Ancestral Body Plan and Adaptive Radiation of Sauropterygian Marine Reptiles

Abstract

SUMMARYMesozoic was the age of reptiles that not only occupied the land and sky but also adapted to the sea. Sauropterygia, from turtle-like placodonts, lizard-like pachypleurosaurs, and predatory nothosaurs to long-necked pistosaurs including plesiosaurs, is one of the most predominant lineages among secondarily aquatic reptiles. However, we know little about the early evolution of sauropterygians involving the morphological gaps among aforementioned sub-groups and their ancestral body-plan, because the Early Triassic fossil remains are scarce shortly after their origin. Here we report a skeleton from the Early Triassic in South China, representing the oldest known complete specimen related to Sauropterygia. It can be referred to Hanosaurus hupehensis and shows a mosaic morphology combining the characters of multiple sauropterygian sub-lineages. We constructed an updated character matrix obtaining the hitherto most comprehensive phylogeny of Triassic sauropterygians, and Hanosaurus hupehensis is stably resolved as the basal-most member of the Sauropterygiformes (clade nov.). Sauropterygians were previously considered using limb propulsion for the movement that is distinguishable from ichthyosaurs and other marine reptiles, but this skeleton reflects an ancestral body plan of sauropterygians unexpectedly developing an elongate trunk and four short limbs, being different from most sauropterygians but more convergently similar to the basal members of other marine reptilian lineages in body shape. After this convergence, we confirm the rapid adaptive radiation of sauropterygiform reptiles following the end-Permian mass extinction. Our results provide an evolutionary framework of sauropterygian marine reptiles and highlight the integrations of both convergences and divergences when an emerging animal lineage arises.

An armored marine reptile from the Early Triassic of South China

“…Pomolispondylus. This result is in contrast to previous studies which recovered these taxa as either the outgroup to Saurosphargidae + Sauropterygia (Hanosaurus; Wang et al 2022), pachypleurosaurs (Rieppel 1998a;Neenan et al 2015;Lin et al 2021) or successive outgroups to a clade comprising Pachypleurosauria + Nothosauroidea to the exclusion of Pistosauroidea (Li and Liu 2020). Suprisingly, the type specimen of Hanosaurus is not recovered in a clade with the referred specimen.…”

“…124.0]). The type (Rieppel 1998a) and referred (Wang et al 2022) Within Diapsida, the clade comprising Helveticosaurus + Sauropterygomorpha was recovered forming a clade with Ichthyosauromorpha, Thalattosauria and Archosauromorpha, a result similar to that recovered in some other recent broad-scale analyses of diapsid phylogenetic interrelationships (Neenan et al 2015;Martínez et al 2021).…”

“…The type (Rieppel 1998a) and referred (Wang et al 2022) specimens of Hanosaurus (Wang et al 2022) and Majiashanosaurus (Jiang et al 2014) are also recovered in the paraphyletic grade leading to Eosauropterygia, but in a position more derived than Pomolispondylus. This result is in contrast to previous studies which recovered these taxa as either the outgroup to Saurosphargidae + Sauropterygia (Hanosaurus; Wang et al 2022), . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.…”

“…The type (Rieppel 1998a) and referred (Wang et al 2022) specimens of Hanosaurus (Wang et al 2022) and Majiashanosaurus (Jiang et al 2014) are also recovered in the paraphyletic grade leading to Eosauropterygia, but in a position more derived than…”

“…Prosaurosphargis represents the stratigraphically oldest occurrence of Saurosphargidae, extending their fossil record back by approximately 4.5 Ma from the Middle (Pelsonian) to the Early (Olenekian) Triassic (Figure 8). Saurosphargids are thus the fourth major marine reptile lineage known from the Early Triassic Nanzhang-Yuan'an fauna, which also includes as many as seven species of hupehsuchians (Chen et al 2015;Qiao et al 2020), one species of ichthyosauriform (Chen et al 2013), and as many as five taxa representing the sauropterygian lineage leading to Eosauropterygia (Young 1965;Rieppel 1998a;Li and Liu 2020;Cheng et al 2022;Wang et al 2022). Measuring approximately 1.5 m in total body length, Prosaurosphargis is one of the larger marine reptiles known from this ecosystem (despite the fact that the holotype likely represents an osteologically immature individual; see above), smaller only than an unnamed eosauropterygian (but possibly representing the same taxon as 'Lariosaurus' sanxiaensis/Hanosaurus hupehensis [referred specimen]) (body length of 3-4 meters; Chen et al 2014bChen et al , 2016 and an indeterminate hupehsuchian (body length of ~2.3 meters; Qiao et al 2020).…”