The oldest record of Saurosphargiformes (Diapsida) from South China could fill an ecological gap in the Early Triassic biotic recovery

Abstract

Diversification following the end-Permian mass extinction marks the initiation of Mesozoic reptile dominance and of modern marine ecosystems, yet major clades are best known from the Middle Triassic suggesting delayed recovery, while Early Triassic localities produce poorly preserved specimens or have restricted diversity. Here we describe Pomolispondylus biani gen. et sp. nov. from the Early Triassic Nanzhang-Yuan’an Fauna of China assigned to Saurosphargiformes tax. nov., a clade known only from the Middle Triassic or later, which includes Saurosphargidae, and likely is the sister taxon to Sauropterygia. Pomolispondylus biani is allied to Saurosphargidae by the extended transverse processes of dorsal vertebrae and a low, table-like dorsal surface on the neural spine; however, it does not have the typical extensive osteoderms. Rather an unusual tuberous texture on the dorsal neural spine and rudimentary ossifications lateral to the gastralia are observed. Discovery of Pomolispondylus biani extends the known range of Saurosphargiformes and increases the taxic and ecological diversity of the Nanzhang-Yuan’an Fauna. Its small size fills a different ecological niche with respect to previously found species, but the overall food web remains notably different in structure to Middle Triassic and later ecosystems, suggesting this fauna represents a transitional stage during recovery rather than its endpoint.

An armored marine reptile from the Early Triassic of South China

“…They regarded the lowermost base of the thick vitric tuff as the boundary of the second and third members. Thus, the typical Nanzhang-Yuan’an fauna was placed into the newly defined second member of the Jialingjiang Formation (Cheng et al 2022; Zhao et al 2022). We argue that this new definition and division of different members of the Jialingjiang Formation contradicts the official geological guide (Bureau of Geology and Mineral Resources of Hubei Province 1990) and causes confusion.…”

“…The copyright holder for this preprint this version posted October 21, 2022. ; https://doi.org/10.1101/2022.10.17.512493 doi: bioRxiv preprint bleekeri (Neenan et al 2013), Paraplacodus broilii (Peyer 1935;Rieppel 2000b), Pomolispondylus biani (Cheng et al 2022) and Prosaurosphargis yingzishanensis gen. et sp.…”

“…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).…”

“…130.1); total number of carpal ossifications more than three(ch. 194.0).Pomolispondylus, recently proposed as a sister-taxon of Saurosphargidae(Cheng et al 2022), was recovered as the most basal member of the grade leading to Eosauropterygia (Pomolispondylus + eosauropterygian lineage supported by a single unambiguous synapomorphy: transverse processes of neural arches of the dorsal region relatively short [ch.124.0]). The type(Rieppel 1998a) and referred(Wang et al 2022) specimens of Hanosaurus were not recovered in a monophyletic group -both specimens were recovered alongside Majiashanosaurus in a polytomy at the base of Eosauropterygia (clade comprising Hanosaurus, Majiashanosaurus and Eosauropterygia supported by a single unambiguous synapomorhpy: osteoderms absent [ch.…”

“…It is possible that the lineage leading to placodonts was represented in the Early Triassic by reptiles morphologically more similar to Atopodentatus than to placodonts and that the specialised placodont body plan did not evolve until the Middle Triassic. New discoveries of Early Triassic sauropterygians are likely to introduce new morphological data needed to test this hypothesis.Pomolispondylus is not recovered as a sister-taxon of Saurosphargidae within Saurosphargiformes (contraCheng et al 2022), but as the most basal member of a grade of sauropterygians leading to Eosauropterygia. Such a phylogenetic position is supported by the presence in Pomolispoindylus of dorsal transverse processes that are relatively short mediolaterally and broad anteroposteriorly, more similar in proportions to the dorsal transverse processes of 'Lariosaurus' sanxiaensis(Li and Liu 2020:fig.…”