Frankowiak Katarzyna, Roniewicz Ewa, Stolarski Jarosław
Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland.
PeerJ. 2021 Mar 16;9:e11062. doi: 10.7717/peerj.11062. eCollection 2021.
During the Carnian, oligotrophic shallow-water regions of the western Tethys were occupied by small, coral-rich patch reefs. Scleractinian corals, which already contributed to the formation of the reef structure, owed their position most probably to the symbiosis with dinoflagellate algae (zooxanthellae). Using microstructural (regularity of growth increments) and geochemical (oxygen and carbon stable isotopes) criteria of zooxanthellae symbiosis, we investigated whether this partnership was widespread among Carnian scleractinians from the Italian Dolomites (locality Alpe di Specie). Although corals from this locality are renowned from excellent mineralogical preservation (aragonite), their skeletons were rigorously tested against traces of diagenesis Irrespective of their growth forms, well preserved skeletons of corals from the Dolomites, most frequently revealed regular growth bands (low values of coefficient of variation) typical of modern zooxanthellate corals. Paradoxically, some Carnian taxa ( and sp.)with highly integrated thamnasterioid colonies which today are formed exclusively by zooxanthellate corals, showed irregular fine-scale growth bands (coefficient of variation of 40% and 41% respectively) that could suggest their asymbiotic status. However, similar irregular skeletal banding is known also in some modern agariciids (Leptoseris fragilis) which are symbiotic with zooxanthellae. This may point to a similar ecological adaptation of Triassic taxa with thamnasterioid colonies. Contrary to occasionally ambiguous interpretation of growth banding, all examined Carnian corals exhibited lack of distinct correlation between carbon ( C range between 0.81‰ and 5.81‰) and oxygen ( O values range between -4.21‰ and -1.06‰) isotope composition of the skeleton which is consistent with similar pattern in modern zooxanthellates. It is therefore highly likely, that Carnian scleractinian corals exhibited analogous ecological adaptations as modern symbiotic corals and that coral-algal symbiosis that spread across various clades of Scleractinia preceded the reef bloom at the end of the Triassic.
在卡尼阶时期，特提斯海西部的贫营养浅水区分布着小型的、富含珊瑚的斑块礁。已经对礁体结构形成有贡献的石珊瑚，其地位很可能归功于与甲藻（虫黄藻）的共生关系。利用虫黄藻共生关系的微观结构标准（生长增量的规律性）和地球化学标准（氧和碳稳定同位素），我们研究了这种共生关系在意大利多洛米蒂山脉（阿尔佩迪物种地区）的卡尼阶石珊瑚中是否普遍存在。尽管该地区的珊瑚以出色的矿物保存状态（文石）而闻名，但它们的骨骼经过了严格的成岩作用痕迹检测。无论其生长形态如何，多洛米蒂山脉保存完好的珊瑚骨骼，最常显示出典型现代虫黄藻珊瑚的规则生长带（变异系数较低）。矛盾的是，一些具有高度整合的 thamnasterioid 群体的卡尼阶分类群（ 和 种），如今这些群体仅由虫黄藻珊瑚形成，却显示出不规则的精细生长带（变异系数分别为 40%和 41%），这可能表明它们处于非共生状态。然而，一些与虫黄藻共生的现代鹿角珊瑚科（脆弱鹿角珊瑚）也存在类似的不规则骨骼带。这可能表明具有 thamnasterioid 群体的三叠纪分类群有类似的生态适应。与生长带偶尔模糊的解释相反，所有检测的卡尼阶珊瑚骨骼的碳（C 范围在 0.81‰至 5.81‰之间）和氧（O 值范围在 -4.21‰至 -1.06‰之间）同位素组成之间均缺乏明显的相关性，这与现代虫黄藻珊瑚的类似模式一致。因此，极有可能卡尼阶石珊瑚表现出与现代共生珊瑚类似的生态适应，并且在三叠纪末期礁体大量繁殖之前，珊瑚 - 藻类共生关系就已在石珊瑚的各个分支中传播开来。
