsequently, many morphologies were unified under one taxon (Dhondt and Dieni,1988), leading to many

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wastebasket taxa. Therefore, in order to avoid further confusion regarding this already complicated scenario,

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a conservative taxonomic approach is advisable along with detailed bed-by-bed collection of a reasonable

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number of specimens.

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4. Systematic Palaeontology

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Class Bivalvia Linnaeus, 1758

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Subclass Pteriomorphia Beurlen, 1944

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Order Ostreida Férussac, 1822

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Superfamily Ostreoidea Rafinesque, 1815

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Family Gryphaeidae Vialov, 1936

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Subfamily Exogyrinae Vialov, 1936

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Tribe Nanogyrini Malchus, 1990

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Genus Aetostreon Bayle, 1878

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Type species: Gryphaea latissima Lamarck, 1801, p. 399; figured by Bourguet, 1742, pl. 14, figs. 84, 85 (by

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subsequent designation of Douvillé, 1879).

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Diagnosis (modified from Stenzel, 1971 and Cooper, 1995): shell medium-sized to large, thick-shelled,

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inequivalve. Opysthogyrate umbo, ligament area varies between gyrostreoid and exogyroid. Left valve convex,

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deep; right valve flat or slightly concave. Left valve with a pronounced keel in the posterior third, which may

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be rounded or acute, surmounted by knobs. A shallow groove running parallel to the keel separates a slightly

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more convex posterior flange. No ornamentation except for growth lines or occasionally radial wrinkles, no

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chomata. Paradontal apparatus well developed.

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Remarks: The genus Aetostreon was first introduced by Bayle in 1878 and although it was placed as a

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subgenus in Exogyra Say, 1820 by Pervinquière (1912), this was later dismissed by Stenzel (1971), who

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differentiated Aetostreon as a separate genus because of the lack of radial ribs and chomata, present in

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Exogyra (Pugaczewska, 1975).

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Aetostreon latissimum (Lamarck, 1801)

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Fig. 5

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1801 Gryphaea latissima n. sp.; Lamarck, p. 399, figured by Bourguet, 1742, pl. 14, figs. 84, 85.

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1819 Gryphaea latissima Lamarck, 1801; Lamarck, p. 199.

183 184

1821 Gryphaea couloni n. sp.; Defrance, p. 534.

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1822 Gryphaea sinuata n. sp.; Sowerby, p. 43, pl. 336.

187 188

1822 Gryphaea aquila n. sp.; Brongniart, p. 96 and 399, pl. 9, fig. 11.

189 190

1834 Exogyra aquila (Brongniart, 1822); Goldfuss, p.36, pl. 87, fig. 3.

191 192

pars 1840 Exogyra sinuata (Sowerby, 1822); Leymerie, p. 121-126; sinuata n.var., Leymerie, p. 124; latissima,

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n.var., Leymerie, p. 124; elongata n.var., Leymerie, p. 124. Non subsinuata n.var., Leymerie, p. 124; dorsata,

194

n.var., Leymerie, p. 124; falciformis n.var., Leymerie, p. 124; aquilina, n.var., Leymerie, p. 124.

195 196

1841 Exogyra sinuata (Sowerby, 1822); Roemer, p. 47.

197 198

1842 Exogyra sinuata (Sowerby, 1822); Leymerie, p.16, 17, pl. 12, figs. 1, 2.

199 200

pars 1847 Ostrea aquila (Brongniart, 1822); d’Orbigny, p. 706, pl. 470, figs. 3, 4; non figs. 1, 2.

201 202

1853 Ostrea aquila (Brongniart, 1822); Pictet and Roux, p. 520, p. 48, figs. 1, 2.

203 204

1869 Ostrea aquila (Brongniart, 1822); Coquand, p.180, pl. 65, figs. 4-9.

205 206

pars 1869 Ostrea couloni (Defrance, 1821); Coquand, p. 180, pl. 75, figs. 4-6, non figs. 1-3, 22.

207 208

pars 1871 Ostrea couloni (Defrance, 1821); Pictet and Campiche, p. 287; pl. 187, figs. 1-3; pl. 188, fig. 1, non

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fig. 2. Non pl. 192, fig. 1.

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1878 Aetostreon latissimum (Lamarck, 1801); Bayle, pl. 139, figs, 1-3.

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pars 1900 Exogyra couloni (Defrance, 1821); Burckhardt, p. 18, pl. 22, fig. 3. Non pl. 21, figs. 7,8.

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pars 1910 Gryphaea latissima Lamarck, 1801; Pervinquière, p. 194c, figs. 194, 194a. Non fig. 194b.

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pars 1913 Exogyra sinuata (Sowerby, 1822); Woods, p. 395, text-figs. 194-202, 204, 205. Non text-figs. 203,

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206-214.

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1924 Exogyra sinuata (Sowerby,1822); Newton, p. 144, pl. 6, figs. 1, 2.

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non 1926 Exogyra latissima (Lamarck, 1801), forma typica; Renngarten, p. 60, pl. 3, fig. 6; pl. 4, fig. 1.

223 224

1931 Exogyra couloni (Defrance, 1821), var. leufensis n. var; Weaver, p. 228, pl. 19, figs. 93, a, b.

225 226

1931 Exogyra couloni (Defrance, 1821); Weaver, p. 229, pl. 19, figs. 88-91.

227 228

1933 Exogyra cf. couloni (Defrance, 1821); Groeber, p. 19-20.

229 230

1948 Exogyra latissima (Lamarck, 1801); Tavani, p. 109, pl. 6, figs. 1, 2, 7; pl. 7, figs. 6, 10; pl. 8, fig. 3.

231 232

1954 Exogyra latissima (Lamarck, 1801); Cox, p. 629.

233 234

1961 Exogyra latissima (Lamarck, 1801); Prozorovskii et al., p. 126, pl. 12, fig. 1; pl. 13, fig. 3.

235 236

1971 Aetostreon latissimum (Lamarck, 1801); Stenzel, p. N117, fig. J92.1.

237 238

1972 Exogyra latissima (Lamarck, 1801); British Museum, p.152, pl. 54, fig. 4.

239 240

pars 1979 Aetostreon latissimum (Lamarck, 1801), morphotype 2, 3 and 4; Damborenea et al., p. 40-41, pl.7.,

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figs. 1-6. Non morphotypes 1, 1a, pl. 6, figs. 5-8.

242 243

1980 Aetostreon latissimum (Lamarck, 1801); Fischer, p. 216, pl. 101, figs. 1, 2.

244 245 246

1985 Aetostreon couloni (Defrance, 1821); Guzmán, p. 7-8, pl. 1, figs. 1-6.

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pars 1988 Aetostreon latissimum (Lamarck, 1801); Dhondt and Dieni, p. 38, pl. 8, fig. 7, non figs. 1-6; pl. 9, figs.

248

4-6, non figs. 1-3.

249 250

1994 Aetostreon (Exogyra) latissima (Lamarck, 1801); van Diggelen, p. 88, fig. 7.

251 252

1995 Aetostreon latissimum (Lamarck, 1801); Cooper, p. 11, 12, 18, figs. 10-14.

253 254

2007 Aetostreon sp.; Lazo, p. 42, pl. 4, figs. H, I.

255 256

2007 Aetostreon latissimum (Lamarck, 1801); Scott, p. 14, 17, pl. 6, figs. A-F.

257 258

2011 Aetostreon sp.; Aguirre-Urreta et al., p. 469, pl. 3, figs. C1-2.

259 260

Diagnosis: sub-oval to irregularly triangular outline, higher than long. Left valve with acute keel, which

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becomes shallower approximately from the dorsal-third onwards, following a dorso-anterior to postero-

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ventral trajectory. No ornamentation except for coarse growth lines or even wrinkles and shallow tubercles

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where these cross the keel.

264 265

Material: 108 specimens collected from the Spiticeras damesi ammonite Zone (late Berriasian), Vaca Muerta

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Formation (MCNAM-PI 24928.1-24928.95) and from the Olcostephanus atherstoni ammonite zone (early

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Valanginian), Mulichinco Formation (MCNAM-PI 24929.1-24929.13), Sierra de la Cara Cura locality (Fig. 4). Six

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specimens collected from the Olcostephanus atherstoni ammonite Zone (early Valanginian), Mulichinco

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Formation (MCNAM-PI 24930.1-24930.6), Puesto Sierra de Reyes locality.

270

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Stratigraphic and geographic range: This species was previously recorded from the Upper Jurassic (Tithonian)–

272

Lower Cretaceous (Albian) of Europe (France, Switzerland, Spain, England, Germany, Italy), Asia (Russia,

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Turkmenistan), North America (Texas and Mexico), South America (Trinidad Island, Colombia, Argentina, Chile)

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and Africa (Algeria, Egypt, Tunisia, Madagascar, Morocco, Somalia, South Africa).

275 276

Description: Shell medium to large in size, inequivalve, sub-oval to triangular outline, opisthogyrate umbo.

277

Cementation area located postero-dorsally. Dorsal third of left valve with acute keel, becoming shallower and

278

almost disappearing towards the ventral margin (Fig. 5 A, C, D). The keel has an almost straight trajectory

279

(although following the curvature of the valve), uncoiling with the umbo at first and crossing the valve,

280

reaching the mid-ventral margin, and dividing the left valve in two sub-equal halves. From the keel towards

281

the anterior margin, shell convexity high, whereas from the keel to the posterior margin, shell convexity less

282

steep. Growth lines lamellose, occasionally almost wrinkles, ornamented with tubercles where growth lines

283

cross the keel (Fig. 5 C). Adductor muscle scar comma-shaped to biconcave-shaped (Fig. 5 E). Ligament area

284

varying between exogyroid and gyrostreoid (Fig. 5 B, E, F). Since most specimens are either articulated or

285

poorly preserved, the paradontal apparatus was difficult to observe. The paradontal recess was preserved but

286

the accompanying paradontal buttress was either broken or eroded (Fig.5 E). As no isolated right valves were

287

found, the paradontal process was not observed. Right valve flat to slightly concave, presenting commarginal

288

growth lines (Fig. 5 G).

289 290

Microstructure: Right valve with a middle-outer layer of complex-cross foliated microstructure, followed by an

291

inner middle layer of regular foliated microstructure (Fig. 6 A, C). Lenticular hollow chambers secondarily filled

292

by granular calcite cement present. Regular prismatic outer layer not preserved. Left valve with an outer-

293

middle layer of “mosaic” complex cross-foliated microstructure with several thin calcitic bands. This grades

294

into a regular foliated to complex cross-foliated microstructure that may also have thin calcitic bands. Inner-

295

middle layer with complex cross-foliated microstructure (Fig. 6 A, B). Lenticular hollow chambers secondarily

296

filled with granular calcite cement conspicuous throughout the middle layer.

297 298

Dimensions: Mean H: 79.71±16.39 mm; mean L: 63.02±11.40 mm; mean H/L: 1.34±0.26.

299 300

Discussion: A. latissimum has a long and complicated taxonomic history and has been considered to have a

301

very plastic morphology (see Rubilar and Lazo, 2009). The identity of A. latissimum is here restricted to the

302

original intention of Lamarck’s description (1801) and Bourguet’s drawings (1742). The original definition was

303

subsequently supported by other publications were the “typical” A. latissimum was figured (Bayle, 1878;

304

Stenzel, 1971; Cooper, 1995). It is important to note that Renngarten (1926) mistakenly used the species to

305

name material that presents plications or shallow ribs and an almost quadrate outline, therefore, it does not

306

correspond to A. latissimum.

307

Considering previous records in Argentina, Burckhardt (1900) was the first to describe specimens belonging to

308

A. latissimum in the vicinity of the Las Lajas locality and in Sierra de la Vaca Muerta, Neuquén Province. He

309

differentiated two forms of ‘E’. couloni, which he distinguished by the presence of an acute keel (here A.

310

subsinuatum, see below) or the lack of it (here A. latissimum). Weaver (1931) recorded specimens of

311

‘E.’couloni var. leufensis and ‘E.’couloni in the vicinity of the Sierra de la Vaca Muerta, Agrio and Cerro El

312

Salado localities, which are here identified as A. latissimum due to the presence of an initially acute and later

313

shallower keel and a general suboval outline. Also, Groeber (1933) mentioned the presence of “oyster banks”

314

and of “big E. cf. couloni” above the Lissonia riveroi Zone and above “35-40 m of green marlstones” (p. 19-20)

315

in the Sierra de la Cara Cura, which probably also correspond to A. latissimum (Fig. 4). Also recorded from the

316

same basin but during the late Valanginian, the species A. pilmatuegrossum differs from A. latissimum because

317

of its general subtrigonal outline, its truncated postero-ventral margin and the presence of a prominent

318

secondary keel in the ventral half of the valve, which is absent in A. latissimum. However, these species are

319

probably closely related.

320 321

Aetostreon subsinuatum (Leymerie, 1842)

322

Figs. 7- 8

323 324

pars 1840 Exogyra sinuata (Sowerby, 1822), Leymerie, p. 121-126; subsinuata n.var., Leymerie, p. 124;

325

dorsata, n.var., Leymerie, p. 124; falciformis n.var., Leymerie, p. 124 (non Goldfuss, 1834, pl. 80, fig. 4);

326

aquilina, n.var., Leymerie, p. 124. Non sinuata n.var., Leymerie, p. 124; latissima, n.var., Leymerie, p. 124;

327

elongata n.var., Leymerie, p. 124.

328 329

1842 Exogyra subsinuata n.sp.; Leymerie, p.16-17, pl. 12, figs. 3-7.

330 331

pars 1847 Ostrea couloni (Defrance, 1821); d’Orbigny, p. 698; pl. 466, figs. 3, 4, non figs. 1, 2; pl. 467, figs. 1-3.

332 333

pars 1847 Ostrea aquila (Brongniart, 1822); d’Orbigny, p. 706; pl. 470, figs. 1, 2, non figs. 3, 4.

334 335

pars 1869 Ostrea couloni (Defrance, 1821); Coquand, p. 180; pl. 71, figs. 8-10; pl. 74, figs. 1-5; pl. 75, figs. 3, 22;

336

non 1, 2, 4-6.

337 338

pars 1871 Ostrea couloni (Defrance, 1821); Pictet et Campiche, p. 287; pl. 188, fig. 2; non fig. 1. Non pl. 187, pl.

339

192, fig. 1.

340 341

1878 Aetotreon aquilinum (Leymerie, 1840); Bayle, pl. 140, figs. 3-5.

342 343 344

1890 Ostrea (Exogyra) haueri n. sp., Toula, p. 340, pl. 5, fig. 3.

345

pars 1900 Exogyra couloni (Defrance, 1821); Burckhardt, p.18; pl. 21, figs. 7, 8. Non pl. 22, fig. 3.

346 347

pars 1910 Gryphae latissima Lamarck, 1801; Pervinquiére, p. 194c, figs. 194b; non figs. 194, 194a

348 349

pars 1913 Exogyra sinuata (Sowerby, 1822); Woods, p. 395, text-figs. 203, 206-214. Non text-figs. 194-202,

350

204, 205.

351 352

pars 1926 Exogyra subsinuata Leymerie, 1842, forma typica; Renngarten, p. 61; pl. 4, fig. 4, non figs. 1- 3; pl. 5,

353

fig. 1.

354 355

1926 Exogyra subsinuata Leymerie, 1842, var. falciformis Leymerie, 1842; Renngarten, p. 62, pl. 4, fig. 5; pl. 6,

356

fig. 1.

357 358

non 1926 Exogyra subsinuata Leymerie, 1842, var. carinato-plicata, n.var.; Renngarten, p. 62, pl. 3, fig. 7; pl. 4,

359

figs. 2, 3.

360 361

1937 Exogyra reedi n. sp.; Imlay, p. 566, pl. 77, figs. 1-6, pl. 78, figs, 1-7.

362 363

1940a Exogyra reedi Imlay, 1937; Imlay, p. 148-149, pl. 9, figs. 4-6; pl. 12, figs. 1-2; pl. 13, figs. 1-5.

364 365

non 1961 Exogyra falciformis Leymerie, 1842; Prozorovskii et al., p. 125, pl. 10, fig. 1; pl. 11, fig. 1.

366 367

1974 Exogyra couloni (Defrance, 1821); Oekentorp and Siegfrid, p. 151, pl. 13, fig. 7.

368 369

1975 Aetostreom latissimum (Lamarck, 1801); Pugaczewska, p. 51, pl. 7, fig. 1; pl. 8, figs. 1-7; pl. 9, figs. 1-4.

370 371

pars 1979 Aetostreon latissimum (Lamarck, 1801), morphotype 1 and 1a; Damborenea et al., p. 38, pl. 6, figs.

372

5-10. Non morphotypes 2, 3, 4, pl. 7, figs. 1-6.

373 374

pars 1988 Aetostreon latissimum (Lamarck, 1801); Dhondt and Dieni, p. 38, pl. 8, figs. 1-6, non fig. 7; pl. 9, figs.

375

1-3, non figs. 4-6.

376 377 378

Diagnosis: Shell sickle-shaped, generally higher than long, with acute keel on left valve that divides it in two

379

anterior and one posterior part. The keel starts at the umbo, uncoiling with it, and traverses the valve in a

380

postero-ventral direction. No ornamentation except for tubercles where growth lines cross the keel. Large

381

specimens may display an anterior fold.

382 383

Material: 305 specimens collected from the Argentiniceras noduliferum ammonite Zone, (early to mid

384

Berriasian age) of the Vaca Muerta Formation (MCNAM-PI 24926.1-24926.302 and 24927.1-24927.3), Sierra

385

de la Cara Cura locality. This taxon was not recorded in the Puesto Sierra de Reyes locality (Fig. 4).

386 387

Stratigraphic and geographic range: This species was previously recorded from the Lower Cretaceous

388

(Berriasian–Aptian) of America (Mexico, Chile and Argentina) and Eurasia (France, Switzerland, England,

389

Poland, Italy, Bulgaria, Russia and Turkmenistan).

390 391

Description: Shell medium to large in size, inequivalve, sickle-shaped; opisthogyrate umbo. Attachment area

392

postero-dorsally located. Left valve with well-developed keel, crossing the entire valve, first following the

393

uncoiling of the umbo and later approaching the ventroposterior margin. Keel acute in the dorsal half,

394

becoming less conspicuous and wider in the ventral half (Figs. 7 C, D; 8 B, C, F, G, H); carrying small tubercles at

395

intersection points with growth lines (occasionally almost wrinkles) (Figs. 7 C, 8 B). Keel dividing left valve in

396

two parts, the anterior twice as wide as the posterior one. In the posterior third, a shallow groove runs parallel

397

to the keel, deepening in the ventral third (Fig. 8 A). Anteriorly of keel, shell very convex, posteriorly of keel,

398

shell straight to slightly concave. Large, adult specimens with anterior fold (Figs. 7 A, C, D; 8 B, H). Some

399

specimens with an anterior globoid convexity towards anteroventral margin, delimited by the anterior fold

400

and posteriorly by the keel (Fig. 8 H). Adductor muscle scar, comma-shaped to slightly biconcave (Fig. 7 B).

401

Ligament areas are variable, most commonly of the exogyroid type, followed by the gyrostreoid type (Fig. 7 E,

402

F). Left valve with a paradontal recess and a paradontal buttress, both accessories of the anodontal hinge (Fig.

403

7 F). Parodontal process possibly present on the right valves, but since most of the studied specimens are

404

either left valves or articulated specimens, this was observed with doubt in the only right valve available. Right

405

valve flat to slightly concave; crossed by commarginal growth lines. Some specimens with an anterior ridge of

406

elevated growth laminae (Figs. 7 A; 8 D, E).

407 408

Microstructure: Right valve with outer prismatic layer. Outer-middle layer with a high-angle complex-crossed

409

foliated microstructure that grades into a regular foliated microstructure. Lastly, the inner-middle layer

410

presents a cone complex-crossed foliated microstructure (Fig. 9 A, B). Left valve with complex crossed-foliated

411

microstructure, containing numerous hollow, lenticular shell chambers, secondarily filled with granular calcite

412

cement (Fig. 9 A, D). Near the umbo, the valves display a low angle complex crossed-foliated microstructure

413

(Fig. 9 A, C).

414 415

Dimensions: Mean H: 40.39±12.95 mm; mean L: 24.78±8.85 mm; mean H/L: 1.68±0.33.

416 417

Discussion: The material closely resembles specimens identified as various species in the past (‘E.’subsinuata,

418

‘E.’coulouni, ‘E.’ sinuata, ‘E. ’reedi, ‘G.’ latissima and many subspecies of these species). These species have

419

been eventually synonymized with A. latissimum (Dhondt and Dieni, 1988; Scott, 2007), largely ignoring many

420

identifiable and consistent differences that are hard to attribute only to plasticity.

421

Therefore, under such a lax definition, probably more than one species is involved (Rubilar and Lazo, 2009).

422

Here, the studied morphology is considered a distinct species, different from A. latissimum in many ways,

423

including a general sickle-shape, acute keel dividing the shell unevenly and the presence of an anterior fold in

424

large specimens, although these two species are probably closely related.

425

Considering nomenclature, priority is given to Leymerie (1840, 1842). Initially, he proposed ‘E.’sinuata

426

subsinuata as one of many “varieties” within ‘E.’ sinuata (Leymerie, 1840), which had previously been erected

427

by Sowerby (1822). However, in a subsequent publication, Leymerie (1842) emended this by erecting ‘E.’

428

subsinuata as a species distinct from ‘E.’ sinuata and dividing the former “varieties” between these two

429

species. As a result, ‘E.’ sinuata included the “varieties” sinuata, latissima and elongata and ‘E.’ subsinuata

430

included the “varieties” subsinuata, dorsata, falciformis and aquilina. However, the varieties included in ‘E.’

431

subsinuata by Leymerie (1842) do not exhibit any real differences nor with ‘E.’ subsinuata. According to article

432

45.6.4 of the International Commission on Zoological Nomenclature (2000), all these “varieties” should be

433

considered of subspecific rank and are therefore considered here as junior synonyms of Aetostreon

434

subsinuatum (Leymerie, 1842) which is the valid name of the species.

435

‘E.’ subsinuata, var. carinato-plicata (Renngarten, 1926) does not correspond to A. subsinuatum because of

436

the presence of rounded, thick, low but clearly visible ribs on the left valve. Also, one of the varieties of

437

Leymerie (1842) formerly included in ‘E’. subsinuata was elevated to species rank, ‘E.’ falciformis, by

438

Prozorovskii et al. (1961), assigning to it specimens that lack the pronounced keel and the opisthogyrate umbo

439

and have a general ovoid outline. This material should not be assigned to ‘E.’ falciformis as it is a junior

440

synonym of A. subsinuatum and lacks the diagnostic features of ‘E.’ falciformis. A. imbricatum (Krauss, 1843, p.

441

129) is another species usually considered similar or closely related to A. latissimum. It differs from A.

442

subsinuatum because although it displays a similar sickle-shaped outline, A. imbricatum lacks the characteristic

443

keel of A. subsinuatum.

444

Considering previous records in Argentina, Burckhardt (1900) was the first to describe this species from the

445

Neuquén Basin. He distinguished two forms (under the name ‘E.’couloni), one with a shallow keel and suboval

446

outline, which corresponds to A. latissimum (see above), and a second form, sickle-shaped and with a very

447

pronounced keel, which he associated to the material figured by Coquand (1869). This is the first record of A.

448

subsinuatum in the region (specifically in the Sierra de la Vaca Muerta). Later on, Weaver (1931) recorded

449

specimens of ‘E.’couloni var. leufensis and ‘E.’couloni, which do not correspond to the species described here

450

due to the lack of an acute keel traversing the shell. They also correspond to A. latissimum (see above).

451

Another species recorded in the Lower Cretaceous of Argentina, A. pilmatuegrossum Rubilar and Lazo, differs

452

from the material studied here because of its general subtrigonal outline and, although it does have a keel, it

453

is weaker than the one developed in A. subsinuatum. Also, the former has an anterior geniculation and an

454

anterior globoid convexity, which is absent in the later.

455 456

Genus Ceratostreon Bayle, 1878

457 458

Type species: Exogyra spinosa Mathéron, 1843 by subsequent designation of Douvillé (1879).

459 460 461

Diagnosis (modified from Stenzel, 1971): Shell inequivalve, small- to medium-sized; elongated to crescentically

462

curved or comma-shaped to ovate. Attachment area generally large. Left valve with keel, although it might be

463

obscured by the surface sculpture that crosses it. Keel separating the narrow and very steep anterior slope

464

from the larger and with a more gently sloping posterior one. Right valve flat or slightly concave, occasionally

465

also with a keel, albeit shallow, formed by raised growth lines. Either both valves or only the left one with

466

many dichotomous, unequal, rounded ribs and approximately equal large and rounded interspaces. Ribs more

467

or less continuous, with or without spines and of different strength depending on the species. Occasionally,

468

with a series of contiguous transverse puckers. Both valves exhibit slender and well developed chomata.

469 470

Remarks: This genus has been considered as a subgenus of Amphidonte (Hayami,1965; Malchus, 1990).

471

Hayami (1965, p. 343) argued that the two groups could not be always sharply separated due to the existence

472

of “intermediate” species that would not strictly belong to either genus. Additionally, Malchus (1990, p. 110)

473

thought that the two genera share some of the most prominent diagnostic features (general sickle- or cup-

474

shaped left valve, slightly convex to concave right valve, chomata surrounding both valves, presence of ribs,

475

shape of the adductor and shell structure), supporting the idea of Ceratostreon being a subgenus. However,

476

this view has not been universally accepted (Cooper, 1995; Dhondt and Jaillard, 2005; Scott, 2007), and even

477

Malchus himself treated Ceratostreon as a genus in subsequent publications (Dhondt et al., 1999). Here,

478

Ceratostreon is considered as a distinct genus, because of compelling morphological differences with

479

Amphidonte, such as the smaller size, the more pronounced sickle shape and the presence of pronounced ribs,

480

which are largely absent in Amphidonte.

481 482

Ceratostreon hilli (Cragin, 1893)

483

Fig. 10

484 485

pars 1888 Ostrea franklini Coquand, 1869; Hill, p. 131, pl. 5, figs. 1-10, non figs. 11-18; pl. 7, fig. 30, non figs.

486

28, 29. Non pl. 6.

487 488

1893 Exogyra hilli n.sp.; Cragin, p. 186.

489 490

1947 Exogyra hilli Cragin, 1893; Stanton, p. 33, pl. 24, fig. 7, pl. 25, figs. 1-14.

491 492

Diagnosis: Shell small-sized, suboval to crescentic. Left valve with marked keel, dividing valve in a posterior

493

third and anterior two-thirds. With coarse, rounded ribs that start on the keel and traverse the valve towards

494

the anterior margin of the valve with a somewhat diagonal trajectory. Occasionally, tuberculated ribs.

495

Chomata present on both valves.

496 497

Material: 729 specimens from the Spiticeras damesi ammonite Zone (late Berriasian), Vaca Muerta Formation

498

(MCNAM-PI 24934.1-24934.38) and from the Lissonia riveroi ammonite Zone (early Valanginian), Mulichinco

499

Formation (MCNAM-PI 24935.1-24935.7 and 24936.1-24936.), Sierra de la Cara Cura locality. 195 specimens

500

from the Lissonia riveroi ammonite Zone (early Valanginian), Mulichinco Formation (MCNAM-PI 24937.1-

501

24937.195), Puesto Sierra de Reyes locality (Fig. 4).

502 503

Stratigraphic and geographic range: The species has been recorded from the Early Cretaceous (Aptian–Albian)

504

of USA (Texas), and now in the late Berriasian–early Valanginian of Argentina.

505 506

Description: Shell small-sized, inequivalve, sub-oval to crescentic with opisthogyrate umbo. attachment area

507

small, located antero-dorsally. Left valve with prominent keel, dividing the valve in approximately one anterior

508

and two posterior parts (Fig. 10 F, L). Valve descending smoothly from the keel towards the posterior margin,

509

whereas sloping steeply from the keel towards the anterior margin. Anterior part of the valve with coarse,

510

rounded, irregular ribs separated by wide interspaces that start at keel (Fig. 10 J). Number of ribs variable

511

between specimens. Ribs initially narrow, becoming thicker towards the anterior margin. They are intersected

512

by growth lines, which gives them a scaly appearance, and might present tubercles throughout (Fig. 10 J). In

513

some specimens, one of the ribs wider and larger than the others, protruding slightly (Fig. 10 C, G). Keel also

514

with tubercles where crossed by ribs (Fig. 10 F, L). Posterior part of the valve with thick scaly growth lines.

515

Ligament area predominantly gyrostreoid and secondarily exogyroid (Fig. 10 B, C, M, N). Paradontal recess and

516

buttress present, but developed variably (Fig. 10 B, C, M, N). Adductor muscle scar comma-shaped to

517

biconcave (Fig. 10 A, B, I, K, N). Chomata surrounding the valve margins, although occasionally absent on the

518

ventral part (Fig. 10 C, N). Right valve generally slightly concave, also with a keel near the anterior margin,

519

formed by raised growth lines (Fig. 10 D, J). From the keel towards the posterior margin, only thick,

520

commarginal, almost scaly growth lines are visible. Shell interior, below ligament area towards the posterior

521

margin, with paradontal process, although variably developed (Fig. 10 A, E, H, K, O). Commonly, this structure

522

is followed by an alate projection with very sculptured chomata (Fig. 10 A, E, G, K, O). In some specimens, this

523

projection is accompanied by a postero-dorsal platform (Fig. 10 G). When this is the case, left valve with a

524

corresponding depressed projection on the posterior margin where the right valve alate projection articulates

525

(Fig. 10 E). Also, to accommodate the right valve postero-dorsal platform, the left valve posterior margin tends

526

to be slightly flatter and straight, generating a more sub-oval outline of the shell. Chomata surround the whole

527

right valve, although occasionally absent on the ventral part. When relict chomata cross the raised growth

528

lines on the anterior margin of the valve, they generate thin striae, perpendicular to growth lines (Fig. 10 J, P).

529 530

Microstructure: Left valve with a relatively thick prismatic outer layer (Fig. 11 A, B). The outer-middle layer

531

with a complex herringbone cross-foliated microstructure, whereas the inner-middle layer displays a complex

532

cross-foliated to chomata-influenced cross-foliated microstructure (Fig. 11 A-D). Thin lenticular chambers

533

secondarily filled with granular calcite cement appear near the umbo and towards the ventral commissure.

534 535

Dimensions: Mean H: 28.86±6.43 mm; mean L: 18.38±3.94 mm; mean H/L: 1.58±0.14.

536 537

Discussion: Ceratostreon encompasses many species that in some cases are not clearly defined, primarily due

538

to high phenotypic plasticity.

539

C. flabellatum (Goldfuss, 1834, p. 38, pl. 87, fig. 6) from the Early Cretaceous of Germany, differs from C. hilli

540

because of its thinner and more numerous ribs on the left valve. Also, the ribs from C. hilli emerge from the

541

keel towards the anterior margin, whereas no ribs are present on the posterior margin. In C. flabellatum, there

542

is no visible keel and ribs may also appear on the posterior region of the valve.

543

C. spinosum (Mathéron, 1843, p. 264, pl. 32, figs. 6 and 7) from the Late Cretaceous of France, has similar

544

coarse ribs as C. hilli, but its ribs have hollow, scaly extensions forming spines, which are absent in C. hilli.

545

D’Orbigny (1846) changed the name of the species to ‘O.’ matheroniana without providing any reason

546

(Stenzel, 1971); hence it is regarded as a junior synonym of C. spinosum. C. pliciferum (Dujardin, 1837), closely

547

related to C. spinosum (by some authors considered as a variety, (Coquand, 1869); or even as a synonym,

548

(Malchus, 1990) differs from C. hilli because it has more numerous and thinner ribs which may also produce

549

spines (Aqrawabi, 1993).

550

C. boussingaulti (d’Orbigny, 1842, p. 91, pl. 18, fig. 20, pl. 20, figs. 8, 9) from the Early Cretaceous of Colombia

551

and Venezuela, is similar to C. hilli in general outline, presence of a keel and coarse ribs but it differs by having

552

ribs also on the posterior region of the left valve (pl. 18, fig. 20; pl. 20, figs. 8,9).

553

In the original description of C. tuberculiferum (Koch and Dunker, 1837, p. 54, pl. 6, fig. 8) from the Berriasian–

554

Barremian of Germany, the only valve available for description, a right one, has rows of small knobs, which has

555

been regarded as a diagnostic character. Although the authors discarded the possibility of a xenomorphic

556

sculpture, this is highly likely (as also noted by Coquand, 1869). However, in further publications, the knobs of

557

the original designation of C. tuberculiferum have been largely ignored (Calzada-Badía and Botero-Arango,

558

1979; Dhondt and Dieni, 1988; Scott, 2007). C. hilli differs from C. tuberculiferum because it lacks the knobby

559

ornament of the right valve but also because it does not have ribs on the posterior region of the left valve that

560

have been mentioned in later publications of C. tuberculiferum (Calzada-Badía and Botero-Arango, 1979).

561

Coquand (1869, p. 183, pl. 64, figs. 13, pl. 73, figs. 5-9, pl. 74, figs. 14 and 15) erected the species ‘O.’ minos

562

from the Berriasian–Barremian of France, differentiating it from ‘O.’ tuberculiferum and ‘O.’ boussingaulti

563

because of its larger size, its more numerous ribs and the presence of ribs also on the right valve. The figured

564

specimens vary between having a well-developed keel (Coquand, 1869, pl. 74, fig. 15) or not (op. cit., pl. 73,

565

figs. 6 and 9). Therefore, C. minos (Coquand, 1869) has an overall similar outline and, presumably, a marked

566

keel on the left valve as C. hilli but differs by having ribs on the right valve. Also, C. texanum (Roemer, 1852,

567

p.69, pl.10, fig.1) from the Barremian of Texas, very similar to C. minos, differs from C. hilli because of the

568

presence of dichotomous ribs on its right valve.

569

C. pauperculum (Cragin, 1893, p. 186, pl. 30, figs. 7, 8) from the Early Cretaceous of USA differs from C. hilli

570

because, although the original description does not state it explicitly, the figured specimens seem to have ribs

571

on the postero-dorsal region of the left valve, instead of on the anterior region as in C. hilli.

572

C. lanchum (Stoyanow, 1949, p. 66, pl.10, figs. 4-6) from the Albian of USA differs from C. hilli because it lacks

573

an acute keel, having instead an “obtuse angulation” and shallow ribs in the ventral region of the right valve.

574

C. yabei (Nagao, 1934, p. 202, pl. 25, fig. 7, pl. 26, fig.1, pl. 27, figs.1, pl. 28, figs.1 and 2, pl.29, fig. 1, 14.) from

575

the Aptian–Albian of Japan seems to differ from C. hilli because its description does not mention the acute

576

keel that characterizes the latter species. C. yabei has ribs on the left valve, but it is not clear, neither from the

577

description nor from the figured specimens whether they are present only in the anterior region of the valve

578

(as C. hilli) or cover the whole valve.

579

C. acuticosta (Nyst and Galeotti, 1840, p. 213, fig. 2) from the Jurassic of Mexico differs from C. hilli because it

580

lacks a marked keel and, consequently, ribs start at the umbo. This species was figured wrongly in its original

581

publication, with a prosogyrate umbo, while it was described as opisthogyrate. However, in subsequent

582

publications, figured specimens (photographed, not drawn) show a clear opisthogyrate umbo (Alencáster de la

583

Cserna, 1956).

584 585

Genus Nanogyra Beurlen, 1958

586 587

Type species: Gryphaea nana Sowerby, 1822; p. 114, pl. 383, fig. 3.

588 589

Diagnosis (modified from Stenzel, 1971): Shell small, variable in outline and shape, inequivalve; opisthogyrate,

590

although variably spiraled. Left valve globular to moderately convex, suborbicular, subtrigonal, elliptical, ovate

591

to comma-shaped; smooth or with fine radial ribs or rough commarginal growth squamae with local puckers

592

or constrictions in some places. Right valve flat to partly concave or gently convex, outline suborbicular, ovate

593

or comma-shaped. Chomata present in some species. Posterior bourrelet reduced in length.

594 595

Remarks: Mirkamalov (1963) erected the genus Palaeogyra largely overlapping Nanogyra Beurlen. The former

596

was later considered as a junior synonym (Stenzel, 1971). Koppka (2015) considered that significant

597

differences existed between them (i.e. presence of ornamentation and chomata), therefore proposing two

598

subgenera: N. (Palaeogyra) encompasses all species with chomata and ornamentation, whereas N. (Nanogyra)

599

includes species without these features.

600

Subgenus Nanogyra (Nanogyra) Beurlen, 1958

601 602 603

Diagnosis (modified from Koppka, 2015): Shell oval to subrectangular, occasionally elongated. Opisthogyrate

604

umbo and exogyroid hinge. Ribs and chomata absent.

605 606

Nanogyra (Nanogyra) brevisulcata n. sp.

607

Fig. 12

608 609

Etymology: Word combination derived from Latin brevis: shallow, superficial; and sulcata: sulcate, with a

610

sulcus.

611 612

Age: Late Berriasian, top of the Argentiniceras noduliferum Zone to Spiticeras damesi Subzone.

613 614

Type locality and stratotype: Sierra de la Cara Cura (Mendoza, Argentina), 22 m below the Intra-Valanginian

615

Discontinuity.

616

617

Holotype: MCNAM-PI 24932.122

618 619

Paratypes: MCNAM-PI 24932.18, 24932.20, 24932.27, 24932.32, 24932.52, 24932.64, 24932.67, 24932.73,

620

24932.105, 24932.123, 24932.141, 24932.157, 24932.174 and 24932.175.

621 622

Additional material: MCNAM-PI 24932.1-24932-175, except for the numbers of the holotype and paratypes;

623

MCNAM-PI 24933.1-24933.19; MCNAM-PI 24938.1-24938.4.

624 625

Diagnosis: Shell small size, triangular to elongate-ellipsoidal. Left valve with a shallow rounded posterior keel

626

that divides it into an anterior part that is twice the size of the posterior one. Development of an anterior fold

627

separated from the posterior keel by a shallow to very shallow sulcus. No ornamentation except for smooth

628

growth lines. No chomata.

629 630

Description: Valves small in size, inequivalve, generally triangular but occasionally elongate-ellipsoidal or

631

reniform, opisthogyrate umbo (Fig. 12 B, D, I). Attachment area small, postero-dorsally located. Left valve with

632

a rounded but pronounced keel, dividing the valve in a posterior third and two anterior parts (Fig. 12 A, G, J, K,

633

L, M, N). Keel starting slightly below the umbo and extending to the postero-ventral margin following a

634

somewhat diagonal trajectory. Anterior fold at variable heights within the anterior two-thirds of the valve, but

635

always well beneath the umbo (Fig. 12 A, C, G, J, K, M). This fold is generally rounded and shallow, following an

636

antero-dorsal to antero-ventral trajectory. The spatial arrangement of both the posterior keel and the anterior

637

fold generates the triangular shape of the valve (Fig. 12 B, K, M, N). Valve with a more or less pronounced

638

sulcus between keel and fold (Fig. 12 A-C, G, J-N). In some specimens, sulcus very shallow, but always

639

noticeable as a change in the curvature of the growth lines. No ornamentation except growth lines (Fig. 12 G,

640

K). Small, rare tubercles may appear where growth lines intersect posterior keel and anterior fold (Fig. 12 K).

641

From the keel towards the posterior margin, valve descending steeply but generally straight. In some

642

specimens, however, this part of the valve is slightly concave or convex (Fig. 12 F). Anteriorly of the keel, valve

643

descending gently, exhibiting its main convexity. Ligament area gryphaeoid to gyrostreoid (Fig. 12 E, O).

644

Adductor muscle scar not visible in any of the studied specimens. Right valve flat to slightly concave, with

645

commarginal, lamellose growth lines (Fig. 12 D, E, H, I). Some specimens with an anterior ridge of raised

646

growth lamellae (Fig. 12 H).

647 648

Microstructure: Valves with thick simple-prismatic outer layer, an outer-middle complex-cross foliated layer

649

and, lastly, an inner-middle regular-foliated layer (Fig. 13 A-C). Hollow shell chambers present at least in left

650

valve, which are secondarily filled with granular calcite cement (Fig. 13 D).

651 652

Dimensions: Mean H: 22.79±5.49 mm; mean L: 14.55±3.80 mm; mean H/L: 1.62±0.39.

653 654

Discussion: The specimens belong to the genus Nanogyra because of their small size, general outline,

655

exogyroid ligament area with reduced posterior bourrelet and general microstructure. They belong to the

656

subgenus Nanogyra because of the lack of ornamentation and chomata. The following species included in

657

Nanogyra (Nanogyra) are considered to differ from the material studied here:

658

N. (N.) nana (Sowerby, 1822; p. 114, pl. 383, fig. 3) from the Kimmeridgian of England, differs from N. (N.)

659

brevisulcata because of its generally ovate to suborbicular outline and globular, capacious left valve.

660

Moreover, it has a more pronounced opisthogyrate umbo and lacks the diagnostic posterior keel and anterior

661

fold of N. (N.) brevisulcata. Koppka (2015) considered N. praevirgula (Douvillé and Jourdy, 1874; subsequently

662

published by Jourdy, 1924, p. 65, pl. 9, fig.2) as a “doubtfully valid species” and a xenomorphic N. (N.) nana.

663

However in the material available online (MNHN.F.R52889 and 52890; Jourdy collection, syntypes; Muséum

664

National d’Histoire Naturelle, Paris, 2019), the specimens assigned to N. praevirgula seem to present exhibit in

665

the left valve, and not what may be considered as xenomorphic sculpture. Also, these specimens seem to have

666

chomata near the umbo and in the ventral area. If these features are confirmed, the species N. praevirgula

667

should be included in the subgenus Paleogyra and not Nanogyra.

668

N. (N.) auricularis (Münster in Goldfuss, 1834; p. 20, pl. 79, fig.7a-b) from the Hettangian of Germany differs

669

from N. (N.) brevisulcata in its overall oval shape and the lack of a visible sulcus in the left valve. As pointed

670

out by Koppka (2015), the original drawings in Goldfuss (1834) portrait a prosogyrate umbo rather than an

671

opisthogyrate one. However, this is attributed to an error in the drawing. Considering the material available

672

online (MNHN.F.R52868; Jourdy collection, Muséum National d’Histoire Naturelle, Paris, 2019), this specimen

673

apparently has a posterior keel as N. (N.) brevisulcata, but lacks the anterior fold.

674

N. (N.) crassa (Smith, 1819; p. 30, fig. 6) from the Bathonian of England, differs from N. (N.) brevisulcata in its

675

overall very elongated outline (almost linguliform sensu Koppka, 2015) and in lacking both, the posterior keel

676

and the anterior fold. This is corroborated both by figures in Fischer (1969; p. 95, pl. 10, figs. 21-22) and by the

677

material available online (MNHN.F.R05040; Fischer collection, Muséum national d’Histoire naturelle, Paris,

678

2019).

679

N. (N.) monoptera (Eudes-Deslongchamps and Eudes-Deslongchamps, 1858; p. 159, pl. 5, figs.1-4) from the

680

Toarcian of France, differs from N. (N.) brevisulcata because of the presence of a sickle-shaped projection on

681

the postero-ventral margin of the shell, which generates an overall sickle-shaped-like outline. Also, the

682

presence of a postero-dorsal to slightly postero-mesial auricle differentiates N. (N.) monoptera from N. (N.)

683

brevisulcata. These two species appear to have in common a smooth keel in both the right and left valve.

684

However, in N. (N.) monoptera, the left valve keel has a more anterior position than in N. (N.) brevisulcata.

685

N. rivelensis (de Loriol, 1904; p. 256, pl. 25, figs. 11-13; MNHN.F.A26394, syntypes, Maire collection; Muséum

686

National d’Histoire Naturelle, Paris, 2019) from the Oxfordian of France, seems to be more subrectangular in

687

shape and has a slightly elevated, relatively big adductor muscle scar, which is not seen in for N. (N.)

688

brevisulcata. These species have in common a shallow keel on the right valve, although in N. rivelensis it has a

689

more mesial position.

690

N. (N.) roederi (de Loriol, 1904; p. 254, pl. 20, figs. 14-21) from the Oxfordian of France, differs from N. (N.)

691

brevisulcata because of its generally oval outline and a protruding auricle on the posterior margin of the right

692

valve. This auricle might be in a mesial or slightly dorsal position. N. (N.) roederi seems to have a keel in the

693

left valve, although in a mesial, not posterior position as in N. (N.) brevisulcata, but lacks the anterior fold.

694

Both species carry a shallow keel on the right valve, formed by raised growth lines.

695

N. (N.) tramauensis (Cox, 1952; p. 94, pl. 10, figs. 5a-c, 6a-c) from the Oxfordian of India, differs from N. (N.)

696

brevisulcata due to its reniform to subrectangular outline. This species has a keel in the left valve, although

697

situated towards the anterior margin. It lacks the anterior fold of N. (N.) brevisulcata.

698

N. fourtaui (Stefanini, 1925; p. 168, pl. 39, fig. 3) from the Callovian–Oxfordian of Somalia, has been

699

synonymized with N. nana by Kiessling et al. (2011) and later considered as part of the Subgenus Paleogyra,

700

although with doubt, because the presence of chomata was never determined (Koppka, 2015). N. fourtaui

701

lacks the rounded posterior keel and anterior fold diagnostic of N. (N.) brevisulcata.

702

N. (N.) sp. (Koppka, 2015, pl. 7, fig. 1a-d) from the Kimmeridgian of France, differs from N. (N.) brevisulcata

703

because of a more markedly opisthogyrate umbo and more pronounced growth wrinkles, which carry small

704

scattered tubercles and more or less continuous radial furrows that give the valves a more irregular aspect.

705 706 707

5. Oyster records from the Lower Cretaceous of the Neuquén Basin and its paleobiogeographic implications

708 709

5.1. Revision of oyster records from the marine transgression of the Mendoza Group (Tithonian–Hauterivian)

710

of the Neuquén Basin

711 712

Oysters from the Mendoza Group of the Neuquén Basin have an abundant and quite diverse record at the

713

genus and species levels. These records are here revised in terms of stratigraphic occurrence, ages and type

714

considering oyster abundance, bed geometry, lateral extension and thickness. On one hand, there are many

715

isolated oyster records that correspond to beds with dispersed oysters, mostly single individuals or small

716

clusters. On the other hand, there are records of dense to loose shell-packed oyster massive occurrences

717

(OMOs) of higher abundance, that in turn are classified here as follows: (1) Tabular OMOs of >50 km lateral

718

extension and up to 13 m thick, excellent stratigraphic markers because of their regional extension (type-1

719

OMOs); (2) lenticular to tabular OMOs recorded at different localities extending for 10-50 km laterally, up to 6

720

m thick, important stratigraphic markers because of their regional extension (type-2 OMOs); and (3) small

721

local OMOs of lenticular to tabular geometry, recorded at just one locality, 1 km maximum lateral extent and <

722

1 m in thickness (type-3 OMOs).

723

Oldest records of oysters from the Mendoza Group are from the late Tithonian of the Vaca Muerta Formation

724

and correspond to the genera Liostrea Douvillé and Deltoideum Rollier, recorded in the Windhauseniceras

725

internispinosum and Corongoceras alternans ammonoid zones (Damborenea et al., 1979; Rubilar et al., 2000;

726

Kietzmann et al., 2014). These correspond to isolated records and type-3 OMOs.

727

The genus Aetostreon is recorded in the Tithonian–Hauterivian time interval in the Vaca Muerta, Mulichinco

728

and Agrio formations. There are records of at least four Aetostreon species and all types of OMOs as well as

729

isolated records (Damborenea et al., 1979; Lazo, 2007; Rubilar and Lazo, 2009; Kietzmann, et al. 2014; this

730

paper). A. subsinuatum appears in the lower to middle Berriasian of the Vaca Muerta Formation

731

(Argentiniceras noduliferum zone) as a type-3 tabular OMO (Fig. 14). A. latissimum is recorded from the upper

732

Berriasian to the upper Valanginian of the Vaca Muerta, Mulichinco and Agrio formations (although older

733

records from the Tithonian are proposed by Damborenea et al. (1979) and Kietzmann et al. (2014)). Both

734

isolated records and the three types of OMOs occur (Fig. 14). In the upper Berriasian of the Vaca Muerta

735

Formation (Spiticeras damesi Zone), A. latissimum is recorded as tabular type-3 OMOs. In the lower

736

Valanginian of the topmost part of the Vaca Muerta Formation (Lissonia riveroi Zone), it forms several

737

lenticular type-2 OMOs. In the lower Valanginian of the Mulichinco Formation and its lateral equivalent, the

738

Chachao Formation (O. (O.) atherstoni Zone), A. latissimum conforms a tabular type-1 OMO of great lateral

739

continuity (over 100 km of lateral extent). In the upper Valanginian of the Pilmatué Member of the Agrio

740

Formation (Pseudofavrella angulatiformis Subzone), A. latissimum appears as a type-3 lenticular OMO. A.

741

pilmatuegrossum is recorded within the upper Valanginian of the Pilmatué Member of the Agrio Formation

742

(throughout the Pseudofavrella angulatiformis Zone) (Fig. 14). It occurs as isolated records and lenticular to

743

lentiform type-2 OMOs. Towards the upper Hauterivian of the Agua de la Mula Member of the Agrio

744

Formation (Crioceratites diamantensis Zone), scattered Aetostreon records and type-2 tabular OMOs up to 0.5

745

m in thickness and at least 1 km of lateral extension are recorded, but the species has not been determined

746

(Fig. 14).

747

The genus Nanogyra is restricted to the Berriasian–early Valanginian including scattered records and a tabular

748

type-3 OMO. Only one species (N. (N.) brevisulcata) is recorded (Fig. 14).

749

The genus Ceratostreon is recorded from the early Valanginian–late Hauterivian time interval in the Vaca

750

Muerta, Mulichinco and Agrio formations (Schwarz, 1999; Lazo, 2007; Aguirre-Urreta et al., 2008; this paper).

751

There are records of at least two Ceratostreon taxa, but abundant Ceratostreon records from the Agrio

752

Formation are in need of major revision. All types of OMOs plus isolated records are known, being particularly

753

common as hard substrate encrusters (Fig. 14). In the lower Valanginian of the top Vaca Muerta and the basal

754

Mulichinco formations (Neocomites wichmanni and Lissonia riveroi zones), C. hilli occurs as isolated records

755

and type-2 tabular OMOs. In the lower Valanginian of the Mulichinco Formation (O. (O.) atherstoni Zone), a

756

tabular type-1 OMO occurs. In the Valanginian of the Mulichinco Formation (Karakaschiceras attenuatum

757

Subzone) several tabular type-2 OMOs have been recorded. In the lower Hauterivian of the Pilmatué Member

758

of the Agrio Formation (O. (O.) laticosta Subzone), a lenticular to tabular type-2 OMOs has been recorded.

759

Lastly, in the upper Hauterivian of the Agua de la Mula Member of the Agrio Formation (Crioceratites

760

diamantensis Zone), lenticular type-3 OMOs occur.

761

All these records show that oysters were an important element in the benthic fauna of the Mendoza Group of

762

the Neuquén Basin, forming diverse concentrations. It is remarkable that the OMOs with the widest lateral

763

extension occurring in numerous localities (type-1 OMOs) are restricted to the O. (O.) atherstoni Subzone, in

764

the middle member of the Mulichinco Formation and its lateral equivalent, the Chachao Formation. This

765

interval corresponds to a relative sea-level rise within the lowstand wedge represented by the Mulichinco

766

Formation, during which siliciclastic influx was greatly diminished, allowing the development of carbonate

767

facies (Schwarz, 1999). Also, variations in salinity, well-oxygenated waters and a high nutrient input could have

768

favoured the development of these tabular OMOs that dominated the inner shelf (Schwarz et al., 2011).

769

Hence, type-1 OMOs represent basin-wide palaeoenvironmental changes, whereas type-2 and type-3 OMOs

770

represent subtler variations at a regional or local scale that could be related to discrete sources of nutrient or

771

freshwater input or to shorter but significant arrests of the siliciclastic influx.

772 773

5.2. Palaeobiogeographic distribution of Aetostreon, Ceratostreon and Nanogyra during the Early Cretaceous

774 775

During the Early Cretaceous, the species studied here had mostly a very wide geographic distribution.

776

Aetostreon subsinuatum has been recorded throughout Europe and western of Asia towards the east

777

(Turkmenistan), southern North America and part of the western margin of South America (Fig. 15). Thus, this

778

taxon has mainly a Tethyan Realm distribution, with a significant extension towards the Southern Temperate

779

Realm along the Andean margin, but also towards the southern margin of the Northern Temperate Realm

780

(sensu Kauffman, 1973). This is the oldest record of this species, i.e. Berriasian, since its records in Europe and

781

Asia are younger, ranging from Valanginian to Aptian (Woods, 1913; Dhondt and Dieni, 1988) and in North

782

America (Mexico), it has been recorded from the Valanginian (Imlay, 1937, 1940a).

783

Aestostreon latissimum has a similar distribution pattern to A. subsinuata, but it extends even further back in

784

time, with its oldest records from the Tithonian–Hauterivian of South America (Damborenea et al., 1979; Lazo,

785

2007; Kietzmann et al., 2014) and, later, from the Valanginian–Aptian of the Caribbean (Cox, 1954; Guzmán,

786

1985) and Valanginian–Albian of the eastern and southern margins of Africa (Dhondt et al., 1999). In Europe

787

and Asia its records date from the Valanginian–Aptian interval (Dhondt and Dieni, 1988). This also coincides

788

with a typical Tethyan distribution, extending towards the Southern Temperate Realm (Fig. 15).

789

The oldest records of Aetostreon are from the Oxfordian–Tithonian of Chile ( Rubilar, 2008; Rubilar, 2009;

790

Rubilar and Lazo, 2009), which could indicate that this genus originated in the southeastern Pacific, specifically

791

in the Andean Subprovince (sensu Kauffman, 1973) and later expanded towards the north and through the

792

Proto-Caribbean Sea and the Tethys Ocean. A. latissimum and A. subsinuatum records from the Tithonian and

793

the Berriasian, respectively, of the Neuquén Basin are consistent with this. In the rest of America, Aetostreon

794

is recorded from the Valanginian–Aptian of Colombia (Gerhardt, 1898; Guzmán, 1985); Valanginian–early

795

Aptian of Mexico (Imlay, 1940a; González-León et al., 2008); Valanginian to Hauterivian–Albian of Peru

796

(Sommermeier, 1913; von Hillebrandt, 1970); the Barremian–Aptian interval of Trinidad Island (Cox, 1954) and

797

the Aptian–Albian of USA (Stanton, 1947; Scott, 2007). There is an earlier record of an ‘Exogyra’ potosina

798

Castillo and Aguilera (1895) from the Tithonian of Mexico and USA (Cragin, 1905; Imlay, 1940b; Torres et al.,

799

1999), but it is not clear whether this species truly belongs to Aetostreon (Rubilar and Lazo, 2009). Also,

800

Pugaczewska (1975) mentioned records of Aetostreon from the Tithonian of Poland, but no further

801

information was given.

802

Ceratostreon hilli has been recorded in the southwestern margin of South America and North America (Fig.

803

15), which corresponds to a Trans Temperate Pacific distribution (Damborenea et al., 2012). In South America,

804

this species has been recorded from the Berriasian to early Valanginian time interval, whereas in North

805

America occurs during Aptian–Albian times. This could indicate that the species originated in South America

806

and dispersed later towards the north. This is also consistent with the oldest records of the genus

807

Ceratostreon which are from the Oxfordian–Tithonian of Chile (Rubilar, 2008, 2009). In the rest of the

808

Americas, Ceratostreon is recorded from the Valanginian–Hauterivian of Colombia (d’Orbigny, 1843; Guzmán,

809

1985); Aptian–upper Albian of Peru (Sommermeier, 1913; Dhondt and Jaillard, 2005); upper Albian of Ecuador

810

(Dhondt and Jaillard, 2005); upper Aptian of Venezuela and Mexico (Gerhardt, 1898; Sutton, 1946; Hernández-

811

Ocaña et al., 2015) and Aptian–Coniacian of Brazil (White, 1888; Seeling and Bengston, 1999). In Eurasia,

812

Ceratostreon is recorded from Valanginian–Maastrichtian strata and in Africa, from Albian–Maastrichtian

813

strata (Dhondt and Dieni, 1988; Dhondt et al., 1999).

814

Aetostreon and Ceratostreon distributions are consistent with the observed Tethyan-wide distribution of the

815

Early Cretaceous bivalve fauna, present in the Caribbean, North America, western margin of South America

816

and eastern margin of Africa, with typical pandemic oyster taxa such as A. latissimum, C. boussingaulti, C.

817

tuberculiferum, etc. (Dhondt, 1992; Dhondt et al., 1999). This is related to the position of the major continents

818

during the Early Cretaceous: the Tethys Ocean was the main connection between America and Eurasia,

819

especially considering that the Atlantic Ocean was closed (except for its central part), global sea level was high,

820

major flooding of continents occurred and a circumglobal eastern equatorial circulation existed (Barron, 1987;

821

Poulsen et al., 2001; Miller et al., 2005). As a result, the marine shelf areas were well-connected, which

822

allowed a free larval exchange and resulted in a generally homogeneous bivalve fauna (Kauffman, 1973).

823

Nanogyra (N.) brevisulcata, in contrast, was endemic to the Neuquén Basin (Fig. 15). Previous records of the

824

genus date from the Middle to Late Jurassic of Europe (Bajocian–Kimmeridgian) and, in America, from the

825

middle Oxfordian of Cuba (Pugaczewska, 1978) and from the middle Kimmeridgian of Mexico (Vega and

826

Lawton, 2011). Doubtful records from Chile date from the Sinemurian and Pliensbachian (Malchus and

827

Aberhan, 1998; Rubilar, 2008), but it is not clear whether they belong to Nanogyra or to some other genus

828

(Koppka, 2015). These occurrences suggest a European origin of the genus and a posterior dispersion to the

829

southeastern Pacific margin through the Hispanic Corridor.

830

6. Conclusions

831 832 833

Oysters are recorded abundantly in Upper Jurassic–Lower Cretaceous strata of the Neuquén Basin.

834

Particularly, around the boundary between the Vaca Muerta and Mulichinco formations, four species

835

corresponding to three genera from the Family Gryphaeidae, Subfamily Exogyrinae were recorded and

836

described.

837

Within the genus Aetostreon, two species were recorded. A. subsinuatum is distinguished by an acute keel

838

that traverses the entire valve and a sickle-shaped outline. A. latissimum is distinguished by a shallow keel that

839

almost disappears in the ventral half of the valve and a sub-oval outline.

840

This is the first record of the genus Nanogyra from the Neuquén Basin and Argentina. The new species N. (N.)

841

brevisulcata is characterized by its small size, triangular to elongate-ellipsoidal outline and the presence of a

842

shallow keel and an anterior fold.

843

Within the genus Ceratostreon, the species C. hilli is distinguished by its small size, crescentic outline, coarse

844

ribs and chomata.

845

Throughout the Mendoza Group of the Neuquén Basin, oysters occur scattered or forming OMOs of different

846

morphologies, dimensions and stratigraphic relevance. These OMOs probably represent different scales of

847

palaeoenvironmental changes, of basin-wide to local extent, and could be useful tools to identify variations in

848

sedimentation rate, siliciclastic and nutrient input and/or salinity variations.

849

During the Early Cretaceous, these species had a very wide geographic distribution: A. subsinuatum and A.

850

latissimum had a mainly Tethyan distribution, extending southwards to the Southern Temperate Realm,

851

whereas C. hilli had a Trans-Temperate Pacific distribution, and N. (N.) brevisulcata was endemic to the

852

Neuquén Basin.

853

The oldest records of the genera Aetostreon and Ceratostreon (Oxfordian–Tithonian of Chile) indicate that

854

they originally appeared within the Andean subprovince and later dispersed through the Hispanic Corridor

855

towards the Tethys Ocean. The wide distribution of these genera is related to the well-connected marine shelf

856

areas, a result of the continental distribution and the high global sea level at the time. The records of

857

Nanogyra, instead, indicate an European origin of the genus and its posterior dispersion to the southeastern

858

Pacific margin through the proto-Caribbean Sea.

859

860

Acknowledgements

861

We thank R.E.G. Ezquerro for help during the field work, C.S. Cataldo for help with taxonomic dilemmas; R.M.

862

Palma for providing the means to photograph the oyster microstructure and two anonymous reviewers and

863

the editor for their careful revisions. This work was supported by the Agencia Nacional de Promoción Científica

864

(grant PICT 2015-1381 awarded to D.G.L. and PICT-2013-1413 awarded to M.B. Aguirre-Urreta (Universidad de

865

Buenos Aires), by the Universidad de Buenos Aires (grants UBACyT 2014–2017 awarded to M.B. Aguirre-

866

Urreta) and by the Consejo Nacional de Investigaciones Científicas y Técnicas (grant CONICET PIP 2013-2015

867

awarded to Victor A. Ramos (Universidad de Buenos Aires).

868

869

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870

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892

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898

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Roemer, F.A., 1841. Die Versteinerungen des Norddeutschen Kreidegebirges. Hahnsche Hofbuchhandlung,

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Rollier, L., 1917. Fossiles nouveaux ou peu connus des terrains secondaires (Mésozoïques) du Jura et des

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contrées environnantes. Mémoires de la Société Paléontologique Suisse 42, 501-634.

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Rubilar, 2009. Ostras fósiles del Jurásico Superior (Oxfordiano) en la Cordillera de la Costa de Iquique (I

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REGIÓN): breve caracterización e implicancia de su registro. Congreso Geológico Chileno 12, 10-34.

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Rubilar, A., Damborenea, S., Manceñido, M., 2000. Ostras del Tithoniano-Valanginiano en el sur de Mendoza

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(Argentina). Actas del IX Congreso Geológico Chileno 1, 549.

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Rubilar, A.E., Lazo, D.G., 2009. Description of Aetostreon pilmatuegrossum sp. nov. from the Lower Cretaceous

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of Argentina (Neuquén Basin), and significance of the conservative left valve morphology in oysters of the

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genus Aetostreon Bayle. Cretaceous Research 30, 727-748.

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Rubilar, A.R., 2008. Las ostras fósiles del Mesozoico en Chile. Libro de actas 1º Simposio de Paleontología en

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Chile, 33-38.

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Sacco, F., 1897. I molluschi dei terreni terziarii del Piemonte e della Liguria. Musei Zoologia e Anatomia

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Schwarz, E., 1999. Facies sedimentarias y modelo deposicional de la Formación Mulichinco (Valanginiano),

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Cuenca Neuquina Septentrional. Asociación Argentina de Sedimentología Revista 6 (1-2), 37-59.

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Schwarz, E., Álvarez-Trentini, G., Valenzuela, M.E., 2013. Ciclos mixtos carbonáticos/silicoclásticos en el

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Figure Captions

1204

Figure 1: Map of Argentina and of the Mendoza province (inset) and regional map showing the location of the

1205

fossil localities, Sierra de la Cara Cura (36°40'38.90"S; 69°40'20.70"O) and Puesto Sierra de Reyes

1206

(36°57'40.70"S; 69°38'5.30"O).

1207

Figure 2: Schematic stratigraphic section of the studied area. The rectangle shows the lithostatigraphic interval

1208

section studied. Based on Schwarz and Buatois (2012).

1209

Figure 3: General view of the studied section. A. Approximate stratigraphic position of the Intra-Valanginian

1210

Discontinuity between the Vaca Muerta and the Mulichinco formations. B. Detail of the firmground with

1211

Glossifungites ichnofacies that represents the Intra-Valanginian Discontinuity in the Sierra de la Cara Cura

1212

section.

1213

Figure 4: Stratigraphic section of the top of the Vaca Muerta and the base of the Mulichinco formations at the

1214

Sierra de la Cara Cura section showing units, ammonoid zonations, lithology and oyster records.

1215

Figure 5: Aetostreon latissimum (Lamarck, 1801). Specimens from Sierra de la Cara Cura. All scale bars

1216

represent 1 cm. A, MCNAM-PI 24928.96, left valve, external view. B, MCNAM-PI 24928.85, left valve, internal

1217

view, dorsal area. C, MCNAM-PI 24928.30, left valve, external view. D, MCNAM-PI 24928.43, left valve,

1218

external view. E, MCNAM-PI 24928.59, left valve, internal view. The adductor muscle scar corresponds to the

1219

right valve and is preserved within the sedimentary filling. F, MCNAM-PI 24928.88, left valve, internal view,

1220

dorsal area. G, MCNAM-PI 24928.58, articulated valves, view of right valve. ams: adductor muscle scar; ela:

1221

exogyroid ligament area; gl: growth lines; gla: gyrostreoid ligament area; k: keel; pr: paradontal recess; t:

1222

tubercles.

1223

Figure 6: Microstructure of A. latissimum shell. A. Thin section of articulated specimen the from Sierra de la

1224

Cara Cura, dorsoventrally sectioned. DM, dorsal margin. B. Detail of the microstructure of the left valve. C.

1225

Detail of the microstructure of the right valve. The arrows point towards the inner side of the valve. CB, calcite

1226

bands; CCF, complex cross-foliated; HC, hollow chamber; RF, regular foliated; M-CCF, mosaic complex cross-

1227

foliated.

1228

Figure 7: Aetostreon subsinuatum (Leymerie, 1842). Specimens from Sierra de la Cara Cura. All scale bars

1229

represent 1 cm. A, MCNAM-PI 24926.98, articulated valves, view of right valve. B, MCNAM-PI 24926.125, left

1230

valve, internal view. C, MCNAM-PI 24926.39, articulated valves, view of left valve. D, MCNAM-PI 24926.179,

1231

left valve, external view. E, MCNAM-PI 24926.68, left valve, internal view. F, MCNAM-PI 24926.67, left valve,

1232

internal view. af: anterior fold; ams: adductor muscle scar; ela: exogyroid ligament area; gla: gyrostreoid

1233

ligament area; k: keel; pr: paradontal recess; pb: paradontal buttress; t: tubercles.

1234

Figure 8: Aetostreon subsinuatum (Leymerie, 1842). Specimens from Sierra de la Cara Cura. All scale bars

1235

represent 1 cm. A, MCNAM-PI 24926.226, articulated valves, posterior view of left valve. B, MCNAM-PI

1236

24926.59, left valve, external view. C-D, MCNAM-PI 24926.78, articulated valves. C, view of left valve. D, view

1237

of right valve. E, MCNAM-PI 24926.130, articulated valves, view of right valve. F, MCNAM-PI 24926.11,

1238

articulated valves, view of left valve. G, MCNAM-PI 24926.169, articulated valves, view of left valve. H,

1239

MCNAM-PI 24926.104, left valve, external view. af: anterior fold; agc: anterior globoid convexity; ar: anterior

1240

ridge; gl: growth lines; k: keel; sg: shallow groove; t: tubercles.

1241

Figure 9: Microstructure of A. subsinuatum . A. Thin section of articulated specimen from Sierra de la Cara

1242

Cura, dorsoventrally sectioned. DM, dorsal margin. B. Detail of the microstructure of the right valve. C. Detail

1243

of the microstructure of the umbonal area of the left valve. D. Detail of the microstructure of the left valve.

1244

The arrows point towards the inner side of the valve. CCF, complex-cross foliated; C-CCF, cone complex-cross

1245

foliated; Ha-CCF, high angle complex-cross foliated; HC, hollow chamber; La-CCF, low angle complex-cross

1246

foliated; PL, prismatic layer; RF, regular foliated.

1247

Figure 10: Ceratostreon hilli (Cragin, 1893). Specimens from Sierra de la Cara Cura. All scale bars represent 1

1248

cm. A, MCNAM-PI 24936.1, right valve, internal view. B, MCNAM-PI 24936.5, left valve, internal view. C,

1249

MCNAM-PI 24936.8, left valve, internal view. D, MCNAM-PI 24936.10, articulated valves, view of right valve. E,

1250

MCNAM-PI 24936.9, articulated valves, view of posterior area. F, H, MCNAM-PI 24936.12, articulated valves,

1251

view of left valve and of posterior area, respectively. G, J, MCNAM-PI 24936.11, articulated valves, general

1252

view of right valve and view of anterior area, respectively. I, MCNAM-PI 24936.2, right valve, internal view. K,

1253

O, MCNAM-PI 24936.4, right valve, general internal view and internal view of dorsal area. L, MCNAM-PI

1254

24936.13, articulated valves, view of left valve. M, MCNAM-PI 24936.6, left valve, internal view. N, MCNAM-PI

1255

24936.7, left valve, internal view. P, MCNAM-PI 24936.3, right valve, internal view of dorsal area. ams:

1256

adductor muscle scar; ap: alate projection; ch: chomata; ela: exogyroid ligament area; gla: gyrostreoid

1257

ligament area; k: keel; pb: paradontal buttress; pdp: postero-dorsal platform; pp: paradontal process; pr:

1258

paradontal recess; pro: protruding rib; r: rib; rch: relict chomata; t: tubercles.

1259

Figure 11: Microstructure of C. hilli shell. A. Thin section of articulated specimen from Sierra de la Cara Cura,

1260

dorsoventrally sectioned. DM, dorsal margin. B, C. Detail of the microstructure left valve. D. Detail of

1261

microstructure of the umbonal area. The arrows point towards the inner side of the valve. CCF, complex cross-

1262

foliated; Ci-CCF, chomata-induced complex-cross foliated; Hb-CCF, herringbone complex-cross foliated; PL,

1263

prismatic layer; RF, regular foliated. RV, right valve; LV, left valve.

1264

Figure 12: Nanogyra (N.) brevisulcata, n. sp. Specimens from Sierra de la Cara Cura. All scale bars represent 1

1265

cm. A, F, J, MCNAM-PI 24932.122. Holotype, left valve. Anterior view, posterior view and general external

1266

view, respectively. B, MCNAM-PI 24932.52. Paratype, left valve, external view. C, MCNAM-PI 24932.27.

1267

Paratype, left valve, anterior view. D, MCNAM-PI 24932.174. Paratype, articulated valves, view of right valve.

1268

E, MCNAM-PI 24932.163, articulated valves, view of right valve. G, MCNAM-PI 24932.67. Paratype, left valve,

1269

external view. H, MCNAM-PI 24932.123. Paratype, articulated valves, view of right valve. I, MCNAM-PI

1270

24932.175. Paratype, articulated valves, view of right valve. K, MCNAM-PI 24932.32. Paratype, left valve,

1271

external view. L, MCNAM-PI 24932.141. Paratype, articulated valves, view of left valve. M, MCNAM-PI

1272

24932.73. Paratype, left valve, external view. N, MCNAM-PI 24932.20. Paratype, left valve, external view. O,

1273

MCNAM-PI 24932.153, fragment of left valve, internal view. af: anterior fold; ar: anterior ridge; gl: growth

1274

lines; grla: gryphaeoid ligament area; gyla: gyrostreoid ligament area; k: keel; s: sulcus; t: tubercle.

1275

Figure 13: Microstructure of N. (N.) brevisulcata. A. Thin section of articulated specimen from Sierra de la Cara

1276

Cura, dorsoventrally cut. DM, dorsal margin. B. Detail of the microstructure of the umbonal area of left valve.

1277

C. Detail of the microstructure of the right valve. D. Detail of the microstructure of the left valve, antero-

1278

posteriorly sectioned. The arrows point towards the inner side of the valve. CCF, complex-cross foliated; HC,

1279

hollow chamber, PL, prismatic layer; RF, regular foliated.

1280

Figure 14: Biostratigraphic scheme of oyster occurrences in the Mendoza Group of the Neuquén Basin

1281

showing units, ammonoid zonations and type of oyster record (isolated records, type-1, type-2 and type-3

1282

OMOs).

1283

Figure 15: Distribution of A. latissimum, A. subsinuatum, C. hilli and N. (N.) brevisulcata during the Early

1284

Cretaceous. Realm boundaries according to Kauffman (1973). Palaeo-coastline reconstruction after Smith et

1285

al. (1994).

1286

1287

Toscano and Lazo: material collection and study; conceptualization, visualization and writing of the manuscript. Lazo: project supervision and funding acquisition.