An armoured marine reptile from the Early Triassic of South China and its phylogenetic and evolutionary implications
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eLife assessment
This paper describes an important new marine reptile specimen. A solid ostelogical description of the saurosphargid Prosaurosphargis yingzishanensis, the earliest known member of this group, combined with a large-scale phylogenetic analysis enhances our understanding of the affinities of a wide range of Triassic reptiles. As such the relevance of this paper goes far beyond the immediate importance of this remarkable fossil - it also sheds light on the position of several important Triassic groups, including Testudinata and Archosauromorpha.
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Abstract
Sauropterygia was a taxonomically and ecomorphologically diverse clade of Mesozoic marine reptiles spanning the Early Triassic to the Late Cretaceous. Sauropterygians are traditionally divided into two groups representing two markedly different body plans – the short-necked, durophagous Placodontia and the long-necked Eosauropterygia – whereas Saurosphargidae, a small clade of armoured marine reptiles, is generally considered as the sauropterygian sister-group. However, the early evolutionary history of sauropterygians and their phylogenetic relationships with other groups within Diapsida are still incompletely understood. Here, we report a new saurosphargid from the Early Triassic (Olenekian) of South China – Prosaurosphargis yingzishanensis gen. et sp. nov. – representing the earliest known occurrence of the clade. An updated phylogenetic analysis focussing on the interrelationships among diapsid reptiles recovers saurosphargids as nested within sauropterygians, forming a clade with eosauropterygians to the exclusion of placodonts. Furthermore, a clade comprising Eusaurosphargis and Palatodonta is recovered as the sauropterygian sister-group within Sauropterygomorpha tax. nov. The phylogenetic position of several Early and Middle Triassic sauropterygians of previously uncertain phylogenetic affinity, such as Atopodentatus , Hanosaurus , Majiashanosaurus, and Corosaurus , is also clarified, elucidating the early evolutionary assembly of the sauropterygian body plan. Finally, our phylogenetic analysis supports the placement of Testudines and Archosauromorpha within Archelosauria, a result strongly corroborated by molecular data, but only recently recovered in a phylogenetic analysis using a morphology-only dataset. Our study provides evidence for the rapid diversification of sauropterygians in the aftermath of the Permo-Triassic mass extinction event and emphasises the importance of broad taxonomic sampling in reconstructing phylogenetic relationships among extinct taxa.
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eLife assessment
This paper describes an important new marine reptile specimen. A solid ostelogical description of the saurosphargid Prosaurosphargis yingzishanensis, the earliest known member of this group, combined with a large-scale phylogenetic analysis enhances our understanding of the affinities of a wide range of Triassic reptiles. As such the relevance of this paper goes far beyond the immediate importance of this remarkable fossil - it also sheds light on the position of several important Triassic groups, including Testudinata and Archosauromorpha.
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Reviewer #1 (Public Review):
The paper is based around one very nice new marine reptile fossil from South China, but the authors make an excellent case in their Introduction that this can shed light on a wide range of fundamental phylogenetic problems around a whole array of Early and Middle Triassic marine reptiles. The description of the fossil is detailed and thorough and makes constant reference to comparative material of other taxa of saurosphargids. The phylogenetic analysis smartly adds some Triassic turtles and some other Early Triassic marine reptiles to a published cladistic data matrix and then can provide some really significant phylogenetic conclusions around Sauropterygia origins and Archelosauria.
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Reviewer #2 (Public Review):
The study describes and names a new marine reptile taxon on the basis of an incomplete postcranial skeleton from the early Triassic of China. The morphologial description and comparison is well concucted/informative and very detailed. The paper and results (phylo. analyseis and hypothesis on ancestral body shape) of Wang et al. 2022 should be discussed in more detail.
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