Friday 14 June 2024

A new specimen of the Hupehsuchian Nanchangosaurus throws light on the origin of the earliest Mesozoic Marine Reptiles.

The first groups of Mesozoic Marine Reptiles returned to the seas early in the Triassic, quickly gaining suites of specialist adaptations to pelagic life which have made it hard to determine their closest terrestrial relatives. The most notable of these groups is the Ichthyopterygia (Ichthyosaurs and close relatives), a group which for a long time were considered to be of uncertain affinities, and which today are placed within the Diapsida, but with no widely accepted hypothesis on their relationship to other members of this group. The sister group to the Ichthyopterygia is considered to be the Omphalosauridae, another group of highly modified pelagic Marine Reptiles, with the two groups forming the Ichthyosauriformes.

The Hupehsuchians are a group of small Mesozoic Marine Reptiles known only from the Early Triassic deposits of the Nanzhang-Yuan’an region of Hubei Province, South China. As with other Marine Reptile groups, their affinities were for a long time hard to determine, though in the last decade it has been realised that they are the sister group to the Ichthyosauriforms, with the two groups together being referred to as the Ichthyosauromorphs. Hupehsuchians had elongate bodies with slender heads, dermal plates on their dorsal surfaces, dense ribs and tightly packed gastralia. Importantly, they were fairly small, ranging from 40 cm to about 2.3 m in length, and restricted to shallow-marine environments, suggesting that they may be closer to the terrestrial origins of the Ichthyosauromorphs than other members of the group.

A large number of Hupehsuchian specimens have been found, and the anatomy of the group reasonably well understood. However, almost all of these specimens are preserved in lateral view, with only three specimens known with their skulls preserved in ventral view, which presents challenges when comparing Hupehsuchians to other groups

In a paper published in the journal Historical Biology on 25 May 2024, Jun Liu, Fan Wu, and Yu Qiao of the Division of Geology at Hefei University of Technology describe a new Hupehsuchian specimen from the Early Triassic Jialingjiang Formation in Yuan’an County, in the west of Hubei Province, China, which is preserved in ventral view, and discuss the implications of this for our understanding of the origins of the Hupehsuchians and related groups.

The specimen, HFUT YAV-10-001, is a small, well-preserved Hupehsuchian on a slab and counter-slab in ventral view, and held in the collection of the Geological Museum of Hefei University of Technology. This comprises the skull, a set of 36 articulated vertebrae and associated ribs, and a partial appendicular skeleton, including both pectoral girdle, forelimbs and partial hindlimbs. The majority of the bones are preserved on the main slab, while vertebral and rib fragments and the right pectoral girdle and forelimb elements are preserved on the counter slab. 

The Hupehsuchian HFUT YAV-10-001 from the Early Triassic of Hubei Province, South China. (A) Photograph of the skeleton on the main slab. (B) Photograph of the skeleton on the counter slab. (C) Interpretive drawing of the skeleton on the main slab. (D) Interpretive drawing of the skeleton on the counter slab. Scale bars are 1 cm. Abbreviations: 2 and 4, distal carpals; I, IV and V, metacarpals; ax, axis; axr, axis rib; bo, basioccipital; Cl, clavicle; cna#, cervical neural arch; Co, coracoid; da, dermal armour; dna#, dorsal neural arch; dns#, dorsal neural spine; dnss, dorsal neural spine second (distal) segment; dr#, dorsal rib; F, femur; Fi, fibula; H, humerus; i, intermedium; mand, mandibular ramus; pm, premaxilla; R, radius; r, radiale; Sc, scapula; Ti, tibia; U, ulna; u, ulnare. Jun et al. (2024).

Jun et al. consider HFUT YAV-10-001 to be referable to the genus Nanchangosaurus, having the same number of cervical vertebrae, as well as distinctively low neural spines, as well as lacking a parapophysis, and having subequally-sized scapula and coracoid bones. However, it differs from the previously described Nanchangosaurus suni in a number of features, including having forelimbs which are longer compared to the size of the Animal, and a smaller overall size, with Nanchangosaurus suni reaching about 20 cm in length, while HFUT YAV-10-001, which is clearly an adult, has an estimated size of only 15 cm. This could be a sign that HFUT YAV-10-001 represents a new species of Nanchangosaurus, but it may also indicate that Nanchangosaurus suni was sexually dimorphic, something which has been suggested in early Ichthyosauriforms and Sauropterygians, and which might therefore be predicted in an early Hupehsuchian. For this reason, Jun et al. assign specimen HFUT YAV-10-001 to Nanchangosaurus cf. suni.

The different aspect in which HFUT YAV-10-001 enables significant extra features to be added to a matrix used for the phylogenetic analysis of Diapsidans. Jun et al. recover HFUT YAV-10-001 as a Hupehsuchian, and the Hupehsuchians as the sister group to the Ichthyosauriformes, together  forming the Ichthyosauromorphs, as with previous studies. Their analysis further suggests that the Ichthyosauromorphs form the sister to the Sauropterygomorpha (a diverse group of Mesozoic Marine Reptiles which included groups such as the Nothosaurs and Plesiosaurs), and that this larger grouping forms a sister group to the Thallatosauria, a group of Lizard-like Marine and semi-Marine Reptiles, again restricted to the Triassic.  This grouping of the Ichthyosauromorphs, Sauropterygomorphs, and Thalattosaurs was in turn found to be the sister group to the Archosauromorphs, the group that includes the Archosaurs (Pterosaurs, Crocodilians, and Dinosaurs) plus close outgroups such as the Rhynchosaurs and Tanystrophids.

Simplified phylogeny of Sauria showing the relationships of Ichthyosauromorphs to other Reptiles. Jun et al. (2024).

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