Data from: Fish and tetrapod communities across a marine to brackish salinity gradient in the Pennsylvanian (early Moscovian) Minto Formation of New Brunswick, Canada, and their palaeoecological and palaeogeographical implications

Euryhaline adaptations in Pennsylvanian vertebrates allowed them to inhabit the marine to freshwater spectrum. This is illustrated by new assemblages of fish and tetrapods from the early Moscovian Minto Formation of New Brunswick, Canada. Fish include chondrichthyans (xenacanthids and the enigmatic Ageleodus), acanthodians (gyracanthids and acanthodiforms), sarcopterygians (rhizodontids, megalichthyids and dipnoans), and actinopterygians (eurynotiforms). Tetrapods include small- to medium-sized, and largely aquatic, stem tetrapods (colosteids) and anthracosaurs (embolomeres). A key finding is that the parautochthonous fossil assemblages are preserved across a salinity gradient, with diversity (measured by the Simpson Index) declining from open marine environments, through brackish embayments, and reaching a nadir in tidal estuaries. Chondrichthyans dominate the entire salinity spectrum (65% of fossils), a distribution that demonstrates a euryhaline mode of life, and one large predatory chondrichthyan, Orthacanthus, may have practised filial cannibalism in coastal nurseries because its heteropolar coprolites contain juvenile xenacanthid teeth. In contrast, other fish communities were more common in open marine settings while tetrapods were more common in coastal brackish waters. While all these faunas were also likely to have been euryhaline, their osmoregulation was, perhaps, less versatile. The demonstration of widespread euryhalinity among fish and aquatic tetrapods explains why Pennsylvanian faunas generally show a cosmopolitan biogeography because taxa were able to disperse via seaways. It also resolves the paradox of enriched strontium isotopic signatures observed in these faunas because organisms would have been, at times, exposed to continental water bodies as well. Therefore, our new findings contribute to the long-running debate about the ecology of Pennsylvanian fishes and tetrapods.

宾夕法尼亚纪 (Pennsylvanian) 脊椎动物的广盐性 (euryhaline) 适应使其能够栖息于从海洋到淡水的完整梯度生境。这一结论可通过加拿大新不伦瑞克省莫斯科阶 (Moscovian) 早期明顿组 (Minto Formation) 中发现的全新鱼类与四足类化石组合得到佐证。本次发现的鱼类类群包括：软骨鱼纲 (chondrichthyans)——异棘鲨科 (xenacanthids) 与神秘的阿吉莱齿鱼 (Ageleodus)；棘鱼纲 (acanthodians)——旋棘鱼科 (gyracanthids) 与棘鱼形目 (acanthodiforms)；肉鳍鱼纲 (sarcopterygians)——根齿鱼科 (rhizodontids)、巨颌鱼科 (megalichthyids) 与肺鱼类 (dipnoans)；以及辐鳍鱼纲 (actinopterygians)——宽颌鱼目 (eurynotiforms)。四足类则包含中小型、以水生为主的茎四足类 (stem tetrapods)——骨鳞鱼科 (colosteids)，以及炭螈类 (anthracosaurs)——栓塞亚目 (embolomeres)。一项核心研究结果显示，准原地 (parautochthonous) 化石组合保存在盐度梯度环境中，其多样性以辛普森多样性指数 (Simpson Index) 衡量，从开阔海洋环境经半咸水海湾逐渐降低，在潮汐河口达到最低值。软骨鱼纲占所有化石的65%，在整个盐度梯度中均占主导地位，这一分布模式证明其具备广盐性生活模式；其中大型掠食性软骨鱼类直棘鲨属 (Orthacanthus) 可能在沿海育苗场存在亲子相食 (filial cannibalism) 行为，因为其异极性粪化石 (heteropolar coprolites) 中发现了幼体异棘鲨的牙齿。与之相反，其他鱼类群落在开阔海洋环境中更为常见，而四足类则更多栖息于沿海半咸水环境。尽管所有这些动物群大概率也具备广盐性，但其渗透压调节 (osmoregulation) 能力或许稍逊一筹。本次研究证实鱼类与水生四足类普遍具备广盐性，这解释了为何宾夕法尼亚纪动物群总体呈现广布生物地理分布 (cosmopolitan biogeography)——各类群可通过海道进行扩散。同时这也解决了此前观测到的这些动物群中富集锶同位素特征 (strontium isotopic signatures) 的悖论：生物有时会接触大陆水体。因此，本次新发现为学界长期争论的宾夕法尼亚纪鱼类与四足类生态问题提供了新的研究支撑。