Delsett Lene Liebe, Faure-Brac Mathieu Gabriel, Engelschiøn Victoria Sjøholt, Houssaye Alexandra, Chinsamy Anusuya, Hurum Jørn Harald, Kear Benjamin P
Natural History Museum, University of Oslo, Sars' gate 1, Oslo, 0562, Norway.
Département Adaptations du vivant, UMR 7179 CNRS/Muséum National d'Histoire Naturelle, 57 rue Cuvier CP-55, Paris, 75005, France.
Sci Rep. 2025 Sep 5;15(1):30221. doi: 10.1038/s41598-025-14335-y.
Ichthyosaurs were the first fully marine tetrapods, and evolved a streamlined body, flippers, live birth, and endothermy-like physiology. However, the transition to these adaptations and how it relates to divergence into ocean environments is ambiguous. Here, we use vertebral bone microstructure to document the first ontogenetic series of two Early Triassic taxa that include the oldest ichthyosaur foetal fossils. One series is from Grippia, an early ichthyopterygian with a small body, and limbs with some plesiomorphic features. The other is a large, contemporaneous ichthyosaur, Cymbospondylus. Together, they phylogenetically bracket the ichthyopterygian-ichthyosaurian transition. Grippia has a unique microstructure with a distinctive compacted outer layer, whereas Cymbospondylus vertebrae are cancellous throughout, indicating a different ecology and swimming style. The dissimilar distribution of woven-parallel complex in the histology between the two taxa indicates that growth progressed at different speeds. We also document birth lines in ichthyosaurs for the first time. Pelagic, tail-propelled, rapid-growing ichthyosaurs were thus present less than five million years after the End Permian mass extinction, alongside more anguilliform ichthyopterygians. These data capture the ecological and evolutionary transition from reptiles with eel-like swimming to whale-like ichthyosaurs, implying a paradigm shift in ecology and physiology that paved the way for ichthyosaur radiation.
鱼龙是最早完全适应海洋生活的四足动物，进化出了流线型的身体、鳍状肢、胎生以及类似恒温动物的生理特征。然而，向这些适应性特征的转变以及它与向海洋环境分化的关系尚不明确。在这里，我们利用椎骨的微观结构记录了两个早三叠世类群的首个个体发育序列，其中包括最古老的鱼龙胎儿化石。一个序列来自格氏鱼龙，这是一种早期的鱼龙类，身体较小，四肢具有一些原始特征。另一个是同时期的大型鱼龙，杯椎鱼龙。它们在系统发育上界定了鱼龙类向鱼龙的过渡。格氏鱼龙具有独特的微观结构，外层有明显的压实层，而杯椎鱼龙的椎骨则全是松质骨，这表明它们有着不同的生态和游泳方式。两个类群组织学中交织平行复合体的不同分布表明生长速度不同。我们还首次记录了鱼龙的出生线。因此，在二叠纪末大灭绝后不到五百万年，远洋、尾部推进、快速生长的鱼龙就出现了，同时还有更多鳗形的鱼龙类。这些数据捕捉到了从类似鳗鱼游泳的爬行动物到类似鲸鱼的鱼龙的生态和进化转变，意味着生态和生理上的范式转变，为鱼龙的辐射演化铺平了道路。
