Monday, February 26, 2024

[Paleontology • 2024] Dinocephalosaurus orientalis Li, 2003: A remarkable marine archosauromorph (Diapsida: Archosauromorpha) from the Middle Triassic of southwestern China


Dinocephalosaurus orientalis Li, 2003
Restoration of Dinocephalosaurus orientalis depicted among a shoal of the large, predatory actinopterygian fish, Saurichthys.
in Spiekman, Wang, Zhao, et al., 2024. 
Artwork by Marlene Donnelly.
The non-archosauriform archosauromorph Dinocephalosaurus orientalis was first described from the Upper Member of the Guanling Formation (late Anisian, Middle Triassic) of Guizhou Province by Li in 2003 on the basis of a complete articulated skull and the first three cervical vertebrae exposed in dorsal to right lateral view. Since then, additional specimens have been discovered in southwestern China. Here, five newly discovered specimens are described for the first time, and redescriptions of the holotype IVPP V13767 and another referred specimen, IVPP V13898, are provided. Together, these permit the description of the complete skeleton of this remarkable long-necked marine reptile. The postcranial skeleton is as much as 6 metres long, and characterised by its long tail and even longer neck. The appendicular skeleton exhibits a high degree of skeletal paedomorphosis recalling that of many sauropterygians, but the skull and neck are completely inconsistent with sauropterygian affinities. The palate does not extend back over the basisphenoid region and lacks any development of the closed condition typical of sauropterygians. The arrangement of cranial elements, including the presence of narial fossae, is very similar to that seen in another long-necked archosauromorph, Tanystropheus hydroides, which at least in part represents a convergence related to an aquatic piscivorous lifestyle. The long and low cervical vertebrae support exceptionally elongate cervical ribs that extend across multiple intervertebral joints and contribute to a ‘stiffening bundle of ribs’ extending along the entire ventral side of the neck, as in many other non-crocopodan archosauromorphs. The functional significance of the extraordinarily elongate neck is hard to discern but it presumably played a key role in feeding, and it is probably analogous to the elongate necks seen in pelagic, long-necked plesiosaurs. Dinocephalosaurus orientalis was almost certainly a fully marine reptile and even gave birth at sea.

Keywords: late Anisian, marine reptile, non-archosauriform, southern China,   

Diapsida Osborn, 1903
Archosauromorpha von Huene, 1946
Dinocephalosauridae Spiekman et al. , 2021

Dinocephalosaurus Li, 2003
Type species: Dinocephalosaurus orientalis Li,2003. 

Distribution: Upper Member of the Guanling Formation (late Anisian, Middle Triassic) near Xinmin in Panxian County, southwestern Guizhou Province, and in Luoping County, eastern Yunnan Province, P.R. China.

Dinocephalosaurus orientalis Li, 2003

Type locality: Xinmin, Panxian County, Guizhou Province, southwestern China.

Horizon: Upper Member of Guanling Formation, Pelsonian, Anisian, Middle Triassic.

 Restoration of Dinocephalosaurus orientalis depicted among a shoal of the large, predatory actinopterygian fish, Saurichthys.
Artwork by Marlene Donnelly.

The Middle Triassic (latest Anisian) marine reptile Dinocephalosaurus orientalis is fully described in detail on the basis of seven beautifully preserved specimens from southwestern Guizhou Province, southern China, five of which are presented for the first time. Characters in the skull and neck are consistent with Dinocephalosaurus orientalis being included within Archosauromorpha. With 32, mostly elongate, cervical vertebrae, it had an extraordinarily long neck that draws comparison with the neck of Tanystropheus hydroides, another aquatic non-crocopodan archosauromorph that has been recorded from the Middle Triassic of both Europe and China. Both taxa share several other cranial features, including a fish-trap type dentition, a distinct antorbital recess and a wide palatal ramus of the pterygoid. The phylogenetic placement of Dinocephalosaurus orientalis is hampered by high levels of homoplasy, but our analysis suggests that the similarities between Dinocephalosaurus orientalis and Tanystropheus hydroides are largely convergent. Instead, the results corroborate the presence of a monophyletic Dinocephalosauridae outside Tanystropheidae. A greater expression of paedomorphosis in the appendicular skeleton and the presence of paddle-shaped autopodia in Dinocephalosaurus orientalis also suggest an adaptation to more open waters than in Tanystropheus hydroides. Dinocephalosaurus orientalis and Tanystropheus sp. were not contemporaries in the eastern Tethys based on current fossil occurrences: all finds of Tanystropheus sp. to date being from latest Ladinian or earliest Carnian sequences. The exact function of the extraordinary long neck of Dinocephalosaurus orientalis is unclear but it almost certainly aided in catching fish, which are preserved in the stomach contents of one of the specimens.

Stephan N.F. SPIEKMAN, Wei WANG, Lijun ZHAO, Olivier RIEPPEL, Nicholas C. FRASER and Chun LI. 2024. Dinocephalosaurus orientalis Li, 2003: A remarkable marine archosauromorph from the Middle Triassic of southwestern China. Earth and Environmental Science Transactions of The Royal Society of Edinburgh. First View; 1 - 33. DOI: 10.1017/S175569102400001X