Photosymbiosis in Late Triassic scleractinian corals from the Italian Dolomites

Introduction

Scleractinian corals engineer one of the most diverse ecosystems on Earth–coral reefs. Their success in oligotrophic tropical marine habitats was enabled by the emergence of symbiotic association with dinoflagellate algae (zooxanthellae) (e.g., Stanley Jr , 1981; Wood, 1999; Hallock, 2001; Kiessling, 2010). Their mutualistic relationship base on the nutrient exchange, with corals providing shelter and inorganic nutrients to their algal partners, and zooxanthellae supplying their hosts with substantial amounts of photosynthates (review in Allemand et al., 2011; Davy, Allemand & Weis, 2012). Despite the rich fossil record of scleractinian corals beginning about 245 Ma and extensive studies concerning mechanisms underlying coral-zooxanthellae symbiosis, little is known about its origin and early evolution.

The mass emergence of scleractinian corals took place in the Middle Triassic (Anisian), ca. 8–10 Ma after the end-Permian mass extinction (e.g., Flügel & Stanley Jr, 1984; Stanley Jr, 1988; Flügel & Senowbari-Daryan, 2001; Martindale, Foster & Velledits, 2019). Although already diverse in their growth form, the earliest corals only occasionally participated in the reef formation (review in Martindale, Foster & Velledits, 2019). The rapid expansion and diversification of Scleractinia occurred later, during the Late Triassic and appear to coincide with the radiation of modern dinoflagellates (MacRae, Fensome & Williams, 1996; Bucefalo Palliani & Riding, 2000; Shaked & De Vargas, 2006; LaJeunesse et al., 2018; Simpson, 2018; Mantle, Riding & Hannaford, 2020). In fact, Late Triassic coral-algae coevolution is regarded as an impetus for the success of corals as major reef builders (e.g., Stanley Jr , 1981; Stanley Jr, 1988; Stanley Jr, 2003; Flügel, 1994; Stanley Jr & Swart, 1995; Wood, 1995; Rosen et al., 2000; Payne & Van de Schootbrugge, 2007; Stanley Jr & Cairns, 1988; Kiessling, 2009; Kiessling, 2010; Martindale, Foster & Velledits, 2019). Wells (1956) hypothesized that scleractinians were initially symbiotic with algae, and asymbiotic corals appeared much later, in the Jurassic. This concept was originally supported by some molecular data showing that the oldest scleractinian lineages were associated with zooxanthellae (Barbeitos, Romano & Lasker, 2010). Consistently, fossil corals from the Middle and Upper Triassic carbonates were described as photosymbiotic (Morycowa & Szulc, 2007; Stanley Jr & Cairns, 1988; Kiessling, 2010; Stanley Jr & Helmle, 2010; Kiessling & Kocsis, 2015; Kołodziej et al., 2018) and coral-algal partnership played a key role in the Late Triassic global reef bloom—first major expansion event after the end-Permian mass extinction (Flügel & Senowbari-Daryan, 2001; Stanley Jr & Fautin, 2001; Flügel, 2002; Kiessling, 2010; Martindale, Foster & Velledits, 2019).

Although it is most commonly assumed that scleractinian corals have been symbiotic with zooxanthellae for over 230–210 myr, alternative scenarios have also been proposed. For example, Stanley Jr (1981) argued that early scleractinians originally lacked symbionts (which would be consistent with the ancestral state of the clade, see Kitahara et al., 2010; Stolarski et al., 2011), and began to be associated with algae only near the end of the Triassic. Recently, Middle to Late Jurassic (ca. 160 Mya) evolutionary divergence times among Symbiodiniaceae clades (multiple genera previously assigned to the zooxanthella genus Symbiodinium) were proposed by new recalibrated molecular clock based on nuclear large subunit (LSU) rDNA sequences (LaJeunesse et al., 2018). Following the LaJeunesse et al. (2018) hypothesis, the modern-day coral-algal symbioses would first emerge during the second Jurassic scleractinian radiation (not during the first Triassic one) when scleractinian communities began to build reef structures composed of lineages of modern reef corals. However, molecular analyses include only living representatives of Symbiodiniaceae and all extinct clades of the group are, by definition, excluded from consideration. It is therefore of paramount importance to provide time calibration of origin of coral-algal symbiosis based on fossil evidence.

The major difficulty in tracing symbiosis into deep time is a lack of direct evidence: no coral tissues with symbionts have been fossilized nor algal cells are incorporated into the coral skeleton. Among the variety of proposed indirect indicators of coral-algal symbiosis (e.g., Cowen, 1983; Coates & Jackson, 1987; Stanley Jr & Swart, 1995; Insalaco, 1996; Gautret, Cuif & Freiwald, 1997 Cuif et al., 1999a; Cuif, Dauphin & Gautret, 1999b; Wood, 1999; Rosen et al., 2000; Nose & Leinfelder, 1997; Helmle & Stanley Jr, 2003; Muscatine et al., 2005; Stanley Jr & Helmle, 2010; Leinfelder et al., 2006; Martindale, Bottjer & Corsetti, 2012; Frankowiak et al., 2016b; Tornabene et al., 2017), the simplest criteria are based on macromorphological differences between skeletons of modern symbiotic and asymbiotic taxa (Coates & Jackson, 1987): zooxanthellates produce highly integrated colonies with small corallites, while azooxanthellates form relatively large solitary or phaceloid skeletons. However, such simple approach appeared to be problematic because of: (i) modern exceptions like Fungia, Cynarina (both solitary and symbiotic) or Astrangia danae (cerioid and asymbiotic), (ii) questionable assessment of phaceloid and epithecate corals that are rare today but prevailed in the Early Mesozoic (Roniewicz & Stolarski, 1999), and (iii) disputable classification of corals with intermediate corallite size. The reliability of morphological criteria is particularly problematic when applied to fossil scleractinian taxa (e.g., Kiessling & Kocsis, 2015; Frankowiak et al., 2016a). Conversely, geochemical proxies describing variations in the isotopic composition of elements involved in algae photosynthesis (oxygen, carbon, and nitrogen), have no evident exceptions, thus can be successfully applied to ancient scleractinians (Stanley Jr & Swart, 1995; Gruszczyński et al., 1990; Muscatine et al., 2005; Frankowiak et al., 2016b; Tornabene et al., 2017). However, since diagenetic processes may obscure the original isotopic signal, the application of geochemical proxies is reasonable only with the pristine aragonite skeletons (Frankowiak et al., 2013; Tornabene et al., 2017). New opportunities to identify the former photosymbiosis are provided by microstructural observations of the fibrous part of the coral skeleton (Frankowiak et al., 2016a). This recently proposed proxy describes the differences in the regularity of the fine-scale growth increments between zooxanthellates and their asymbiotic counterparts. A considerable advantage of the microstructural criterion over geochemical characteristics is that the banding pattern can be observed even in coralla with some traces of diagenetic alteration which could affect geochemical proxies.

All Late Triassic corals examined geochemically to date were recognized as zooxanthellates (e.g., Stanley Jr & Swart, 1995; Muscatine et al., 2005; Frankowiak et al., 2016a; Frankowiak et al., 2016b; Tornabene et al., 2017), suggesting that symbiosis was common if not exclusive lifestyle among corals occupying the western Tethys. While progress continues to be made in the identification of Triassic symbiosis, the scarcity of primary preserved coral material, followed by the fact that most of the previous studies focus on fossils from one Norian site Alakir Çay Valley, Turkey (Stanley Jr & Swart, 1995; Muscatine et al., 2005; Frankowiak et al., 2016a; Frankowiak et al., 2016b; Tornabene et al., 2017) indicate that further investigations in other localities are needed. Herein, we take advantage of the well-preserved coral fauna from Alpe di Specie, Italy, in order to assess coral-algae symbiosis in the early Carnian. Although symbiosis in Carnian corals was previously addressed by Stanley Jr & Swart (1995), the application of only one symbiotic criterion to a total of five specimens points to the need for more comprehensive studies. Using high-precision analytical tools combined with various skeletal-based criteria, we investigated whether symbiosis was widespread among previously not examined Carnian scleractinian species. We have also estimated how the diagenetic alteration could influence original geochemical signatures of the examined fossil skeletons.

Materials & Methods

Geological setting and coral material

Fossil corals used in the present study were collected in Julian (early Carnian) deposits of the Dürrenstein/Heiligkreuz Formation in the Alpe di Specie (Seelandalpe), NE Dolomites, Italy (see Fig. 2 in Tosti et al., 2014). Despite the abundance of publications concerning the coral-bearing erratic reef boulders, their origin is still a matter of debate. They have been regarded as autochthonous remains of small shallow-water coral patch reefs (Ogilvie, 1893; Salomon, 1895) or as “Cipit-blocks”—downward displacements either from the Cassian Formation (e.g., Wendt, 1982; Sánchez-Beristain et al., 2011) or Dürrenstein/Heiligkreuz Formation (Russo et al., 1991; for a detailed discussion see Sánchez-Beristain & Reitner, 2016; Tosti et al., 2014). Leaving aside the issue of their exact origin, erratic boulders record a broad spectrum of organisms involved in the formation of Carnian reefs in this area, including calcareous sponges, corals, echinoderms, mollusks, and foraminifera, etc. The invertebrate fauna is typically characterized by excellent preservation of original skeletal mineralogy, microstructures, and geochemistry (Cuif, 1973; Montanaro-Gallitelli, Morandi & Pirani, 1974; Scherer, 1977; Laghi, Martinelli & Russo, 1984; Frisia-Bruni & Wenk, 1985; Russo et al., 1991; Nützel, Joachimski & Correa, 2010). Impermeable sediments surrounding corals provided protection from diagenesis, ensuring exceptional preservation of their skeletons (e.g., Wendt, 1977; Russo et al., 1991).

The diversified coral assemblage from the studied site shows a broad spectrum of morphologies (from solitary to complex thamnasterioid) known from modern scleractinians (Figs. 1A–1C). With few exceptions, scleractinian fauna of Alpe di Specie differs taxonomically from previously studied corals from Antalya localities (Frankowiak et al., 2016a; Frankowiak et al., 2016b) but further, in-depth taxonomic description of the coral fauna is a subject of a separate study (Roniewicz et al. in preparation). Based on observations under an optical microscope, corals with well-defined ultrastructures (Rapid Accretion Deposits - RAD and Thickening Deposits—TD; after Stolarski, 2003) were selected (Table S1). Comparative Recent material included Leptoseris fragilis from the Red Sea and Montipora sp. from Mayotte (Indian Ocean). Material is housed at the Institute of Paleobiology, Polish Academy of Sciences, Warsaw (abbreviation ZPAL).

Results

Our results show that corals from Alpe di Specie exhibit both, primary skeletal features and diagenetic alterations (Fig. 1). At the ultrastructural level, two main regions of the skeleton were distinguished: (i) the first region situated in the central regions of septa which in modern corals is recognized as composed of Rapid Accretion Deposits (RADs), and (ii) the main skeletal component recognized in modern corals as Thickening Deposits (TDs) (Fig. 2). SEM observations of Carnian specimens show that the crystal textures in the central region of septa (RADs) are typical of calcite spar (Fig. 2) and under CL exhibit strong red luminescence typical for diagenetic calcite (Fig. S2F). The susceptibility to secondary alterations RADs owe to the high amount of organic matter and nanocrystalline structure (e.g., Cuif & Dauphin, 1998; Stolarski, 2003; Stolarski & Mazur, 2005; Benzerara et al., 2011). Contrary, fibrous crystals of TDs originally consist of denser and organic-poor material that is more resistant to diagenetic processes. As a result, TDs tend to preserve their original mineralogy and microstructure in skeletons as old as Triassic (e.g., Cuif, 1977; Roniewicz, 1989). The TDs of examined Carnian skeletons locally exhibit a distinct pattern of doublets of optically dark and light bands (squares in Fig. 3). Although these bands are regarded as growth increments and thus are expected to occur commonly, their appearance in modern, as well as fossil coralla is often restricted to small areas (Fig. S1). In such optically preserved specimens, the banding pattern of alternating layers with positive and negative etching reliefs was observed under SEM (Fig. 2). Lack of luminescence indicates aragonite mineralogy of fibrous crystals (Figs. S2–S5), an observation supported by chemical staining with Feigl’s solution. Occasionally, TDs were locally altered to calcite, and these secondary areas were of few micrometers in size (Fig. S5D). The aragonite composition and microstructural arrangement argue for the exceptional preservation of the examined corals, which facilitates the study of original O and C isotopic ratios. Nevertheless, some minor altered TD regions together with diagenetic aragonite cements developed on lateral faces of the skeleton (Fig. S3J) and sparry calcite in the corallite infilling (Fig. S2–S5) were also detected.

Fine-scale skeletal banding of Carnian corals that met conditions of good preservation described above is formed by regular and continuous aragonite layers, similar to those observed in modern symbiotic corals. The regularity of these growth increments was expressed using the coefficient of variation (CV) (Table S2 and Fig. S6). Almost all examined corals, regardless of their morphological form (solitary or complex colonial), exhibit low CV between 4% and 17%, which are typical for modern zooxanthellates (green area in Fig. 3). High CV values that are placed in the range of modern asymbiotic species (yellow area in Fig. 3) were calculated only for two scleractinian specimens. Surprisingly, these corals are highly integrated Thamnasteriomorpha sp. (CV of 41%) and Thamnasteriomorpha frechi (CV of 40%). Banding measurements also show that results are repeatable, and similar CVs have been calculated for different specimens of the same species. For example, two specimens of Remismilia sp. have a CV of 5% and 9% respectively, and two specimens of Margarosmilia cf. confluens exhibit CV of 8% and 10%. These variations could result either from researchers’ subjective assessment, different responses to the etching or some secondary alterations that might slightly affect banding pattern –either way, these minor differences do not influence the final interpretation.

Oxygen isotopic composition of corals studied herein ranges between −4.21‰  and −1.06‰ , and carbon signatures vary from 0.81‰  to 5.81‰  (Table S3). As shown in Fig. 4, examined corals tend to group together in carbon and oxygen isotope space. Possible contamination with secondary material was verified by comparison of δ ¹⁸O and δ ¹³C of the coral skeleton and calcite cement, and any similarities were considered as suggestive for diagenesis. However, as shown in Fig. S7 skeletons and the secondary infilling exhibit different isotopic signatures e.g., Margarastraea klipsteini (skeletal δ¹⁸O = −3.77‰  and δ¹³C = 3.19‰, calcite cement δ¹⁸O = −1.85‰  and δ¹³C=1.96‰), and Tropiastraea carinata morphotype A (skeletal δ ¹⁸O = −2.98‰  and δ ¹³C=4.56‰, calcite cement δ ¹⁸O = −5.09‰  and δ ¹³C = 0.93‰). In addition, differences in the preservation state of the skeletons are not always reflected in their isotopic composition. For example, diagenetic patterns within a single colony of the tropiastreiid sp. E range from unaltered bioaragonite to nearly completely recrystallized to calcite (Fig. 1). These regions, however, are not characterized by radically different geochemistry: the aragonite part of the skeleton exhibit δ¹⁸O = −3.30‰  and δ¹³C = 4.59‰, whereas secondary skeleton yield δ¹⁸O = −3.07‰  and δ¹³C = 4.73‰  (Fig. 4).

Discussion

Conclusions

Our findings match the scenario that symbiosis preceded the Late Triassic reef bloom. We suggest that the coral-algae relationship was common among corals living on the patch reefs developed in the present-day Alpe di Specie area. The skeletal evidence of symbiosis was found in taxa that previously were considered as azooxanthellate e.g., Craspedophyllia and Margarophyllia (Kiessling & Kocsis, 2015). This observation suggests that either previous classification of Craspedophyllia and Margarophyllia as asymbiotic was incorrect (new systematic studies of these taxa, possibly from different locations, are needed) or that both corals, similarly to modern Tubastrea, represent ”‘exceptions” from the microstructural criterion. Such possible exceptions from the microstructural criterion call for more in-depth observations of microecology of modern shallow-water corals with mixotrophic nutrition mode. It may appear that in some cases behavioral and/or environmentally controlled factors such as cyclic food supply may influence regularity of biomineralization cycles.

No compelling evidence of asymbiotic species was found among studied Late Triassic coral assemblage. The fossil record of asymbiotic scleractinian corals has long been suggested (macromorphological reconstructions excluded; Stanley & Cairns, 1988; (Gruszczyński et al., 1990; Stanley Jr & Swart, 1995; Gill, Santantonio & Lathuilière, 2004) but only recently, Tornabene et al. (2017) presented reliable geochemical data pointing on azooxanthellate status of Early Miocene Caryophyllia sp. Such a result, however, could be expected considering that all shallow- and deep-water extant species of Caryophyllia lack photosymbionts. In agreement with previous reports of Late Triassic symbiosis, our results suggest that either all corals were symbiotic or that asymbiotic scleractinians inhabited different niches than shallow-water tropics. Data presented in this study support the conclusion that photosymbiotic corals prevailed in the Early Mesozoic oceans and their partners involved most likely entirely extinct lineages of Symbiodiniaceae (LaJeunesse et al., 2018).

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