An Early Cretaceous mesophotic coral ecosystem built by platy corals (middle Aptian, Southern Carpathians, Romania)

Journal Pre-proof An Early Cretaceous mesophotic coral ecosystem built by platy corals (middle Aptian, Southern Carpathians, Romania) Bogusław Kołodziej, Ioan I. Bucur PII:

S0195-6671(19)30211-3

DOI:

https://doi.org/10.1016/j.cretres.2020.104374

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YCRES 104374

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Cretaceous Research

Received Date: 17 May 2019 Revised Date:

30 November 2019

Accepted Date: 5 January 2020

Please cite this article as: Kołodziej, B., Bucur, I.I., An Early Cretaceous mesophotic coral ecosystem built by platy corals (middle Aptian, Southern Carpathians, Romania), Cretaceous Research, https:// doi.org/10.1016/j.cretres.2020.104374. 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. © 2020 Elsevier Ltd. All rights reserved.

Bogusław Kołodziej a, *, Ioan I. Bucur b a, *

Institute of Geological Sciences, Jagiellonian University, ul. Gronostajowa 3a, 30-387 Kraków,

Poland b

Department of Geology (Centre of Integrated Geological Research), Babeş-Bolyai University, Str.

M. Kogălniceanu 1, 400084 Cluj-Napoca, Romania *

Corresponding author

e-mail addresses: [email protected], [email protected]

An Early Cretaceous mesophotic coral ecosystem built by platy corals (middle Aptian, Southern Carpathians, Romania)

Abstract: A low relief middle Aptian (Gargasian) reef built by platy corals is described from the Lower Cretaceous succession of the Reşiţa–Moldova Nouă zone (Southern Carpathians, SW Romania). Two coral-bearing units, 16–17 and 38–42 m thick, discontinuously cover ca. 1100 m. This is an unusually thick fossil reef to be built by platy corals. The coral units are underlain by bioclastic limestones interlayered with thin rudist-chaetetid biostromes, separated by a 15–30 m thick interval of bioclastic limestones and overlain by upper Aptian conglomerates. Mostly dense, rarely sparse, platestones are composed of a low diversity coral assemblage mainly represented by the suborder Microsolenina. Small, branching corals are very rare. The matrix mainly consists of fine bioclastic-peloidal packstones and wackestones. Dominance of microsolenine corals, their flattened morphology, the presence of epibionts on coral undersurfaces and occurrence of red algae Sporolithon rude and Polyastra alba indicate that low-light was the main factor controlling reef growth. As with most fossil reefs dominated by platy corals (e.g., Upper Jurassic microsolenid reefs), the Reşiţa reef can be considered an analogue of modern reefs from mesophotic coral ecosystems (MCEs) in relatively deep water settings. The mesophotic or perhaps oligophotic environment was characterised by low background sedimentation, and high nutrient level as evidenced by abundant bioerosion traces. Matrix sediment and rare fragmentation of coral skeletons indicate moderate water movement. In contrast with common Barremian–lower Aptian coral reefs, younger Lower Cretaceous reefs in Romania are very rare. This reflects the general demise of carbonate platforms in the northern Tethyan domain during the early Aptian Oceanic Anoxic Event (OAE1a).

Keywords: reefs, platy corals, microsolenids, functional morphology, Cretaceous, Carpathians

1. INTRODUCTION

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Light is an important factor structuring coral reef communities, because many species of modern scleractinians cannot photoacclimatize to depths greater than 60 m (Lesser et al., 2010). Depth around 90–100 m at which light is reduced to 1% (lower limit of the euphotic zone) photosynthetically active radiation (PAR) often defines the lower limit of occurrence of zooxanthellate corals (Fricke and Schuhmacher, 1983; Fricke and Meischner, 1985; and Pyle and Copus, 2019 for more references). However, physiological and morphological adaptations allow some photosynthetic corals to inhabit deeper settings. Modern reefs built by light-dependent coral communities developed at depths from 30–40 m to the bottom of the photic zone, which extends to over 150 m in some regions, are referred to as mesophotic coral ecosystems (MCEs) (Kahng et al., 2010, p. 256). Prior to the introduction of this term (MCE) on the beginning of 20th century, this habitat had been referred, among others, to as deep outer reefs, the coral reef twilight zone or simply deep coral reefs (e.g., Schlichter et al., 1986; Fricke et al., 1987; Pyle and Copus, 2019). Corals from mesophotic coral ecosystem commonly have a flattened, plate-like morphology to maximize light capture (e.g., Goreau and Goreau, 1973; Fricke and Schuhmacher, 1983; Lesser et al., 2009; Kahng et al., 2010, 2014; Baker et al., 2016; Muir and Pichon, 2019). Such platy assemblages are known throughout scleractinian history (Rosen et al., 2002; Kołodziej et al., 2018). Based on coral morphology and sedimentological evidence, Rosen et al. (2002) proposed for platy coral assemblages the ‘euphotic floor model’ because they argued that platy coral assemblages are a photoadaptive response to minimum PAR, due to deeper and/or turbid waters. This interpretation is commonly accepted, unless flattened morphology can be attributed to adaptation to an encrusting life style or unstable substrate. Analysis of platy corals from Anisian (Middle Triassic) patch reefs suggests that at least some degree of photosymbiosis occurred (Kołodziej et al., 2018). Other detailed studies of the palaeoecological and sedimentary context of platy scleractinian assemblages have been made in the Upper Triassic (Martindale et al., 2012), Middle Jurassic (Lathuilière, 1982), Upper Jurassic (Roniewicz and Roniewicz, 1971; Insalaco, 1996; Leinfelder et al., 2002; Gill et al., 2004; Lathuilière et al., 2005), Cretaceous (Clack, 2001a, b; Aillud, 2001; Tomás et al., 2008), Eocene (Bosellini, 1998; Morsilli et al., 2012) and Miocene (Novak et al., 2013; Santodomingo et al., 2015; Reuter et al., 2019). For additional references see Rosen et al. (2002) and Kołodziej et al. (2018). Flattened morphology in Paleozoic tabulates and rugosans is usually interpreted as an adaptation to substrate conditions (Young, 1999). Recent analysis of Silurian and Devonian platy coral assemblages suggests, however, that their morphology may also be interpreted as adaptation to low light conditions, providing indirect evidence that some tabulates may have contained photosymbionts analogous to modern zooxanthellae (Zapalski et al., 2017; Zapalski and Berkowski, 2019). This study is a contribution to knowledge of platy coral assemblages from mesophotic coral ecosystems. Its main aims are to describe a unique example of a Cretaceous reef built by platy corals, 2

and to interpret the environmental factors controlling its growth, including possible responses to a contemporaneous Oceanic Anoxic Event (OAE1a).

2. GEOLOGICAL SETTING AND MATERIAL

The Reşiţa-Moldova Nouă Zone of the South Carpathians in Romania (Săndulescu, 1975, 1984, 1994), is situated in the south-western part of the area covered by the Getic domain or Getic Nappe (Fig. 1). The sedimentary deposits of this area consist of Carboniferous–Permian and Jurassic– Lower Cretaceous formations (Bucur, 1997). The Lower Cretaceous deposits are represented by calpionellid-bearing limestones and marls (Marila and Crivina formations, upper Tithonian–lower Valanginian), slope limestones (Valea Lindinei Member, lower part of the Plopa Formation, upper Valanginian–Hauterivian), carbonate platform limestones (Valea Nerei Member, upper part of the Plopa Formation, lower Barremian), carbonate platform limestones with marl intercalations (Valea Minişului Formation, upper Barremian–middle Aptian), and glauconitic sandstone and shales (Valea Golumbului Formation, uppermost Aptian–Albian) (Bucur, 1997; Fig. 2). The Valea Nerei and Valea Minisului formations represent carbonate platform deposits. The section studied (Figs. 1B, 3) was sampled along the Valea Minişului, and corresponds mainly to the Valea Minişului Formation. In the Reşiţa-Moldova Nouă zone the Valea Minişului Formation is 300– 400 m thick and consists of bioclastic and organogenic limestones interlayered with marls. It contains a macrofauna with rudists (Toucasia carinata, T. compressa, Requienia cf. gryphoides), calcified sponges (Cladocoropsis cretacica), echinoids (Salenia prestensis, Heteraster oblongus), brachiopods (Terebratula sella), gastropods (Trochonerita mammaeformis), and rich foraminiferal and calcareous algae associations (Bucur, 1997). Among the foraminifera, the most important are the orbitolinids Paracoskinolina maynci, Montseciella arabica, Palorbitolina lenticularis, Praeorbitolina cormyi, Mesorbitolina parva and M. texana, as well as Neotrocholina friburgensis (Bucur, 1997). The calcareous algae association includes: Angioporella? bakalovae, Cylindroporella ivanovici, Falsolikanella danilovae, Falsolikanella nerae, Kopetdagaria sphaerica, Neomeris cretacea, Pseudoactinoporella fragilis, Rajkaella banatica, Salpingoporella melitae, S. muehlbergii, S. patruliusi, Suppililiumaella praebalkanica, Triploporella carpatica, Arabicodium meridionalis, Boueina hochstetteri, Halimeda? fluegeli, Sporolithon phylloideum, Sporolithon rude, Permocalculus ampulacea, and Polystrata alba. Among the dasyclads, Bakalovaella elitzae is the most common species within this lithostratigraphic unit (Bucur, 1994a, b, 1997, 2001). The age of the Valea Minişului Formation, as indicated by the orbitolinid foraminifera, is late Barremian–middle Aptian (Gargasian). The lower part of the Valea Minişului Formation at its type locality (Minişului Valley, between Bozovici and Anina) is dominated in the lower part by bioclastic and bioclastic-peloidal packstones-grainstones rich in foraminifera and interlayered with thin rudist-chaetetid biostromes that 3

locally contain large branching (phaceloid) corals. In its upper part, the formation is dominated by limestones with the platy corals described in this paper. For the succession as a whole, orbitolinid foraminifers (e.g., Palorbitolina lenticularis and Palaeodictyoconus div. sp.) indicate a late Barremian–Bedoulian age for the lower part, and Mesorbitolina parva and Mesorbitolina texana indicate a Gargasian age for the upper part (Schroeder et al., 2010). In this section, the Valea Minişului Formation represents external platform deposits. The upper coral unit is bounded above by an unconformity and overlain by upper Aptian conglomerates (Fig. 3). The sampling area is located along the road from Bozovici to Anina. A total 42 thin sections were prepared from the coral platestones: 36 of 25 mm x 40 mm size and 6 of 50 mm x 60 mm size.

3. RESULTS

The coral reef from the Miniş Valley – referred to here as the Reşiţa reef – consists of two units respectively 16–17 m (lower unit) and 38–42 m (upper unit) thick (Figs. 4–5) separated by 15–30 m bioclastic limestone. The reef corals virtually exclusively exhibit platy growth forms. Small phaceloid corals, some centimetres in diameter, also occur, but are rare. The reef is laterally discontinuous for approximately 1100 m. The matrix between the corals mainly consists of dark grey bioclastic-peloidal packstone, wackestone, subordinate marly calcimudstone, marl and microbialite (Fig. 6). The corals are mainly poorly preserved and strongly recrystallized, especially the largest specimens, which makes taxonomic identification difficult. Thin skeletons are inconspicuous in both outcrop and hand specimens, because intraskeletal space is filled with micrite, and the contrast between the coral and the matrix is poor. The taxonomic diversity (both on genus and family level) of the coral assemblage seems to be low (even compared with relatively poorly diversified Upper Aptian coral communities; e.g., Tomás et al., 2008; Löser, 2013) and dominated by representatives of the suborder Microsolenina Morycowa and Roniewicz, 1995. Most of these with porous skeletons and more or less developed pennular ornamentation of the septa, remain unidentified due to poor preservation and lack of good sections (Fig. 7A–B). These which could be determined to genus level include representatives of the family Latomeandridae: Dimorphastrea sp. (Fig. 7C), ?Astraeofungia sp. (Fig. 7D), Thalamocaeniopsis sp. (Fig. 7E) and ?Latomeandra sp. (Fig. 7F). Less common are: Latusastrea sp. (family Heterocoeniidae; Carolastraeidae according to Löser, 2016; Fig. 7G); Eocolumastrea sp. (family Columnastraeidae; Fig. 7H), and Ahrdorffia sp. (Thamnasteriidae; Fig. 7I) (for taxonomy of coral genera see Löser, 2016). Nearly all the corals have platy and undulose morphology (Fig. 5), and are 8–40 cm in diameter and 1–15 cm in thickness. The reef consists of dense, locally sparse platestone sensu Insalaco (1998, his fig. 11), with high coral density, 50–90% of outcrop area. In platestones, by definition, the width-to-height ratio of the dominant corals ranges is 5:1–30:1. In the reef studied, the values are 5:1– 4

18:1. Sheetstones (width-to-height ratio >30:1) were not recognized. Ragged vertical margins occur locally in the coral plates. Fragmentation of platy corals was rarely observed. Although coral density is high, corals that encrust one another are very rare (Fig. 8B). Encrustations of the coralline red alga Sporolithon rude, the peyssonneliacean red alga Polyastra alba (Fig. 8A–D), and, although much more rarely, the problematic microencruster Koskinobullina socialis (Fig. 8E), as well as rare bacinellid microbial textures occur on the upper surfaces of the corals. Coral undersides are locally encrusted by serpulids, bryozoans (Fig. 8A) and large agglutinated foraminifera. Microbial crusts are occasionally present on, between and below the corals. Bioerosion (sponges, bivalves, worms?) of corals is moderate to high (Fig. 8F–G). In addition to encrusting and boring organisms, the associated biota include small fragments of bryozoans, brachiopods, echinoderms, molluscs, crustaceans, serpulids, ostracodes, orbitolinid foraminifers and rare fragments of the discoidal dasycladalean alga Terquemella sp.

4. DISCUSSION 4.1. Reef type and environmental controls

Late Jurassic microsolenid biostromes (see discussion below) have commonly been accepted as a type of reef (Insalaco, 1996). In this paper, following most authors who study fossil reefs, we also use the term “reef” in a broad sense (e.g., Wood, 1999, Riding, 2002; Kiessling, 2009). The Reşiţa reef, in terms of its thickness and lateral extent, is arguably the most spectacular fossil reef built by platy corals anywhere in the world, recorded so far. Rosen et al. (2002) show that most fossil platy coral assemblages form units (which they collectively termed lithosomes, not reefs) often less than a few metres thick, although there are examples of 20-m-thick lithosomes in the Oxfordian of France (Insalaco, 1996) and of 16-m-thick lithosomes in the Albian of France (Clack, 2001a, see Rosen et al., 2002). In the Upper Jurassic of Western Europe platy coral lithosomes are up to 350–400 m in lateral extent (Insalaco, 1996; Insalaco et al., 1997). Platy coral lithosomes in the Miocene of Indonesia extend for 1600 m, but outcrops are discontinuous and lithosomes can be less than 3 m-thick (BouDager-Fadel and Wilson, 2000; see Rosen et al., 2002). Reef development is governed by numerous environmental and ecological factors, primarily light, sedimentation, water movement, nutrient level, topography and temperature (e.g., Wood, 1999; Sheppard et al., 2009). In the following discussion we focus on the first four factors. Light, particularly PAR, is an important factor structuring coral reef communities, especially for the photosynthetic members of their biota. Moreover, many modern scleractinians cannot photoacclimatize at depths greater than 60 m (Lesser et al., 2010). Nonetheless, there are exceptions, such as Leptoseris fragilis that has been found photosynthesizing as deep as 145 m (Fricke and Schuhmacher, 1983; Schlichter, 1992). This species is common in mesophotic coral ecosystems.

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As noticed Pyle and Copus (2019, p. 9) “term mesophotic coral ecosystem was intentionally defined broadly, to accommodate the degree of variation in both depth and composition in different regions”. Criteria to define mesophotic coral ecosystems are poorly established. The prefix „meso” indicates intermediate light conditions. Mesophotic reefs should therefore be expected to occur where light conditions were midway between well-illuminated and where light is absent or photosynthetically active radiation (PAR) approaches zero, i.e. the euphotic floor (see above). Therefore, the latter reefs should be called „oligophotic” (anonymous Reviewer pers. comm.). Term mesophotic has been used in the geological literature to refer to a light range roughly between 1% and 20% PAR (Pomar, 2001; Pomar et al., 2017; Renema, 2019 and references therein). In some cases the deepest zone, if irradiance levels are 1–5% of subsurface levels, is referred to as the “oligophotic” (Pomar et al., 2017 and references therein), but estimating these values in the fossil record is difficult or doubtful. According to Kirk (2011; see Tamir et al., 2019) the euphotic zone ends at the depth where 1% of surface PAR remains. This is the light level where photosynthesis is believed to equal respiration (Lesser et al., 2009). However, light values for modern reefs are rarely provided (Fricke et al., 1987; Lesser et al.; 2009, Tamir et al., 2019). To our knowledge the distinction between mesophotic (sufficient light for coral growth, commonly below normal wave base) and oligophotic (sufficient light for coralline red algal growth) zones is more commonly used in the geological literature to analyze carbonate platforms and reef development (Pomar, 2001; Morsilli et al., 2012; Pomar et al., 2017 and references therein; see also Renema, 2019), but has not been used so far in papers on modern, deeper water, photic-zone reef ecosystems. The modern reef, Pulley Ridge (Florida Keys), called mesophotic, is developed at light level that is typically 95% below that of subsurface levels (Slattery et al., 2018), and thus could be classified as an oligophotic reef. Until hard data on PAR conditions in modern examples of platy coral assemblages becomes available, the indirect evidence (e.g., Rosen et al., 2002) that such coral assemblages occur at or close to the euphotic floor must be used. Based on literature, it seems that platy forms of zooxanthellate corals are especially common at lower to lowest light (PAR) values, that is in oligophotic conditions rather than in mesophotic conditions in the strictest sense of what latter word. Moreover, the word “mesophotic” (“middle light”), implies that the primary environmental factor of relevance is light intensity. However, the boundaries incorporated into the definition of mesophotic coral ecosystems are based on depth. These two environmental parameters are generally correlated in clear tropical waters, but the correlation is not consistent on a global scale (Pyle and Copus, 2019). Mesophotic coral communities extend significantly deeper in clear oligotrophic waters, where euphotic zone depth may extend down to ca. 170 m (Loya et al., 2019 and references therein). It is in contrast to low-light habitats in shallower waters, which are subjected to terrestrial input or deepwater outfalls. The term, mesophotic reefs, has recently also been applied to reefs developed in inshore, turbid-water settings (Morgan et al., 2016, 2017). While this seems to contradict the depth criterion for ‘mesophotic’, it is consistent with the light-based criterion of Rosen et al. (2002) who 6

emphasized that attenuation of light (PAR) can be due either to increasing water depth alone, or to increasingly turbid conditions, or some combination of both. Corals from shallow-water, turbid environments are morphologically more diverse as they must be adapted both to sediment load and decreased light level (Browne et al., 2012). The problem with definition of mesophotic coral ecosystem, for example the conflict between light levels and depth, was recently discussed by Pyle and Copus (2019). Fossil coral assemblages dominated by forms with platy morphology are therefore commonly attributed to poorly-lit environments in deeper and/or turbid settings. The earliest platy scleractinian assemblages recently described from Middle Triassic of Poland inhabited shallow, turbid-water environment (Kołodziej et al., 2018). In the euphotic floor model, the photoadaptive origin of the platy coral growth form is supported by sedimentological and palaeoecological evidence (Rosen et al., 2002). Estimating water depth and light level is difficult for ancient environments. Based on dominance of platy corals, sediment characteristics and associated biota we interpret that the Reşiţa reef grew in a relatively low light mesophotic or even oligophotic environment. Physiological and morphological photoadaptation can include a variety of attributes, e.g., flattened morphology; mono-layered zooxanthellae packaging to increase photosynthetic efficiency; specialized zooxanthellae, zooxanthellae densities, pigment concentrations, decreasing metabolic demands, and endolithic algae that supply photosynthates to coral hosts; thin skeletons requiring less calcification; slow rates of growth; and enhanced heterotrophic nutrition (Lesser et al., 2009; Kahng et al., 2010, 2014, 2012, 2019; Baker et al., 2016). These physiological adaptations may never be accurately deciphered in fossil material, but functional analysis of the coral skeleton can provide important information about the environment. Bathymetric interpretations can be based on the skeleton morphology in recent corals, because most of the zooxanthellate scleractinians from mesophotic to oligophotic communities have an intrinsic plate-like morphology or develop it due to ecophenotypic plasticity. In these conditions, some coral species grow in large flat plates to maximize light capture (review in Todd, 2008). Phototrophic microencrusters in the Reşiţa reef include the red algae Sporolithon rude and Polyastra alba, rare microproblematicum Koskinobullina socialis (Pleș et al., 2017 for review), and rare bacinellid textures interpreted as structures produced by cyanobacteria (Schlagintweit and BoverArnal, 2013). The red algae species are eurytopic. For example, the presence of Polystrata alba in lower Aptian coral rubble deposits in Spain was interpreted as partial evidence for a poorly illuminated environment (Bover-Arnal et al., 2011). Mesophotic or even oligophotic conditions of the Reşiţa reef are further supported by a lack of dasycladalean algae, except for very rare fragments of Terquemella. Most fossil and modern platy coral assemblages represent deeper settings, below normal wave base. Late Jurassic (mostly Oxfordian) microsolenid reefs are commonly interpreted as having developed at a water depth between 20 and 80 m (Insalaco, 1996). Cretaceous reefs built of platy 7

corals show similarities to Upper Jurassic examples, but have generally been much less explored. However, Tomás et al. (2008) provide detailed analysis of a Lower Cretaceous example (see also Clack, 2001a, b; Aillud, 2001). In addition, taxonomic details of Cretaceous platy coral assemblages are presented in Tomás et al. (2008) and Löser and Bilotte (2017). The upper Aptian carbonate succession from the southern Maestrat Basin (eastern Spain) contains coral-algal sheetstones and platestones-domestones dominated by a Microsolenina−Faviina coral association (Tomás et al., 2008). The proportion of the rock occupied by coral skeletons is 20−30 %. The environment of the first coral assemblage was interpreted as proximal outer ramp, below storm wave base (ca. 20 m deep); and middle ramp in the case of second coral assemblage, between storm and fair-weather wave-bases (10 to 20 m) (Tomás et al., 2008). Platy coral assemblages described by Clack (2001a, b; see also Rosen et al., 2002) from the upper Barremian of France, upper Aptian of France, and lower Albian of Spain are located in the mid to inner ramp (first three case studies) and outer ramp (in the case of Albian example). Platy coral lithosomes representing outer platform have been recorded in the Santonian of Vilanoveta, in the Tremp Basin by Skelton et al (1997; see also Rosen et al., 2002). Bucur (1997) interpreted the Valea Minişului Formation containing the Reşiţa reef as deposited in the external carbonate platform. Heterotrophic encrusters observed on aboral parts of some platy corals indicate that these portions of plates were probably cantilevered suprastratally (i.e. above the substrate), instead of resting on it (Rosen et al., 2002, their fig. 1), implying low sedimentation rate and suprastratal growth. Flattened morphology per se is not evidence of low sedimentation input, because some platy corals reject sediment from the surface, or their angled, tiered or whorled morphology facilitate the shedding of sediment (Rosen et al., 2002; Wilson and Lokier, 2002; Browne et al., 2012). However, reefs dominated by dense platestones and high bioerosion strongly suggest low net sedimentation. Matrix sediment and rare fragmentation of coral skeletons point to moderate water movement. Episodes of increased sedimentation and energy are inferred from changes in sediment matrix (from calcimudstones-wackestones to packstones and rare grainstones) and ragged vertical margins of some colonies. Lack of rudstone indicates that the reef studied had low syndepositional relief, with constratal growth fabric, sensu Insalaco (1998). Similarly, Rosen et al. (2002) assumed by default that platy coral assemblages grew without elevation above the substrate. Redeposition from shallow-water settings appears to be subordinate, because photophilic biota, such as dasycladalean algae (apart from very rare Terquemella sp.) are absent. Differences in the thickness of platy corals may reflect light availability, fluctuations in the sedimentation rate (resulting in the death of young individuals), and/or coral growth rate. For example, in the upper Aptian of Spain, Tomás et al. (2008) reported thinner platy specimens (sheetstones) from deeper setting (proximal outer ramp), and thicker specimens (platestones, domestones) from the middle ramp. Wilson and Lokier (2002) showed that Miocene platy corals from Indonesia became larger and thicker

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as the amount of siliciclastic material decreased. Thinner coral plates did not form sheetstones in the Reşiţa reef. Their growth may be related to episodes of increased turbidity or sediment input. Corals of the suborder Microsolenina, with porous skeletons and pennular septal ornamentation, display similarities with modern Leptoseris, a zooxanthellate coral from what would now be called mesophotic reef ecosystems, with unique filtering gastrovascular system (Schlichter, 1992). Platy microsolenine corals were particularly common in Late Jurassic reefs of deeper settings (microsolenid biostromes). Some authors suggested that microsolenine corals, alike L. fragilis, could also develop improved (by filtering) system of heterotrophic nutrition (Lathuilière and Gill, 1995; Morycowa and Roniewicz, 1995; Insalaco, 1996; Gill et al., 2004) which would be consistent with evidence of increased nutrient levels, such as moderate to high bioerosion. Increased activity of endolithic organisms on carbonate platforms is commonly observed during nutrient-enhanced periods (Hallock, 1988; Dupraz and Strasser, 2002; Tomás et al., 2008). This may be partly related to episodes of increased sediment input that supplied siliciclastic components (clay minerals) in the reef matrix, resulting in the locally marly character of the limestones studied. However, as mentioned above zooxanthellate corals may adapt and thrive at relatively extreme depths using diffrent strategies, for example decreased biomass, metabolism and growth rate (Kahng et al., 2012, 2019). Decreased coral growth rates have also been observed in Upper Jurassic microsolenid reefs (Insalaco, 1996). In summary, the Reşiţa reef – in terms of both composition and sedimentary setting – shows many similarities with other reefs built by platy corals, especially Upper Jurassic microsolenid reefs. The similarities in interpretation with these Upper Jurassic examples include (i) low coral diversity and dominance of Microsolenina, (ii) low light intensity, (iii) low background sedimentation rate, (iv) low energy levels, (v) increased nutrient level, (vi) relatively deep-water environment (see Insalaco, 1996). Comprehensive synthesis of the state of knowledge of modern MCEs is given by Loya et al. (2019). Although light-dependent corals from what are now widely referred to as mesophotic environments have been known since the nineteenth century (including studies by Linnaeus and Darwin), 70% of the research on MCEs has been published within just the past 7 years (see Pyle and Copus, 2019). Platy scleractinian assemblages attributed to low-light environments are well known from the fossil record (review in Rosen et. al., 2002; Kołodziej et al., 2018). Usually they were not termed mesophotic, although Upper Eocene coral buildups from Spain have recently been described as mesophotic. Mesophotic or mesophotic-oligophotic environments have recently commonly been inferred for the growth of some Cenozoic coral reefs and buildups (Nowak et al., 2013; Santodomingo et al, 2015; Pomar et al., 2017). Zapalski et al. (2017) and Zapalski and Berkowski (2019) described Silurian reefs from Sweden and Devonian reefs from Poland as the oldest mesophotic coral ecosystems.

4.2. Reşiţa reef and relations to global environmental perturbations 9

Increasing interest in modern mesophotic coral ecosystems is helpful in deciphering environmental controls of fossil platy assemblages. According to the deep water refugia hypothesis, deep reefs are potential refugia for shallow, clear-water reef organisms in times of environmental stress (Lesser et al., 2009; Bongaerts et al., 2010). Shallow-water reefs developed in naturally turbid environments (in consequence with reduced illumination) are considered as an alternative status of reef development (Perry and Larcombe, 2003; Browne et al., 2012; Morgan et al., 2016, 2017). Coral assemblages representing turbid-water environments are more common in the fossil record (Sanders and Baron-Szabo, 2005) than coral assemblages representing deeper settings (Rosen et al., 2002). The Reşiţa reef was controlled by local factors, but it is tempting to hypothesise that reef growth in the mesophotic or oligophotic environment was a response to global environmental perturbation. The Barremian–Albian was a time of prolific carbonate platform development containing varied types of coral and rudist bioconstructions, as well as diversified level-bottom coral assemblages (Masse and Philip, 1981; Simo et al., 1993; Höfling and Scott, 2002; Skelton, 2003; Skelton and Gili, 2012). However, extensive platform growth in the earliest Aptian on the northern Tethyan margin was interrupted during the mid-early Aptian (middle Bedoulian). The platform demise has been linked to oceanic anoxic event, OAE 1a (‘Selli event’), and to the later mid-late Aptian warming interval (Heldt et al., 2010; Skelton and Gili, 2012; and references therein). In contrast to common Barremian–lower Aptian coral reefs in Romania (Eastern and Southern Carpathians, Apuseni Mts., Moesian Platform, Dobrogea; Bucur, 2008; Kołodziej et al., 2011, 2012; Marian and Bucur, 2012; Pleş et al., 2017), younger Lower Cretaceous (middle and upper Aptian) reefs are very rare. This reflects the general demise of many carbonate platforms in the northern Tethyan domain during this time. Apart from the Reşiţa reef, the only other case of post-early Aptian coral-bearing buildups in Romania is in the Pădurea Craiului Massif (Apuseni Mountains, NW Romania). Upper Aptian coral biostromes with poorly diversified corals occur within a carbonate succession dominated by bacinellid microbial structures (“Lithocodium aggregatum–Bacinella irregularis”) (Daoud et al., 2011; Bucur et al., 2010; Kołodziej et al., 2011; Papp et al., 2013). Extensive development of the early Aptian bacinellid-dominated buildups has been interpreted as a response to environmental perturbations linked to OAE 1a on the southern Tethyan platforms, such as in Oman and Croatia (Immenhauser et al., 2005; Huck et al., 2010) and in a proto–North Atlantic setting in Portugal (Huck et al., 2012). It was hypothesised that the dominance of bacinellid-dominated buildups in the Pădurea Craiului may represent similar responses to environmental perturbations during the middle and the late Aptian (Kołodziej et al., 2011). The mesophotic Reşiţa reef and coralbearing, bacinellid-dominated buildups from the Pădurea Craiului may therefore be differing responses to environmental perturbations in the carbonate production factory during the middle and late Aptian, respectively. This hypothesis could be tested by studies of other middle and upper Aptian coral buildups. 10

5. Conclusions The Reşiţa reef is interpreted as a fossil example of a mesophotic coral ecosystem. In terms of thickness, it is globally the most spectacular fossil reef built by platy coral assemblages. Light (PAR) was a main factor controlling reef growth, indicated by the platy coral morphology which is an adaptation of photosynthetic corals to very low light conditions (mesophotic or oligophotic conditions). Other supporting arguments include low coral diversity, dominated by the suborder Microsolenina (adapted to deeper environment and more heterotrophic mode of feeding), and scarcity of other photophilic biota, apart from the opportunistic red algae Sporolithon rude and Polyastra alba. This biotic composition, together with the nutritional mode inferred for microsolenids are consistent with other evidence of increased nutrient levels inferred from abundant bioerosion. The matrix sediment (mostly bioclastic packstone and wackestone) and rare fragmentation of coral skeletons suggest low-moderate water movement. The platy coral morphology, presence of encrusting organisms on some coral undersides, and intense bioerosion indicate a dominantly low sedimentation rate. As in Upper Jurassic microsolenid biostromes, these Romanian platy coral assemblages developed in a low-light, nutrient-rich environment between the proximal outer ramp (below storm wave base) and middle ramp. The Reşiţa reef also shows similarities with upper Aptian sheetstones and platestones in Spain, but in the Reşiţa reef the coral-bearing intervals are much thicker and the coral aggregations are denser. Middle and upper Aptian carbonate platforms and reefs in Romania are much rarer than Barremian–lower Aptian ones. This is consistent with environmental perturbations related to OAE1a and the mid-late Aptian warming event. Development of the Reşiţa reef reflects local factors, but it is tempting to hypothesise that reef growth in the mesophotic ecosystem was also influenced by global environmental perturbations.

Acknowledgements We thank Hannes Löser (Universidad Nacional Autónoma de México, Hermosillo) for discussion of coral taxonomy. Robert Riding (University of Tennessee, Knoxville) kindly checked the English. The helpful corrections and comments of two anonymous reviewers are kindly acknowledged.

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Figure captions Fig. 1. A. Location of the Mesozoic formations of the Resiţa-Moldova Nouă zone in Romania. B. Location of the Reşiţa reef at the Miniş Valley section, scale 1:50000 (Năstăseanu and Savu, 1970, modified). Fig. 2. Succession and biozonation of the uppermost Jurassic–Lower Cretaceous deposits of the Reşiţa-Moldova Nouă Zone (modified from Bucur, 1997). Fig. 3. Barremian-Aptian succession of the Miniş Valley section. V.N.M. – Valea Nerei Member of the Plopa Formation; G.F. – Golumbu Formation. Fig. 4. The upper unit of the Reşiţa reef. A. General view of the reef with dense coral aggregations. B. General view showing distinct inclination of the coral unit. Fig. 5. Dense (A–C) and sparse (D) coral platestones. Image C is a close up of B. E. Polished slab showing two thin platy corals (arrowed) located between two thicker platy colonies. Fig. 6. Matrix sediment within the Reşiţa reef. A. Bioclastic-peloidal packstone. B. Bioclastic packstone (pac) below the skeleton of Latusastrea sp., and calcimudstone to wackestone (c–w) above the coral skeleton. C. Wackestone and microbialite crust. Scale bars 2 mm. Fig. 7. Main coral taxa involved in the Reşiţa reef. A−B. Undetermined species of the suborder Microsolenina. C. Dimorphastrea sp. D. ?Astraeofungia sp. E. Thalamocaeniopsis. F. ?Latomeandra sp. G. Latusastrea sp. H. Eocolumastrea sp. I. Ahrdorffia sp. Scale bars 2 mm. Fig. 8. Encrusting organisms and bioerosion traces in corals. A. Sporolithon rude and Polyastra alba encrusting the upper surface of microsolenid corals (a, arrowed). The lower coral surface is 18

encrusted by bryozoans and serpulids (s-b, arrowed). B. Microsolenid coral encrusted by the red algae Sporolithon rude and Polyastra alba, which in turn are encrusted by an undetermined coral. C–D. Coral encrusted by Sporolithon rude (C) and Polyastra alba (D). E. Koskinobullina socialis. F–G. Corals with abundant borings produced by bivalves, worm? (F), and sponges (G).

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1 Author declaration

X No conflict of interest exists. We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

2. Funding X Funding was received for this work. All of the sources of funding for the work described in this publication are acknowledged below: [List funding sources and their role in study design, data analysis, and result interpretation] Institute of Geological Sciences, Jagiellonian Universit, Kraków, Poland – place of employment of B. Kołodziej Department of Geology (Centre of Integrated Geological Research), Babeş-Bolyai University, Cluj-Napoca, Romania – place of employment of Ioan I. Bucur

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X We confirm that the manuscript has been read and approved by all named authors. X We confirm that the order of authors listed in the manuscript has been approved by all named authors.

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6. Contact with the Editorial Office The Corresponding Author declared on the title page of the manuscript is: Bogusław Kołodziej

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Conflicts of Interest Statement Manuscript title: An Early Cretaceous mesophotic coral ecosystem built by platy corals (middle Aptian, Southern Carpathians, Romania).

The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or nonfinancial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.

Author names: Bogusław Kołodziej Ioan I. Bucur 29.11.2019