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First occurrence of Pterotheca (Gastropoda) from the Silurian (Aeronian) of Spain
Accepted Manuscript Title: First occurrence of Pterotheca (Gastropoda) from the Silurian (Aeronian) of Spain Author: Jan Ove R. Ebbestad Juan Carlos Guti´errez-Marco PII: DOI: Reference:
S0016-6995(16)30091-2 http://dx.doi.org/doi:10.1016/j.geobios.2017.02.003 GEOBIO 777
To appear in:
Geobios
Received date: Revised date: Accepted date:
7-10-2016 2-2-2017 21-2-2017
Please cite this article as: Ebbestad, J.O.R., Guti´errez-Marco, J.C.,First occurrence of Pterotheca (Gastropoda) from the Silurian (Aeronian) of Spain, Geobios (2017), http://dx.doi.org/10.1016/j.geobios.2017.02.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
First occurrence of Pterotheca (Gastropoda) from the Silurian (Aeronian) of Spain Jan Ove R. Ebbestad a,*, Juan Carlos Gutiérrez-Marco b
Museum of Evolution, Uppsala University, Norbyvägen 16, 752 36 Uppsala, Sweden
b
Instituto de Geociencias (CSIC, UCM), Facultad de Ciencias Geológicas, José Antonio
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a
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Novais 12, 28040 Madrid, Spain
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Corresponding author. E-mail addresses: [email protected],
Corresponding editor: Bertrand Lefebvre.
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[email protected] (J.O.R. Ebbestad).
Abstract
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Pterotheca hispanica nov. sp. is described from the basal beds (Stimulograptus sedgwickii graptolite Zone) of the Formigoso Fm., in the Cantabrian Zone of the Iberian
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Massif, north-western Spain. This specialized bellerophontoid gastropod is for the first time recorded in the Silurian of peri-Gondwana; it is the only Aeronian species known so far. Its most closely allied relatives are from higher beds placed in the Telychian of Scotland. Pterotheca is considered a highly derived genus within the Pterothecinae of the
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Carinaropsidae, with the strongly reduced to absent coiling and the growth of the septum being synapomorphies. In the present work it is shown that not only the apical angle of the septum can be used to distinguish species, but also the relative length and width of the septum. In some cases, the slit may extend as far back as the anterior part of the internal septum. Silurian species were hitherto only found in marginal Laurentian and Eastern Avalonian settings. Pterotheca is absent from central Laurentia and Baltica, where it could be expected to be present based on its wide Ordovician distribution. In the light of this the occurrence of the genus in the Aeronian of Spain seems difficult to explain, albeit the genus was present in peri-Gondwana prior to the end-Ordovician extinction. The extreme scarcity of
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described taxa recorded from Avalonia, Baltica and Iberia during the Early Silurian may explain this patchy distribution.
Keywords: Bellerophontoidea Carinaropsidae
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Pterothecinae Silurian
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Palaeobiogeography
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Iberian Peninsula
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1. Introduction
Pterotheca Salter, 1853 is a readily identifiable bellerophontoid gastropod with a low ovate to sub-rectangular shell and a raised median carina. Despite its bellerophontoid affinity
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and marked incurved apex it lacks true coiling of the shell (Ulrich and Scofield, 1897; Reed, 1909; Knight, 1941). Internally, the shell contains a triangular septum at the posterior end,
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and this is often exposed or outlined on the fossilized shell. Early workers variably placed the genus with the Brachiopoda, or the gastropods Pteropoda or Heteropoda. A growing consensus eventually placed it within the bellerophontoids based on the close relationship
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with the Ordovician bellerophontoid Carinaropsis Hall, 1847. For further details on the early taxonomic history of Pterotheca, see Teichert (1928, 1935). Most of the more than 30 species of Pterotheca are found in the Late Ordovician of Laurentia and Baltica. A single species is known from the Late Ordovician of Avalonia and
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another one from peri-Gondwana. The genus suffered heavily at the end-Ordovician extinction with only five species hitherto found in the Silurian: four species from Scotland representing marginal Laurentian settings, and one from Shropshire, England, representing Avalonian settings (Table 1). Pterotheca is here recorded for the first time from the Silurian of peri-Gondwana. A single specimen of Pterotheca hispanica nov. sp. was found in the Early Silurian Formigoso Fm. of the Cantabrian Zone, north-western Spain. This is also the only Aeronian species known so far. The new species is compared to other Silurian Pterotheca species, and their morphology and distribution are discussed.
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2. Geological setting The first undisputable record of the genus Pterotheca from SW Europe comes from a single outcrop of Silurian shales located at the southern slope of the Cantabrian Cordillera in northern Spain. This corresponds to the Pontedo section, placed in the Bodón nappe of the Cantabrian Zone of the Iberian Massif (Fig. 1). The locality lies east of the village of Pontedo (province of León), and was discovered by Aramburu (1989: profil 32, appendix 17) who
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reported diverse Llandovery graptolites, trilobites, brachiopods, molluscs, echinoderms, and cornulitids. The fossil assemblage occurs within a 6 m-thick unit of green and grey shales that
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underlies the typical graptolitic black shales that usually characterize the lower part of the
Formigoso Fm. (Llandovery-Wenlock) in the whole Cantabrian Zone. Although Aramburu
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(1989) originally placed the fossiliferous shales towards the upper part of his “Getino Beds” – interpreted by him as a “condensed” sucession from Middle Ordovician to early Silurian
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strata–, Aramburu and García-Ramos (1993) separated these from the preceding unit, naming it the “Pontedo Shales”. According to these authors, the latter are a local and lateral correlative of the uppermost “Getino Beds”. A different stratigraphic view was presented by
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Bernárdez et al. (2006), who placed the highly fossiliferous “Pontedo beds” as bracketed by the glauconitic sandstone that constitutes the base of the Formigoso Fm. at a regional level,
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and the black graptolitic shales of the remaining part of the same unit. The Formigoso Fm. in the Pontedo section rests unconformably on the Late Ordovician (Hirnantian) diamictites of the Pontedo paleovalley (Bernárdez et al., 2006; Gutiérrez-Marco et al., 2010).
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Besides the single specimen of the gastropod Pterotheca and the diagnostic graptolites of the lower Stimulograptus sedgwickii Zone occurring in the same bed (Neolagarograptus tenuis (Portlock), Metaclimacograptus undulatus (Kurck), Parapetalolithus clandestinus Štorch), the associated fauna consists of trilobites, molluscs, brachiopods, echinoderms, and
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cornulitids (Bernárdez et al., 2006). Until now, the genus Pterotheca was only previously cited from the Ordovician of Spain (as “Pteroteca [sic.] consobrina, Ba.”) by Hernández Sampelayo (1915: p. 297), a species restricted to the Kralodvorian (Upper Ordovician) of Bohemia, Czech Republic. Its supposed record in the Middle Ordovician of north-western Spain was reviewed by Gutiérrez-Marco et al. (1999: p. 23), who attributed the specimens to flattened fragments of orthocone nautiloids. Recently Pterotheca sp. was also cited in an unpublished Ph.D. thesis on the Silurian of Central Spain (Lorenzo Álvarez, 2015). However, the generic attribution of the single specimen, recorded from the Monograptus riccartonensis graptolite Zone (Wenlock), is dubious because a clear septum is not recognizable; this material needs re-examination.
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3. Material and methods A single specimen of Pterotheca hispanica nov. sp. is known (MGM 900S), but in addition figured and described type material of other Silurian species has been studied for first hand comparisons. Typically, only two to three specimens are known for each species, the exception being Pterotheca gath Lamont, 1978 which is quite abundant in the Pterotheca
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beds from the Pentland Hills of Scotland (Clarkson et al., 1995, JORE pers. obs.) and thus in the collections of the National Museum of Scotland.
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Five measurements were made on the most complete of the available specimens; the
length (L) and width (W) of the shell, the length (LS) and width (WS) of the internal septum,
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and the septal angle (Fig. 2). Where the septum meets the dorsal shell, a septal suture is formed, which is commonly expressed on the fossil; the angle formed posteriorly by the
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diverging septal sutures is the septal angle (Fig. 2). As pointed out by Wahlman (1992), the septal angle (SA) is a useful measurement for separating species and can be readily measured. Likewise, the tangential length of the septal sutures is easy to measure and often given in the
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literature. Although sometimes given in the literature, the true sagittal length of the septum is more difficult to estimate as the anteriorly curved anterior septal margin may be obscured. To
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avoid conflict between cited measurements, the exsagittal length of the suture from the posterior margin to the anterior tip of the septal suture was considered in this study (Fig. 2; Table 2). Measurements were taken from the literature, or made in Adobe Photoshop® and/or
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CorelDraw® from published drawings or figures as well as on photographs made by the first author on original specimens (Table 2). The data was analysed using Microsoft Excel® and PAST (Hammer et al., 2001).
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4. Systematic palaeontology
The specimens described and illustrated here are housed in the following public collections: MGM, Museo Geominero, Madrid; NMS, National Museum of Scotland; BGS, British Geological Survey from their old museum collections (GSM); NHMUK, Natural History Museum, London. The acronym RSMGY cited by Clarkson et al. (1995) for Pterotheca aff. mullochensis is now NMS (Table 2). Specimens were coated with ammonium chloride sublimate prior to photography. Bellerophontoidea M’Coy, 1851 Carinaropsidae Ulrich and Scofield, 1897
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Pterothecinae Fischer, 1883 Genus Pterotheca Salter, 1853 Type species: By original designation of Salter (1853: p. 61), Atrypa transversa Portlock, 1843 (p. 455), most likely from the Caradoc (Sandbian 2) Bardahessiagh Fm. of Desertcreat, Co. Tyrone, Northern Ireland. Remarks: Fischer (1883: p. 432) erected the family Pterothecidae for Pterotheca and
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tentatively Phragmotheca Barrande, 1867; the latter was interpreted as a bivalve by Horný (1964). Fischer’s classification was followed by Walcott (1886) to which Teichert (1928)
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attributed the family name. Ulrich and Scofield (1897: p. 857, 926, 927) pointed out
similarities between Carinaropsis Hall, 1847 and Pterotheca in their shell form, the strongly
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reduced coiling, the median carina, and in the development of an internal plate; they united them within the family Carinaropsidae Ulrich and Scofield, 1897 of the Bellerophontoidea
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M’Coy, 1851, ranked by Ulrich and Scofield (1897: p. 844) as suborder Bellerophontacea. Subsequent studies have broadly concurred with the close association suggested by Ulrich and Scofield (1897), albeit with different opinions on the higher-level classification. Wenz (1938)
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preferred the two families Carinaropsidae and Pterothecidae within the Bellerophontoidea, while Knight et al. (1960) ranked these as subfamilies within the family Bellerophontidae.
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They also expanded these families by including additional genera so that the subfamily Carinaropsinae encompassed Carinaropsis, Bucanopsis Ulrich in Ulrich and Scofield, 1897, Sphenosphaera Knight, 1945, Phragmosphaera Knight, 1945, and Gyrospira Boucot, 1956,
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while the subfamily Pterothecinae encompassed Pterotheca, Cyclotheca Teichert, 1935, Aspidotheca Teichert, 1935, and Pedasiola Spriestersbach, 1919. Knight et al. (1960: p. 181) changed the taxonomic level of the Pterothecidae to the subfamily level by nomen translatum but as most subsequent authors they attributed the name to Wenz (1938). Horný (1963) could
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not accept a placement of the pterothecids within the family Bellerophontidae because a true selenizone could not be confirmed and the morphology was too highly specialized. He therefore reverted to place pterothecids at the family level sensu Wenz (1938). Wahlman (1992) partly reverted to the original concept of Ulrich and Scofield (1897), using the family Carinaropsidae within the superfamily Bellerophontoidea, but he recognized three subfamilies: the Carinaropsinae with only Carinaropsis and Gyrospira, Pterothecinae with only Pterotheca and Cyclotheca, and the Devonian Pedasiolinae Wahlman, 1992 including Aspidotheca and Pedasiola. The genera Bucanopsis and Sphenosphaera were transferred to the new subfamily Bucanopsinae Wahlman, 1992 of the Bellerophontidae. Clarkson et al.
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(1995) used the subfamily Pterothecinae of the family Bellerophontidae but followed Wahlman (1992) by including only Pterotheca and Cyclotheca. Frýda and Bouchet in Bouchet and Rocroi (2005) followed Wahlman (1992) in recognizing the subfamilies Pterothecinae, Carinaropsinae and Pedasiolinae. These were placed in the family Pterothecidae of the superfamily Bellerophontoidea, correctly identifying Fischer (1883) as the author of the family Pterothecidae. The family placement was
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presumably prompted by the principle of priority as Pterothecidae Fischer, 1883 is an older name for the family group taxon than Carinaropsidae Ulrich and Scofield, 1897. However,
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according to article 35.5 of the International Code of Zoological Nomenclature (ICZN 1999) an older subfamily name does not need to displace a younger family name. The classification
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of Wahlman (1992) is therefore upheld and followed here for reasons outlined below (see
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section 5.1).
Pterotheca hispanica nov. sp. Fig. 3
emphasize its geographical origin.
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Derivation of the name: From Hispania (the old Roman name for the Iberian Peninsula), to
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Material: Holotype specimen only (MGM 900S).
Type locality and horizon: 800 m east-southeast from the San Pelayo Chapel of the village of Pontedo (north of Cármenes, province of León, north-western Spain; Coord.: 42°58’16” N,
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5°37’25” W; elevation: ca. 1,405 m a.s.l.). The locality corresponds to the so-called “Pontedo shales”, placed in the lowermost part of the Formigoso Fm. The fossiliferous bed is late Aeronian in age, as demonstrated by graptolites belonging to the basalmost Stimulograptus sedgwickii Zone.
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Measurements: See Table 2.
Diagnosis: A small species of Pterotheca with an ovate shell, a narrow and deep V-shaped sinus leading into a narrow slit. Median carina narrow at base. Distinct trilobation of the median dorsum is missing. Septum is 40% of shell length and 49% of maximum width, being slightly longer than wide and with a septal angle of 69°. Ornamentation of fine co-marginal growth lines bundle into periodic co-marginal bands. Description: The shell is very low, semi-circular, 16 mm wide and 15 mm long, length thus being 90% of the width. Maximum width is at mid-length. Some dorsoventral compaction is evident, contributing to the very low shell form. The median dorsal carina is broken except for a small part at the anterior apertural sinus where it turns upwards. Its height is unknown but
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the scar of the broken carina further back shows that it was narrow at the base, and a marked trilobation of the shell is therefore not expressed. Instead, the shell is slightly concave adaperturally along the carina. The apertural margin swings sharply backwards medially to form a narrow V-shaped sinus with an angle of 24 deg and reaching back about 24% of shell length where it turns into a narrow slit of unknown length. Formation of a selenizone is not evident because of preservation. The septum is 40% of shell length (based on the LS measure)
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and 49% of maximum width. The width is 88% of the length, with the septal angle at the
posterior being 69°. The growth lines seen at the anterior part of the septum are superimposed
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from the outer shell. Surface ornamentation consists of fine co-marginal growth lines that
form regularly spaced growth increments visible as co-marginal bands; altogether seven bands
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are discernible, but the posterior part is obscured.
Remarks: Pterotheca hispanica nov. sp. differs from the type species foremost in having a
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near ovate shell in contrast to the proportionally much wider shell in P. transversa (Portlock, 1843). A deep anterior median sinus, as seen in P. hispanica nov. sp., is not developed in the type species. Instead the median carinated fold is markedly raised and protruding slightly
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anteriorly. In the compressed condition, the posterior depth of the sinus is to a certain degree accentuated in P. hispanica nov. sp., and it would have extended more upwards in the un-
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compressed form, outlining the anterior of the raised carina, although it would not have been as high as in the type species. The septal angle of the septum in P. transversa is slightly wider than that of P. hispanica nov. sp., and consequently the length:width ratio of the septum is
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slightly larger. Furthermore, the septum in P. transversa extends nearly 2/3 into the shell, whereas that of P. hispanica nov. sp. extends slightly less than half the length of the shell. All Silurian species have an ovate shell form and share many other characteristics. For comparison, exemplars of each hitherto described Silurian Pterotheca are shown in Figs. 4
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and 5. Measurements are presented in Table 2 and Fig. 6. The Spanish species is most similar to P. gath Lamont, 1978 from the Telychian of Scotland in its size, ornamentation and narrow carina (compare Fig. 3 and Fig. 4(A, B)). The median anterior suture is very similarly developed in the two species although the one in P. hispanica nov. sp. may be slightly less deep. The septal angle in P. hispanica nov. sp. is larger and it has a proportionally broader septum compared to that of P. gath. The Scottish species shows a marked asymmetry to the shell, which is not considered a taphonomic artefact (Clarkson et al., 1995). Although the septa develop differently in the two species, they reach about the same distance into the shell. A slit is also present in P. gath (Fig. 4(B), arrow), but it
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is unclear how deep it is as specimens of P. gath are moulds and the contrast to the matrix is low. Despite the size differences, the shell form and ornamentation of P. hispanica nov. sp. and P. aff. mullochensis Reed, 1909 (sensu Clarkson et al., 1995) are very similar but the latter has a much wider and shallower sinus compared to that of P. hispanica nov. sp. (compare Fig. 3 and Figs. 4(C), 6(A)). Besides the size, the major difference between the two
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species is the asymmetry of the shell of P. aff. mullochensis and the proportions of its very long septum (Table 2; Fig. 6(B–D)). A slit is inferred to be present also in P. aff.
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mullochensis. All along the top the median carina is broken, leaving a slightly uneven scar with sub-parallel sides which is a very typical feature, when the top of an open carina is
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broken (Ebbestad, 1999). The slit may thus have reached at least back to the anterior part of the septum.
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The oldest Silurian species is P. mullochensis Reed, 1909; its fossils are the least compacted of the three forms discussed so far. Three specimens were described by Reed (1909), of which one is figured here (Fig. 4(D)). At the anterior margin, the carinated fold is
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markedly raised and the median sinus is much narrower than in P. hispanica nov. sp. and P. gath. The depth of the sinus in the two last species may have become somewhat accentuated
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by dorsal compaction of the carina, but not so much that they would resemble P. mullochensis if uncompacted.
The sinus depth in P. mullochensis seems to include the height of the carina itself, and
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the sinus is followed by a narrow slit. This seems to reach the anterior part of the septum, but preservation obscures further details. At the circumference of the septum, irregular comarginal ridges are developed in P. mullochensis, contrasting the fine growth lines in P. hispanica nov. sp. The L/W ratio of the septum in P. mullochensis is mostly sub-equal with a
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lower septal angle (the exception being specimen NHM 47132; Table 2) and the septum extends deeper into the shell than in P. hispanica nov. sp. Both P. avirostris Pitcher, 1939 and P. multidecorata Reed, 1909 stand out among the Silurian forms. In P. avirostris, the base of the carinated fold is much wider than in other Silurian forms, giving a very high carina (Fig. 5(A)). The septal angle is low (Table 2). P. multidecorata is slightly wider than long compared to other Silurian forms; it is distinguished by the elaborate ornamentation (Fig. 5(B)).
5. Discussion 5.1. Morphological variations in Silurian Pterotheca.
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Table 2 presents linear measurements and septal angles for Silurian Pterotheca specimens. The septal angle gets larger with lower LS/WS ratios as a result of the linear proportions, although the measurements do not always follow this relationship (Table 2). This is because in some specimens the septal suture is not straight but swings out laterally at their anterior ends. As a consequence of how the septal measurements are made (Fig. 2), the septal angle measurement therefore gets bigger than it would if the septal suture was straight (see for
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instance P. avirostris, Fig. 5(A)). Foerste (1924) used the septal angle to differentiate between species of Pterotheca, which was further advocated by Wahlman (1992). Because the septum
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forms an isosceles triangle, the proportional relationship assures that even the length and width measurements are equally good parameters.
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Despite the obvious variations in linear measurements between the species, the length:width ratio of the shell is near equal in most of them (Fig. 6(A)), reflecting the ovate
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shape of the shells (Figs 3–5). The same relationship holds for the length:width ratio of the septum (Fig. 6(B); best fit lines near 1 in both cases), with the exception of P. aff. mullochensis, in which the length of the septum is more than half its width. The width of the
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shell is generally more than twice the width of the septum (best fit line: 0.56; Fig. 6(C)) but the width of the shell against the width of shell:width of septum ratio shows large variations.
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The septum width can range between less than 2/3 of the width, such as in P. gath, to about half the width as in P. hispanica nov. sp. (Fig. 6(D)). These shells are about the same size, but even the large shell of P. aff. mullochensis relative to a large shell of P. avirostris shows this
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relationship. This suggests that the growth of the septum is not an isometric reflection of variation in shell size, but reflects a true biological character. Intraspecific variability of these measurements within a species like P. gath (Fig. 6(D)) strengthens this assumption. Despite some compaction, P. hispanica nov. sp. displays a deep V-shaped sinus leading
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into a narrow slit of unknown length. A similar development of the sinus and slit is also clearly evident in P. gath, P. aff. mullochensis, and P. mullochensis (Fig. 4(A–D)). Knight (1941) tentatively suggested the presence of a slit in the type species P. transversa, and Wahlman (1992: p. 181) gives the presence of a slit in the diagnosis of the genus. The depth of the slit in various species of Silurian Pterotheca is unknown, but it seems that it may reach back to the anterior part of the septum in some taxa (see remarks in section 4). A filled median band is inferred in association with the slit. When a carina is broken in a planispiral shell, as explained in section 4, a median band showing distinct lunulae would typically be revealed. Although the median bands in the investigated Pterotheca are poorly preserved, lunulae are not seen in any of them and it seems likely that they are not present. Presumably
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this is what Horný (1963: p. 69) meant when he commented the “existence of a true selenizone is not confirmed”. Wahlman (1992) explained the septum in Pterotheca and similar forms within his Carinaropsidae as a protrusion of the parietal region and not an extension of the columnellar area as in the modern Crepidula. Similar plate-like extensions of the parietal lip develop in members of the Bucanopsinae Wahlman, 1992, but these develop in a coiled shell and are
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reflexed around the initial coil, they extend less into the shell and the plates are often
carinated (Wahlman, 1992). In the Carinaropsidae the coil is strongly reduced or absent and
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the septum grows and extends into the shell independently of coiling; some taxa have a
median carina on the septum (Wahlman, 1992). The shell muscles in Carinaropsis were
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attached deep in the shell, pulling over the rounded anterior edge of the septum and stability during contractions was provided by the median septal carina (Peel, 1993).
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In the phylogenetic concept of Wahlman (1992), the reduced coiling and septal development would be synapomorphies of the Carinaropsidae, a view that is accepted here. As pointed out by Wahlman (1992), the Pterothecinae would show the most derived
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morphology, and as discussed in this paper the median band probably did not develop lunulae
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as in species of Carinaropsis.
5.2. Temporal and spatial distribution of Silurian Pterotheca. All known Silurian species of Pterotheca are from the Lower Silurian (Llandovery) and
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up to now known from marginal Laurentia and Eastern Avalonia only (Table 1; Fig. 7). Ebbestad et al. (2013) demonstrated that Pterotheca was one of the genera joining Scotland and Eastern Avalonia in the Telychian. Other genera linked these places with the platform areas of North America where Pterotheca is absent. The genus is also absent in the Early
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Silurian of Baltica where it could be expected considering its diverse Ordovician record there. The occurrence of Pterotheca in Iberia is therefore highly unexpected, where it seems like an outlier. The patchy geographical distribution of the Silurian species therefore appears difficult to explain. However, the study by Ebbestad et al. (2013: table 15.7) demonstrated the extreme scarcity of taxa in Avalonia, Baltica and Iberia during the Early Silurian; the distribution pattern discussed here may partly be explained by this. The Pterotheca lineage was present in all of these areas prior to the end-Ordovician extinction.
Acknowledgements
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This contribution is part of SYNTHEYS ES-TAF-5274. The first author extends his thanks to the ES-TAF administration and the helpful staff at the Museo Nacional de Ciencias Naturales, Madrid. JORE also thanks Dr. Paul Shepard and Dr. Mike Howard for facilitating help during the visit to the Keyworth collections of the British Geological Survey, Dr. Sarah Stewart for help with the collections at the National Museums of Scotland, and Dr. Jonathan
reviewers and the editor helped to improve the manuscript.
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Floyd, J.D., Williams, M. 2003. A revised correlation of Silurian rocks in the Girvan district, SW Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 93, 383– 392. Foerste, A.F., 1924. Upper Ordovician fossils of Ontario and Quebec. Memoir of the Geological Survey of Canada 138, 1–255.
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Gutiérrez-Marco, J. C., Ghienne, J.-F., Bernárdez, E., Hacar, M.P., 2010. Did the Late
Gutiérrez-Marco, J.C., Aramburu, C., Arbizu, M., Bernárdez, E., Hacar Rodríguez, M.P.,
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Méndez-Bedia, I., Montesinos López, R., Rábano, I., Truyols, J., Villas, E., 1999.
Revisión bioestratigráfica de las pizarras del Ordovícico Medio en el noroeste de España
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(Zonas Cantábrica, Asturoccidental-leonesa y Centroibérica septentrional). Acta Geologica Hispanica 34, 3-87.
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Hall, J., 1847. Paleontology of New York, v. 1, Containing descriptions of the organic remains of the lower division of the New York System. Albany, N.Y., C. Van Benthuysen, pp xxiii, 338.
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Hammer, Ø., Harper, D.A.T., Ryan, P.D. 2001. PAST: Palaeontological statistics software package for education and data analysis. Palaeontologia Electronica 4(1), 1–9.
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Hernández Sampelayo, P., 1915. Fósiles de Galicia. Nota sobre la fauna paleozoica de la provincia de Lugo. Boletín del Instituto Geológico de España 36, 277–303. Horný, R.J., 1963. Lower Paleozoic Bellerophontina (Gastropoda) of Bohemia. Sborník
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Geologických Věd, Paleontologie 2, 57–164. Horný, R.J., 1964. Phragmotheca Barrande, 1867 je mlž (Bivalvia). Časopis Národního muzea, oddíl přírodovědný 133, 107–108. International Commission on Zoological Nomenclature, 1999. International Code of
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Zoological Nomenclature, ed. 4. The International Trust for Zoological Nomenclature, London, 306 p.
Knight, J.B., 1941. Paleozoic gastropod genotypes. Geological Society of America Special Paper 32, 1–510. Knight, J.B., 1945. Some new genera of the Bellerophontacea. Journal of Paleontology 19, 333–340. Knight, J.B., Cox, L.R., Keen, A.M., Batten, R.L., Yochelson, E.L., Robertson, R., 1960. Systematic descriptions. In: Moore, R.C. (Ed.), Treatise on Invertebrate Paleontology, Pt. I, Mollusca 1. Geological Society of America and University of Kansas Press, Lawrence, pp. I169–I324.
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Lamont, A., 1978. Pentlandian miscellany: Mollusca, Trilobita etc. Scottish Journal of Science 1, 245–302. Lorenzo Álvarez, S., 2015. Bioestratigrafía del Silúrico de la Zona Centroibérica meridional (España). Ph.D. Thesis, Complutense University of Madrid, 435 p. (unpubl.) Loydell, D.K., 2005. Graptolites from the Deerhope Formation, North Esk Inlier. Scottish Journal of Geology 41, 189–190.
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M’Coy, F., 1851. On some new Cambro-Ordovician fossils. Annals and Magazine of Natural History, 2nd Series 8, 387–489.
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Peel, J.S., 1993. Muscle Scars and Mode of Life of Carinaropsis (Bellerophontoidea,
Gastropoda) from the Ordovician of Tennessee. Journal of Paleontology 67, 528–523.
Magazine of Natural History, Series 11 4, 82–132.
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Pitcher, B.L., 1939. The Upper Valentian Gastropod Fauna of Shropshire. Annals and
Tyrone and Fermanagh, Dublin, p. 784.
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Portlock, J.E., 1843. Report on the Geology of the County of Londonderry and of Parts of
Society of Edinburgh 47, 203–222.
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Reed, F.R.C., 1909. Lower Palaeozoic Hyolithidae from Girvan. Transactions of the Royal
Salter, J., 1853. On a few genera of Irish fossils. 22nd Report of the British Association for
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the Advancement of Science, Belfast, 59–61.
Spriesterbach, J., 1919. Neue Versteinerungen aus dem Lenneschiefer. Jahrbuch der Königlich Preussischen Geologischen Landesanstalt zu Berlin 38, 434–512.
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Torsvik, T.H., Cocks, R.L.M., 2013. New global palaeogeographical reconstructions for the Early Palaeozoic and their generation. In: Harper, D.A.T., Servais, T. (Eds.), Early Palaeozoic Biogeography and Palaeogeography, 38, Geological Society, London, Memoirs, pp. 5–24.
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Teichert, C., 1928. Die silurische Familie Pterothecidae Walcott, nov. emend. (Gastrop. Heterop.). Senckenbergiana 10, 136–157. Teichert, C., 1935. Über Pterotheca Salter und verwandte Bellerophontacean. Paläontologische Zeitschrift 17, 167–177. Ulrich, E.O., Scofield, W.H., 1897. The Lower Silurian Gastropoda of Minnesota. In: The Geology of Minnesota, 3, part 2 (Paleontology), 813–1081. Wahlman, G. P., 1992. Middle and Upper Ordovician symmetrical univalved mollusks (Monoplacophora and Bellerophontina) of the Cincinnati Arch Region. United States Geological Survey, Professional Paper, 1066–O, 1–213.
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Walcott, C.D., 1886. Second contribution to the studies on the Cambrian faunas of North America. Bulletin of the United States Geological Survey 30, 1–369. Wenz, W., 1938. Gastropoda, Allgemeiner Teil und Prosobranchia. In: Schindewolf, O.H. (Ed.), Handbuch der Paläozoologie 6. Gebrüder Borntraeger, Berlin, pp. 1–480. Zalasiewicz, J.A., Taylor, L., Rushton, A.W.A., Loydell, D.K., Rickards, R.B., Williams, M.,
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ed
M
an
us
cr
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2009. Graptolites in British stratigraphy. Geological Magazine 146, 785–850.
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Table and Figure captions
Table 1. Alphabetic list of Silurian Pterotheca species and their stratigraphical and geographical occurrences. Graptolite zonation comes from Curtis (1972: Damery Beds), Floyd and Willams (2003: Mulloch Hills Sandstone, Penkill Mudstone Fm.; see also Zalasiewicz et al., 2009), Loydell (2005: Deerhope Fm., Wether Law Linn Fm.), and
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Bernardez et al. (2006: Formigosa Fm.).
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Table 2. Measurements of known species of Silurian Pterotheca. See Fig. 2 for explanation of the various parameters. Linear measurements are in mm. Spec. I and Spec. II for P.
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refers to measurements given in Clarkson et al. (1995).
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multidecorata refers to the designations given in Reed (1909). The means given for P. gath
Fig. 1. A. Geographical map of Spain showing some major cities and the two autonomous
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districts associated with the inset. A’. Detail of the area around the Pontedo section.
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Fig. 2. Generalized dorsal shell morphology of an ovate Pterotheca and measurements used in this study. The shell is removed to view the internal septum from above. L: Length; W: width;
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LS: length of septum; LW: width of septum.
Fig. 3. Pterotheca hispanica nov. sp. (MGM 900S, holotype). Dorsal view (A), detail of septum (B), oblique dorsal (C) and detail of media sinus (D). Arrow in D points to the
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posterior part of the open slit. Scale bars: 5 mm.
Fig. 4. A. Dorsal view of left-skewed form of Pterotheca gath Lamont, 1978 (NMS G.l897.32.746), Wether Law Linn Fm., Pentland Hills, Scotland. B. Detail of anterior median suture and slit of same specimen as in A. Arrow points to open slit area between two parallelsided ridges. C. Pterotheca aff. mullochensis Reed, 1909 (RSM GY 1993.58.70), Deerhope Fm., Pentland Hills, Scotland. D. Pterotheca mullochensis Reed, 1909 (NHM G 47130), Mulloch Hill Sandstone, Girvan, Scotland. Scale bars: 5 mm.
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Fig. 5. A. Pterotheca avirostris Pitcher, 1939 (BGS 23114), Purple Shales and Damery Beds, England. B. Pterotheca multidecorata Reed, 1909, Penkill Mudstone Fm., Scotland. Scale bars: 5 mm.
Fig. 6. Linear measurements of Silurian Pterotheca species. Specimens and measurements are given in Table 2. A. Length vs. width. B. Length of septum vs. width of septum. C. Width of
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septum vs. width of shell. D. Width of shell against the length:width ratio of septum.
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Pterotheca. Map modified from Torsvik and Cocks (2013).
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Fig. 7. Early Silurian (Aeronian) palaeogeographic reconstruction showing the distribution of
Page 16 of 25
ip t cr Occurrence
Stage
Biozone
P. avirostris Pitcher, 1939
Purple Shales and Damery Beds, England
Telychian
P. gath Lamont, 1978
Wether Law Linn Fm., Scotland
Telychian
Monoclimacis griestonensis Stimulograptus sedgwickii/ Cyrtograptus insectus
P. hispanica nov. sp.
Formigoso Fm., Spain
Aeronian
Stimulograptus sedgwickii
P. mullochensis Reed, 1909
Mulloch Hill Sandstone, Scotland
Rhuddanian
Coronograptus cyphus/Monograptus revolutus
P. aff. P. mullochensis Reed, sensu Clarkson et al. (1995)
Deerhope Fm., Scotland
Telychian
Oktavites spiralis–Cyrtograptus lapworthi
P. multidecorata Reed, 1909
Penkill Mudstone Fm., Scotland
Telychian
Monograptus crispus
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ep te
d
M
Taxon
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us
Table 1.
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cr us ed
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Taxon P. avirostris Pitcher, 1939 P. avirostris Pitcher, 1939 P. gath Lamont, 1978 P. gath Lamont, 1978 P. gath Lamont, 1978 P. hispanica nov. sp. P. mullochensis Reed, 1909 P. mullochensis Reed, 1909 P. mullochensis Reed, 1909
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Table 2.
Length 23 27.1 13.5 9.8 9.02 15 23.8 22.6 21.5
Width 24.4 29.1 13.5 11.9 10.31 16 26 22.6 25.8
Length septum 12.2 12.4 5.6 3.9 4.06 6 13.8 10.4 9.3
Width septum 12.3 14.9 4.9 5 4 8 13.9 10 12.7
Septal angle 49.8° 61.8° 52° 62° 60° 69° 54° 57° 65°
NMS G.1993.58.70 NHMUK PI OR 47143. (Spec. I) NHMUK PI OR 47141. (Spec. II)
36 11 16.3
44 13.5 21.5
28.1 6 7.5
17.5 6 8.0
50° 56° 62°
Ac
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P. aff. P. mullochensis Reed, sensu Clarkson et al. (1995) P. multidecorata Reed, 1909 P. multidecorata Reed, 1909
Museum no. BGS GSM23114 BGS GSM23112 NMS G.l897.32.746 NMS G.l995.66.1 Means of 35 specimens MGM 900S NHMUK PI OR 47131 NHMUK PI OR 47130 NHMUK PI OR 47132
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S0016-6995(16)30091-2 http://dx.doi.org/doi:10.1016/j.geobios.2017.02.003 GEOBIO 777
To appear in:
Geobios
Received date: Revised date: Accepted date:
7-10-2016 2-2-2017 21-2-2017
Please cite this article as: Ebbestad, J.O.R., Guti´errez-Marco, J.C.,First occurrence of Pterotheca (Gastropoda) from the Silurian (Aeronian) of Spain, Geobios (2017), http://dx.doi.org/10.1016/j.geobios.2017.02.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
First occurrence of Pterotheca (Gastropoda) from the Silurian (Aeronian) of Spain Jan Ove R. Ebbestad a,*, Juan Carlos Gutiérrez-Marco b
Museum of Evolution, Uppsala University, Norbyvägen 16, 752 36 Uppsala, Sweden
b
Instituto de Geociencias (CSIC, UCM), Facultad de Ciencias Geológicas, José Antonio
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a
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Novais 12, 28040 Madrid, Spain
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Corresponding author. E-mail addresses: [email protected],
Corresponding editor: Bertrand Lefebvre.
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[email protected] (J.O.R. Ebbestad).
Abstract
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Pterotheca hispanica nov. sp. is described from the basal beds (Stimulograptus sedgwickii graptolite Zone) of the Formigoso Fm., in the Cantabrian Zone of the Iberian
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Massif, north-western Spain. This specialized bellerophontoid gastropod is for the first time recorded in the Silurian of peri-Gondwana; it is the only Aeronian species known so far. Its most closely allied relatives are from higher beds placed in the Telychian of Scotland. Pterotheca is considered a highly derived genus within the Pterothecinae of the
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Carinaropsidae, with the strongly reduced to absent coiling and the growth of the septum being synapomorphies. In the present work it is shown that not only the apical angle of the septum can be used to distinguish species, but also the relative length and width of the septum. In some cases, the slit may extend as far back as the anterior part of the internal septum. Silurian species were hitherto only found in marginal Laurentian and Eastern Avalonian settings. Pterotheca is absent from central Laurentia and Baltica, where it could be expected to be present based on its wide Ordovician distribution. In the light of this the occurrence of the genus in the Aeronian of Spain seems difficult to explain, albeit the genus was present in peri-Gondwana prior to the end-Ordovician extinction. The extreme scarcity of
Page 1 of 25
described taxa recorded from Avalonia, Baltica and Iberia during the Early Silurian may explain this patchy distribution.
Keywords: Bellerophontoidea Carinaropsidae
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Pterothecinae Silurian
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Palaeobiogeography
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Iberian Peninsula
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1. Introduction
Pterotheca Salter, 1853 is a readily identifiable bellerophontoid gastropod with a low ovate to sub-rectangular shell and a raised median carina. Despite its bellerophontoid affinity
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and marked incurved apex it lacks true coiling of the shell (Ulrich and Scofield, 1897; Reed, 1909; Knight, 1941). Internally, the shell contains a triangular septum at the posterior end,
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and this is often exposed or outlined on the fossilized shell. Early workers variably placed the genus with the Brachiopoda, or the gastropods Pteropoda or Heteropoda. A growing consensus eventually placed it within the bellerophontoids based on the close relationship
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with the Ordovician bellerophontoid Carinaropsis Hall, 1847. For further details on the early taxonomic history of Pterotheca, see Teichert (1928, 1935). Most of the more than 30 species of Pterotheca are found in the Late Ordovician of Laurentia and Baltica. A single species is known from the Late Ordovician of Avalonia and
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another one from peri-Gondwana. The genus suffered heavily at the end-Ordovician extinction with only five species hitherto found in the Silurian: four species from Scotland representing marginal Laurentian settings, and one from Shropshire, England, representing Avalonian settings (Table 1). Pterotheca is here recorded for the first time from the Silurian of peri-Gondwana. A single specimen of Pterotheca hispanica nov. sp. was found in the Early Silurian Formigoso Fm. of the Cantabrian Zone, north-western Spain. This is also the only Aeronian species known so far. The new species is compared to other Silurian Pterotheca species, and their morphology and distribution are discussed.
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2. Geological setting The first undisputable record of the genus Pterotheca from SW Europe comes from a single outcrop of Silurian shales located at the southern slope of the Cantabrian Cordillera in northern Spain. This corresponds to the Pontedo section, placed in the Bodón nappe of the Cantabrian Zone of the Iberian Massif (Fig. 1). The locality lies east of the village of Pontedo (province of León), and was discovered by Aramburu (1989: profil 32, appendix 17) who
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reported diverse Llandovery graptolites, trilobites, brachiopods, molluscs, echinoderms, and cornulitids. The fossil assemblage occurs within a 6 m-thick unit of green and grey shales that
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underlies the typical graptolitic black shales that usually characterize the lower part of the
Formigoso Fm. (Llandovery-Wenlock) in the whole Cantabrian Zone. Although Aramburu
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(1989) originally placed the fossiliferous shales towards the upper part of his “Getino Beds” – interpreted by him as a “condensed” sucession from Middle Ordovician to early Silurian
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strata–, Aramburu and García-Ramos (1993) separated these from the preceding unit, naming it the “Pontedo Shales”. According to these authors, the latter are a local and lateral correlative of the uppermost “Getino Beds”. A different stratigraphic view was presented by
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Bernárdez et al. (2006), who placed the highly fossiliferous “Pontedo beds” as bracketed by the glauconitic sandstone that constitutes the base of the Formigoso Fm. at a regional level,
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and the black graptolitic shales of the remaining part of the same unit. The Formigoso Fm. in the Pontedo section rests unconformably on the Late Ordovician (Hirnantian) diamictites of the Pontedo paleovalley (Bernárdez et al., 2006; Gutiérrez-Marco et al., 2010).
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Besides the single specimen of the gastropod Pterotheca and the diagnostic graptolites of the lower Stimulograptus sedgwickii Zone occurring in the same bed (Neolagarograptus tenuis (Portlock), Metaclimacograptus undulatus (Kurck), Parapetalolithus clandestinus Štorch), the associated fauna consists of trilobites, molluscs, brachiopods, echinoderms, and
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cornulitids (Bernárdez et al., 2006). Until now, the genus Pterotheca was only previously cited from the Ordovician of Spain (as “Pteroteca [sic.] consobrina, Ba.”) by Hernández Sampelayo (1915: p. 297), a species restricted to the Kralodvorian (Upper Ordovician) of Bohemia, Czech Republic. Its supposed record in the Middle Ordovician of north-western Spain was reviewed by Gutiérrez-Marco et al. (1999: p. 23), who attributed the specimens to flattened fragments of orthocone nautiloids. Recently Pterotheca sp. was also cited in an unpublished Ph.D. thesis on the Silurian of Central Spain (Lorenzo Álvarez, 2015). However, the generic attribution of the single specimen, recorded from the Monograptus riccartonensis graptolite Zone (Wenlock), is dubious because a clear septum is not recognizable; this material needs re-examination.
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3. Material and methods A single specimen of Pterotheca hispanica nov. sp. is known (MGM 900S), but in addition figured and described type material of other Silurian species has been studied for first hand comparisons. Typically, only two to three specimens are known for each species, the exception being Pterotheca gath Lamont, 1978 which is quite abundant in the Pterotheca
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beds from the Pentland Hills of Scotland (Clarkson et al., 1995, JORE pers. obs.) and thus in the collections of the National Museum of Scotland.
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Five measurements were made on the most complete of the available specimens; the
length (L) and width (W) of the shell, the length (LS) and width (WS) of the internal septum,
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and the septal angle (Fig. 2). Where the septum meets the dorsal shell, a septal suture is formed, which is commonly expressed on the fossil; the angle formed posteriorly by the
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diverging septal sutures is the septal angle (Fig. 2). As pointed out by Wahlman (1992), the septal angle (SA) is a useful measurement for separating species and can be readily measured. Likewise, the tangential length of the septal sutures is easy to measure and often given in the
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literature. Although sometimes given in the literature, the true sagittal length of the septum is more difficult to estimate as the anteriorly curved anterior septal margin may be obscured. To
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avoid conflict between cited measurements, the exsagittal length of the suture from the posterior margin to the anterior tip of the septal suture was considered in this study (Fig. 2; Table 2). Measurements were taken from the literature, or made in Adobe Photoshop® and/or
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CorelDraw® from published drawings or figures as well as on photographs made by the first author on original specimens (Table 2). The data was analysed using Microsoft Excel® and PAST (Hammer et al., 2001).
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4. Systematic palaeontology
The specimens described and illustrated here are housed in the following public collections: MGM, Museo Geominero, Madrid; NMS, National Museum of Scotland; BGS, British Geological Survey from their old museum collections (GSM); NHMUK, Natural History Museum, London. The acronym RSMGY cited by Clarkson et al. (1995) for Pterotheca aff. mullochensis is now NMS (Table 2). Specimens were coated with ammonium chloride sublimate prior to photography. Bellerophontoidea M’Coy, 1851 Carinaropsidae Ulrich and Scofield, 1897
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Pterothecinae Fischer, 1883 Genus Pterotheca Salter, 1853 Type species: By original designation of Salter (1853: p. 61), Atrypa transversa Portlock, 1843 (p. 455), most likely from the Caradoc (Sandbian 2) Bardahessiagh Fm. of Desertcreat, Co. Tyrone, Northern Ireland. Remarks: Fischer (1883: p. 432) erected the family Pterothecidae for Pterotheca and
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tentatively Phragmotheca Barrande, 1867; the latter was interpreted as a bivalve by Horný (1964). Fischer’s classification was followed by Walcott (1886) to which Teichert (1928)
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attributed the family name. Ulrich and Scofield (1897: p. 857, 926, 927) pointed out
similarities between Carinaropsis Hall, 1847 and Pterotheca in their shell form, the strongly
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reduced coiling, the median carina, and in the development of an internal plate; they united them within the family Carinaropsidae Ulrich and Scofield, 1897 of the Bellerophontoidea
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M’Coy, 1851, ranked by Ulrich and Scofield (1897: p. 844) as suborder Bellerophontacea. Subsequent studies have broadly concurred with the close association suggested by Ulrich and Scofield (1897), albeit with different opinions on the higher-level classification. Wenz (1938)
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preferred the two families Carinaropsidae and Pterothecidae within the Bellerophontoidea, while Knight et al. (1960) ranked these as subfamilies within the family Bellerophontidae.
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They also expanded these families by including additional genera so that the subfamily Carinaropsinae encompassed Carinaropsis, Bucanopsis Ulrich in Ulrich and Scofield, 1897, Sphenosphaera Knight, 1945, Phragmosphaera Knight, 1945, and Gyrospira Boucot, 1956,
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while the subfamily Pterothecinae encompassed Pterotheca, Cyclotheca Teichert, 1935, Aspidotheca Teichert, 1935, and Pedasiola Spriestersbach, 1919. Knight et al. (1960: p. 181) changed the taxonomic level of the Pterothecidae to the subfamily level by nomen translatum but as most subsequent authors they attributed the name to Wenz (1938). Horný (1963) could
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not accept a placement of the pterothecids within the family Bellerophontidae because a true selenizone could not be confirmed and the morphology was too highly specialized. He therefore reverted to place pterothecids at the family level sensu Wenz (1938). Wahlman (1992) partly reverted to the original concept of Ulrich and Scofield (1897), using the family Carinaropsidae within the superfamily Bellerophontoidea, but he recognized three subfamilies: the Carinaropsinae with only Carinaropsis and Gyrospira, Pterothecinae with only Pterotheca and Cyclotheca, and the Devonian Pedasiolinae Wahlman, 1992 including Aspidotheca and Pedasiola. The genera Bucanopsis and Sphenosphaera were transferred to the new subfamily Bucanopsinae Wahlman, 1992 of the Bellerophontidae. Clarkson et al.
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(1995) used the subfamily Pterothecinae of the family Bellerophontidae but followed Wahlman (1992) by including only Pterotheca and Cyclotheca. Frýda and Bouchet in Bouchet and Rocroi (2005) followed Wahlman (1992) in recognizing the subfamilies Pterothecinae, Carinaropsinae and Pedasiolinae. These were placed in the family Pterothecidae of the superfamily Bellerophontoidea, correctly identifying Fischer (1883) as the author of the family Pterothecidae. The family placement was
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presumably prompted by the principle of priority as Pterothecidae Fischer, 1883 is an older name for the family group taxon than Carinaropsidae Ulrich and Scofield, 1897. However,
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according to article 35.5 of the International Code of Zoological Nomenclature (ICZN 1999) an older subfamily name does not need to displace a younger family name. The classification
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of Wahlman (1992) is therefore upheld and followed here for reasons outlined below (see
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section 5.1).
Pterotheca hispanica nov. sp. Fig. 3
emphasize its geographical origin.
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Derivation of the name: From Hispania (the old Roman name for the Iberian Peninsula), to
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Material: Holotype specimen only (MGM 900S).
Type locality and horizon: 800 m east-southeast from the San Pelayo Chapel of the village of Pontedo (north of Cármenes, province of León, north-western Spain; Coord.: 42°58’16” N,
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5°37’25” W; elevation: ca. 1,405 m a.s.l.). The locality corresponds to the so-called “Pontedo shales”, placed in the lowermost part of the Formigoso Fm. The fossiliferous bed is late Aeronian in age, as demonstrated by graptolites belonging to the basalmost Stimulograptus sedgwickii Zone.
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Measurements: See Table 2.
Diagnosis: A small species of Pterotheca with an ovate shell, a narrow and deep V-shaped sinus leading into a narrow slit. Median carina narrow at base. Distinct trilobation of the median dorsum is missing. Septum is 40% of shell length and 49% of maximum width, being slightly longer than wide and with a septal angle of 69°. Ornamentation of fine co-marginal growth lines bundle into periodic co-marginal bands. Description: The shell is very low, semi-circular, 16 mm wide and 15 mm long, length thus being 90% of the width. Maximum width is at mid-length. Some dorsoventral compaction is evident, contributing to the very low shell form. The median dorsal carina is broken except for a small part at the anterior apertural sinus where it turns upwards. Its height is unknown but
Page 6 of 25
the scar of the broken carina further back shows that it was narrow at the base, and a marked trilobation of the shell is therefore not expressed. Instead, the shell is slightly concave adaperturally along the carina. The apertural margin swings sharply backwards medially to form a narrow V-shaped sinus with an angle of 24 deg and reaching back about 24% of shell length where it turns into a narrow slit of unknown length. Formation of a selenizone is not evident because of preservation. The septum is 40% of shell length (based on the LS measure)
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and 49% of maximum width. The width is 88% of the length, with the septal angle at the
posterior being 69°. The growth lines seen at the anterior part of the septum are superimposed
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from the outer shell. Surface ornamentation consists of fine co-marginal growth lines that
form regularly spaced growth increments visible as co-marginal bands; altogether seven bands
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are discernible, but the posterior part is obscured.
Remarks: Pterotheca hispanica nov. sp. differs from the type species foremost in having a
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near ovate shell in contrast to the proportionally much wider shell in P. transversa (Portlock, 1843). A deep anterior median sinus, as seen in P. hispanica nov. sp., is not developed in the type species. Instead the median carinated fold is markedly raised and protruding slightly
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anteriorly. In the compressed condition, the posterior depth of the sinus is to a certain degree accentuated in P. hispanica nov. sp., and it would have extended more upwards in the un-
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compressed form, outlining the anterior of the raised carina, although it would not have been as high as in the type species. The septal angle of the septum in P. transversa is slightly wider than that of P. hispanica nov. sp., and consequently the length:width ratio of the septum is
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slightly larger. Furthermore, the septum in P. transversa extends nearly 2/3 into the shell, whereas that of P. hispanica nov. sp. extends slightly less than half the length of the shell. All Silurian species have an ovate shell form and share many other characteristics. For comparison, exemplars of each hitherto described Silurian Pterotheca are shown in Figs. 4
Ac
and 5. Measurements are presented in Table 2 and Fig. 6. The Spanish species is most similar to P. gath Lamont, 1978 from the Telychian of Scotland in its size, ornamentation and narrow carina (compare Fig. 3 and Fig. 4(A, B)). The median anterior suture is very similarly developed in the two species although the one in P. hispanica nov. sp. may be slightly less deep. The septal angle in P. hispanica nov. sp. is larger and it has a proportionally broader septum compared to that of P. gath. The Scottish species shows a marked asymmetry to the shell, which is not considered a taphonomic artefact (Clarkson et al., 1995). Although the septa develop differently in the two species, they reach about the same distance into the shell. A slit is also present in P. gath (Fig. 4(B), arrow), but it
Page 7 of 25
is unclear how deep it is as specimens of P. gath are moulds and the contrast to the matrix is low. Despite the size differences, the shell form and ornamentation of P. hispanica nov. sp. and P. aff. mullochensis Reed, 1909 (sensu Clarkson et al., 1995) are very similar but the latter has a much wider and shallower sinus compared to that of P. hispanica nov. sp. (compare Fig. 3 and Figs. 4(C), 6(A)). Besides the size, the major difference between the two
ip t
species is the asymmetry of the shell of P. aff. mullochensis and the proportions of its very long septum (Table 2; Fig. 6(B–D)). A slit is inferred to be present also in P. aff.
cr
mullochensis. All along the top the median carina is broken, leaving a slightly uneven scar with sub-parallel sides which is a very typical feature, when the top of an open carina is
us
broken (Ebbestad, 1999). The slit may thus have reached at least back to the anterior part of the septum.
an
The oldest Silurian species is P. mullochensis Reed, 1909; its fossils are the least compacted of the three forms discussed so far. Three specimens were described by Reed (1909), of which one is figured here (Fig. 4(D)). At the anterior margin, the carinated fold is
M
markedly raised and the median sinus is much narrower than in P. hispanica nov. sp. and P. gath. The depth of the sinus in the two last species may have become somewhat accentuated
ed
by dorsal compaction of the carina, but not so much that they would resemble P. mullochensis if uncompacted.
The sinus depth in P. mullochensis seems to include the height of the carina itself, and
ce pt
the sinus is followed by a narrow slit. This seems to reach the anterior part of the septum, but preservation obscures further details. At the circumference of the septum, irregular comarginal ridges are developed in P. mullochensis, contrasting the fine growth lines in P. hispanica nov. sp. The L/W ratio of the septum in P. mullochensis is mostly sub-equal with a
Ac
lower septal angle (the exception being specimen NHM 47132; Table 2) and the septum extends deeper into the shell than in P. hispanica nov. sp. Both P. avirostris Pitcher, 1939 and P. multidecorata Reed, 1909 stand out among the Silurian forms. In P. avirostris, the base of the carinated fold is much wider than in other Silurian forms, giving a very high carina (Fig. 5(A)). The septal angle is low (Table 2). P. multidecorata is slightly wider than long compared to other Silurian forms; it is distinguished by the elaborate ornamentation (Fig. 5(B)).
5. Discussion 5.1. Morphological variations in Silurian Pterotheca.
Page 8 of 25
Table 2 presents linear measurements and septal angles for Silurian Pterotheca specimens. The septal angle gets larger with lower LS/WS ratios as a result of the linear proportions, although the measurements do not always follow this relationship (Table 2). This is because in some specimens the septal suture is not straight but swings out laterally at their anterior ends. As a consequence of how the septal measurements are made (Fig. 2), the septal angle measurement therefore gets bigger than it would if the septal suture was straight (see for
ip t
instance P. avirostris, Fig. 5(A)). Foerste (1924) used the septal angle to differentiate between species of Pterotheca, which was further advocated by Wahlman (1992). Because the septum
cr
forms an isosceles triangle, the proportional relationship assures that even the length and width measurements are equally good parameters.
us
Despite the obvious variations in linear measurements between the species, the length:width ratio of the shell is near equal in most of them (Fig. 6(A)), reflecting the ovate
an
shape of the shells (Figs 3–5). The same relationship holds for the length:width ratio of the septum (Fig. 6(B); best fit lines near 1 in both cases), with the exception of P. aff. mullochensis, in which the length of the septum is more than half its width. The width of the
M
shell is generally more than twice the width of the septum (best fit line: 0.56; Fig. 6(C)) but the width of the shell against the width of shell:width of septum ratio shows large variations.
ed
The septum width can range between less than 2/3 of the width, such as in P. gath, to about half the width as in P. hispanica nov. sp. (Fig. 6(D)). These shells are about the same size, but even the large shell of P. aff. mullochensis relative to a large shell of P. avirostris shows this
ce pt
relationship. This suggests that the growth of the septum is not an isometric reflection of variation in shell size, but reflects a true biological character. Intraspecific variability of these measurements within a species like P. gath (Fig. 6(D)) strengthens this assumption. Despite some compaction, P. hispanica nov. sp. displays a deep V-shaped sinus leading
Ac
into a narrow slit of unknown length. A similar development of the sinus and slit is also clearly evident in P. gath, P. aff. mullochensis, and P. mullochensis (Fig. 4(A–D)). Knight (1941) tentatively suggested the presence of a slit in the type species P. transversa, and Wahlman (1992: p. 181) gives the presence of a slit in the diagnosis of the genus. The depth of the slit in various species of Silurian Pterotheca is unknown, but it seems that it may reach back to the anterior part of the septum in some taxa (see remarks in section 4). A filled median band is inferred in association with the slit. When a carina is broken in a planispiral shell, as explained in section 4, a median band showing distinct lunulae would typically be revealed. Although the median bands in the investigated Pterotheca are poorly preserved, lunulae are not seen in any of them and it seems likely that they are not present. Presumably
Page 9 of 25
this is what Horný (1963: p. 69) meant when he commented the “existence of a true selenizone is not confirmed”. Wahlman (1992) explained the septum in Pterotheca and similar forms within his Carinaropsidae as a protrusion of the parietal region and not an extension of the columnellar area as in the modern Crepidula. Similar plate-like extensions of the parietal lip develop in members of the Bucanopsinae Wahlman, 1992, but these develop in a coiled shell and are
ip t
reflexed around the initial coil, they extend less into the shell and the plates are often
carinated (Wahlman, 1992). In the Carinaropsidae the coil is strongly reduced or absent and
cr
the septum grows and extends into the shell independently of coiling; some taxa have a
median carina on the septum (Wahlman, 1992). The shell muscles in Carinaropsis were
us
attached deep in the shell, pulling over the rounded anterior edge of the septum and stability during contractions was provided by the median septal carina (Peel, 1993).
an
In the phylogenetic concept of Wahlman (1992), the reduced coiling and septal development would be synapomorphies of the Carinaropsidae, a view that is accepted here. As pointed out by Wahlman (1992), the Pterothecinae would show the most derived
M
morphology, and as discussed in this paper the median band probably did not develop lunulae
ed
as in species of Carinaropsis.
5.2. Temporal and spatial distribution of Silurian Pterotheca. All known Silurian species of Pterotheca are from the Lower Silurian (Llandovery) and
ce pt
up to now known from marginal Laurentia and Eastern Avalonia only (Table 1; Fig. 7). Ebbestad et al. (2013) demonstrated that Pterotheca was one of the genera joining Scotland and Eastern Avalonia in the Telychian. Other genera linked these places with the platform areas of North America where Pterotheca is absent. The genus is also absent in the Early
Ac
Silurian of Baltica where it could be expected considering its diverse Ordovician record there. The occurrence of Pterotheca in Iberia is therefore highly unexpected, where it seems like an outlier. The patchy geographical distribution of the Silurian species therefore appears difficult to explain. However, the study by Ebbestad et al. (2013: table 15.7) demonstrated the extreme scarcity of taxa in Avalonia, Baltica and Iberia during the Early Silurian; the distribution pattern discussed here may partly be explained by this. The Pterotheca lineage was present in all of these areas prior to the end-Ordovician extinction.
Acknowledgements
Page 10 of 25
This contribution is part of SYNTHEYS ES-TAF-5274. The first author extends his thanks to the ES-TAF administration and the helpful staff at the Museo Nacional de Ciencias Naturales, Madrid. JORE also thanks Dr. Paul Shepard and Dr. Mike Howard for facilitating help during the visit to the Keyworth collections of the British Geological Survey, Dr. Sarah Stewart for help with the collections at the National Museums of Scotland, and Dr. Jonathan
reviewers and the editor helped to improve the manuscript.
cr
References
ip t
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Aramburu, C., 1989. El Cambro-Ordovícico de la Zona Cantábrica (NO. de España). Ph.D.
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Cantábrica (NO de España). Trabajos de Geología, Oviedo 19, 45–73. Barrande J, 1867. Système silurien du centre de la Bohème 3, Classe des Mollusques, Ordre des Ptéropodes. I-XV, pp. 1-179.
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Bouchet, P., Rocroi, J.-P. (Ed.), 2005. Classification and nomenclator of gastropod families.
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Washington Academy of Science Journal 46, 46–47. Clarkson, E.N.K., Harper, D.A.T., Peel, J.S., 1995. Taxonomy and palaeoecology of the mollusc Pterotheca from the Ordovician and Silurian of Scotland. Lethaia 28, 101–114. Curtis, M.K.L., 1972. The Silurian rocks of the Tortworth Inlier, Gloucestershire. Proceedings
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of the Geologists Association 83, 1–35. Ebbestad, J.O.R., 1999. Bucaniid gastropods from the Upper Ordovician of Baltica, with a discussion of the Bucaniinae. Palaeontology 42, 149–169. Ebbestad, J.O.R., Frýda, J., Wagner, P.J., Horný, R.J., Isakar, M., Stewart, S., Percival, I.G., Bertero, V., Rohr, D.M., Peel, J.S., Blodgett, R.B., Högström, A.E.S., 2013. Biogeography of Ordovician and Silurian gastropods, monoplacophorans and mimospirids. In: Harper, D.A.T., Servais, T. (Eds.), Early Palaeozoic Biogeography and Palaeogeography, 38, Geological Society, London, Memoirs, pp. 199–220. Fischer, P., 1883. Manuel de conchyliologie et de paléontologie conchyliologique. Fascicule 5. Savy, Paris, 417–512.
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Floyd, J.D., Williams, M. 2003. A revised correlation of Silurian rocks in the Girvan district, SW Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 93, 383– 392. Foerste, A.F., 1924. Upper Ordovician fossils of Ontario and Quebec. Memoir of the Geological Survey of Canada 138, 1–255.
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Lamont, A., 1978. Pentlandian miscellany: Mollusca, Trilobita etc. Scottish Journal of Science 1, 245–302. Lorenzo Álvarez, S., 2015. Bioestratigrafía del Silúrico de la Zona Centroibérica meridional (España). Ph.D. Thesis, Complutense University of Madrid, 435 p. (unpubl.) Loydell, D.K., 2005. Graptolites from the Deerhope Formation, North Esk Inlier. Scottish Journal of Geology 41, 189–190.
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Walcott, C.D., 1886. Second contribution to the studies on the Cambrian faunas of North America. Bulletin of the United States Geological Survey 30, 1–369. Wenz, W., 1938. Gastropoda, Allgemeiner Teil und Prosobranchia. In: Schindewolf, O.H. (Ed.), Handbuch der Paläozoologie 6. Gebrüder Borntraeger, Berlin, pp. 1–480. Zalasiewicz, J.A., Taylor, L., Rushton, A.W.A., Loydell, D.K., Rickards, R.B., Williams, M.,
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2009. Graptolites in British stratigraphy. Geological Magazine 146, 785–850.
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Table and Figure captions
Table 1. Alphabetic list of Silurian Pterotheca species and their stratigraphical and geographical occurrences. Graptolite zonation comes from Curtis (1972: Damery Beds), Floyd and Willams (2003: Mulloch Hills Sandstone, Penkill Mudstone Fm.; see also Zalasiewicz et al., 2009), Loydell (2005: Deerhope Fm., Wether Law Linn Fm.), and
ip t
Bernardez et al. (2006: Formigosa Fm.).
cr
Table 2. Measurements of known species of Silurian Pterotheca. See Fig. 2 for explanation of the various parameters. Linear measurements are in mm. Spec. I and Spec. II for P.
an
refers to measurements given in Clarkson et al. (1995).
us
multidecorata refers to the designations given in Reed (1909). The means given for P. gath
Fig. 1. A. Geographical map of Spain showing some major cities and the two autonomous
M
districts associated with the inset. A’. Detail of the area around the Pontedo section.
ed
Fig. 2. Generalized dorsal shell morphology of an ovate Pterotheca and measurements used in this study. The shell is removed to view the internal septum from above. L: Length; W: width;
ce pt
LS: length of septum; LW: width of septum.
Fig. 3. Pterotheca hispanica nov. sp. (MGM 900S, holotype). Dorsal view (A), detail of septum (B), oblique dorsal (C) and detail of media sinus (D). Arrow in D points to the
Ac
posterior part of the open slit. Scale bars: 5 mm.
Fig. 4. A. Dorsal view of left-skewed form of Pterotheca gath Lamont, 1978 (NMS G.l897.32.746), Wether Law Linn Fm., Pentland Hills, Scotland. B. Detail of anterior median suture and slit of same specimen as in A. Arrow points to open slit area between two parallelsided ridges. C. Pterotheca aff. mullochensis Reed, 1909 (RSM GY 1993.58.70), Deerhope Fm., Pentland Hills, Scotland. D. Pterotheca mullochensis Reed, 1909 (NHM G 47130), Mulloch Hill Sandstone, Girvan, Scotland. Scale bars: 5 mm.
Page 15 of 25
Fig. 5. A. Pterotheca avirostris Pitcher, 1939 (BGS 23114), Purple Shales and Damery Beds, England. B. Pterotheca multidecorata Reed, 1909, Penkill Mudstone Fm., Scotland. Scale bars: 5 mm.
Fig. 6. Linear measurements of Silurian Pterotheca species. Specimens and measurements are given in Table 2. A. Length vs. width. B. Length of septum vs. width of septum. C. Width of
ip t
septum vs. width of shell. D. Width of shell against the length:width ratio of septum.
Ac
ce pt
ed
M
an
us
Pterotheca. Map modified from Torsvik and Cocks (2013).
cr
Fig. 7. Early Silurian (Aeronian) palaeogeographic reconstruction showing the distribution of
Page 16 of 25
ip t cr Occurrence
Stage
Biozone
P. avirostris Pitcher, 1939
Purple Shales and Damery Beds, England
Telychian
P. gath Lamont, 1978
Wether Law Linn Fm., Scotland
Telychian
Monoclimacis griestonensis Stimulograptus sedgwickii/ Cyrtograptus insectus
P. hispanica nov. sp.
Formigoso Fm., Spain
Aeronian
Stimulograptus sedgwickii
P. mullochensis Reed, 1909
Mulloch Hill Sandstone, Scotland
Rhuddanian
Coronograptus cyphus/Monograptus revolutus
P. aff. P. mullochensis Reed, sensu Clarkson et al. (1995)
Deerhope Fm., Scotland
Telychian
Oktavites spiralis–Cyrtograptus lapworthi
P. multidecorata Reed, 1909
Penkill Mudstone Fm., Scotland
Telychian
Monograptus crispus
Ac c
ep te
d
M
Taxon
an
us
Table 1.
Page 17 of 25
cr us ed
pt
Taxon P. avirostris Pitcher, 1939 P. avirostris Pitcher, 1939 P. gath Lamont, 1978 P. gath Lamont, 1978 P. gath Lamont, 1978 P. hispanica nov. sp. P. mullochensis Reed, 1909 P. mullochensis Reed, 1909 P. mullochensis Reed, 1909
M
an
Table 2.
Length 23 27.1 13.5 9.8 9.02 15 23.8 22.6 21.5
Width 24.4 29.1 13.5 11.9 10.31 16 26 22.6 25.8
Length septum 12.2 12.4 5.6 3.9 4.06 6 13.8 10.4 9.3
Width septum 12.3 14.9 4.9 5 4 8 13.9 10 12.7
Septal angle 49.8° 61.8° 52° 62° 60° 69° 54° 57° 65°
NMS G.1993.58.70 NHMUK PI OR 47143. (Spec. I) NHMUK PI OR 47141. (Spec. II)
36 11 16.3
44 13.5 21.5
28.1 6 7.5
17.5 6 8.0
50° 56° 62°
Ac
ce
P. aff. P. mullochensis Reed, sensu Clarkson et al. (1995) P. multidecorata Reed, 1909 P. multidecorata Reed, 1909
Museum no. BGS GSM23114 BGS GSM23112 NMS G.l897.32.746 NMS G.l995.66.1 Means of 35 specimens MGM 900S NHMUK PI OR 47131 NHMUK PI OR 47130 NHMUK PI OR 47132
Page 18 of 25
t ep Ac c
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e pt ce Ac Page 20 of 25
M an d te Ac ce p
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u an M d te ep Ac c
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pt ce Ac
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M ed pt ce Ac
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e pt ce Ac
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