What does the rapid rise of ichthyosauromorphs tell us about marine ecosystem recovery in the Triassic?

This blog is contributed together by Yu Qiao, Qiang Li, Andrzej S. Wolniewicz and Jun Liu.
What does the rapid rise of ichthyosauromorphs tell us about marine ecosystem recovery in the Triassic?

Ichthyosaurs were a group of Mesozoic marine reptiles that became perfectly adapted to life in the open ocean and evolved streamlined bodies strikingly similar to those of modern dolphins. Ichthyosaurs and their close relatives – hupehsuchians and nasorostrans (grouped together within Ichthyosauromorpha) – first appeared around 250 million years ago, shortly after the Permian-Triassic Mass Extinction (PTME). Whereas huephsuchians and nasorostrans are known only from the Early Triassic, ichthyosaurs remained important predators in Mesozoic seas until their extinction in the early Late Cretaceous. Because predators are sensitive to environmental stress, understanding the evolutionary dynamics of ichthyosauromorphs can help us illuminate the tempo and mode of marine ecosystem recovery after the PTME. It is currently thought that marine ecosystems did not fully regenerate from the PTME until the Middle Triassic, but fossils of a diverse array of ichthyosauromorphs discovered in South China in the last decade suggest a more rapid ecosystem recovery, indicating complex food webs were already established in the Early Triassic. However, this view is based mostly on studies of ichthyosauromorphs and other marine reptiles from shallow marine environments, but relatively little is known about ecosystem recovery in the open ocean.

Figure 1. Majiashan Quarry near Chaohu, Anhui Province, South China, where numerous skeletons of Early Triassic ichthyosauromorphs (ichthyosaurs and their relatives, including Sclerocormus) were found.

Chaohu in South China is a locality of great importance for our understanding of marine ecosystem recovery in the earliest Triassic, as rocks deposited at the bottom of the sea some 250 million years ago are exposed in its vicinity. The first ichthyosaur from the Chaohu area – Chaohusaurus – was described by the father of Chinese vertebrate paleontology – Zhongjian Yang (C. C. Young) – in the 1970s. Since 2010, full-scale fieldwork organized in Majiashan Quarry near Chaohu (Fig. 1) by Peking University and Anhui Geological Museum has yielded numerous fossils of Chaohusaurus, as well as skeletons of the short-snouted ichthyosauromorphs Cartorhynchus and Sclerocormus, greatly enhancing our knowledge of the origins and early evolution of ichthyosaurs. In 2014, a field project focussing on ichthyosauromorphs from Chaohu was started by our group at Hefei University of Technology (HFUT), with fossil excavations at Majiashan and other quarries continuing until today. After the first two years of fieldwork, dozens of ichthyosaur fossils were collected, all of which belonged to Chaohusaurus, the most abundant ichthyosaur known from the Chaohu area. Meanwhile, Mr Rongao Cao, a passionate local collector who worked with our team on several occasions, found an unusual fossil that was much larger than Chaohusaurus during a visit to Majiashan Quarry in 2016. Realizing the scientific importance of this specimen, Mr. Cao contacted us and generously donated it to the fossil collection of HFUT. The specimen was then examined by Yu Qiao, a graduate student at HFUT, and identified as the ichthyosauromorph Sclerocormus (Fig. 2).

Figure 2. Reconstruction of the Early Triassic omphalosaurid Sclerocormus from Majiashan Quarry (used with permission from author Nikolay Zverkov).

During the preparation of the fossil, a mouth full of rounded teeth with a characteristic ‘orange-peel-like’ enamel surface was revealed. Such teeth were unique among the marine reptiles from the Chaohu fauna and intrigued our team. As preparation work progressed, a nearly perfect three-dimensional skull was uncovered, and CT-scanning revealed numerous replacement teeth hidden in the upper and lower jaws (Fig. 3),  very similar to Omphalosaurus, an enigmatic marine reptile from the Early and Middle Triassic of North America and Europe. Omphalosaurus fossils were known since the early 1900s, but they were represented only by fragmentary fossils, most often jaw fragments with teeth. As a consequence, paleontologists have not been able to determine exactly which group of reptiles Omphalosaurus was most closely related to. Several groups, such as rhynchosaurs, placodonts and ichthyosaurs were proposed as the closest relatives of Omphalosaurus, but its exact position within the reptile evolutionary tree remained unresolved.

Figure 3. Sclerocormus cf. parviceps (referred specimen HFUT MJS-16-012). a, Photograph of the specimen, scale bar = 10 cm. b, Interpretative drawing of the ventral view of the skeleton, scale bar = 10 cm. c, Interpretative drawing of the left lateral view of the skull, scale bar = 10 cm. d, CT reconstruction of the dentition, scale bar = 1 mm. e, Close-up of the enamel surface, scale bar = 2 mm. f, The tip of the first maxillary tooth, scale bar = 2 mm.

With this goal in mind, we launched a collaborative research with University of Bonn vertebrate paleontology group in late 2019, who has studied Omphalosaurus in details before. We compared the new specimen of Sclerocormus with different species of Omphalosaurus and observed several morphological similarities between them. We also performed a phylogenetic analysis that strongly supported a close relationship between Omphalosaurus, Sclerocormus and Cartorhynchus, all recovered within Omphalosauridae. Our new phylogenetic analysis clearly demonstrated that nasorostrans are synonymous with omphalosaurids and that omphalosaurids were ichthyosauromorphs, solving the 100-year-long controversy surrounding their taxonomic placement. Our results also suggested that Omphalosauridae were broadly distributed in the open oceans of the Northern Hemisphere (Europe, North America, and South China) during the Early and Middle Triassic.

To understand the ecology of Omphalosauridae and the function of their short-snouted skulls and rounded teeth, we performed a functional morphospace analysis of lower jaw characters taken from various marine reptiles. We found that omphalosaurids occupied the area of morphospace also occupied by marine turtles, suggesting that their jaws would be capable of processing a wide range of food items like those of pelagic sea turtles. However, the presence of heavily worn and frequently replaced teeth suggests that omphalosaurids might have at least partially fed on hard-shelled pelagic invertebrates, especially ammonoids that were abundant in Early and Middle Triassic oceans. The peculiar feeding ecology of omphalosaurids likely enabled them to evolve rapidly and greatly expand their geographic range following the PTME. Furthermore, the evolution of omphalosaurids seems to have coincided with the fast recovery of ammonoids and conodonts in the Early and Middle Triassic. This hints at an early establishment of complex and functional trophic webs in the pelagic waters of the Triassic. The wide geographic distribution and high ecomorphological diversity of Early and Middle Triassic ichthyosauromorphs underscore their crucial role in both shallow marine and pelagic ecosystem recovery after the PTME.

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