Archosauromorphs first appeared in the Permian, and became the dominant Vertebrate group in terrestrial ecosystems during the Mesozoic. While Archosauriform groups such as the Pseudosuchians (Crocodile-lineage Archosaurs) and Avemetatarsalians (Pterosaurs and Dinosaurs) became the dominat groups of Archosaurs from the Jurassic onwards, a variety of non-Archosauriform Archosauromorphs, such as the Rhynchosauria, Allokotosauria, and Tanystropheidae played an important role in Triassic ecosystems. The Tanystropheidae are one of a number of groups of highly gracile (slender) Triassic Archosauromorphs, once thought to represent a single clade, but now thought to be a loose group of basal Archosauromorphs, refered to as non-Crocopodans, where the Crocopodans comprise the Rhynchosauria, Allokotosauria, and Archosauriforms.
These gracile non-Crocopodans were a diverse group, inhabiting terrestrial, freshwater, and marine environments, and in one case (Ozimek volans), possibly taking to the air. However, their delicate skeletons did not favour preservation, and many species are known only from compressed and fragmentary remains. One such species is Dinocephalosaurus orientalis, a non-Crocopodan Archosauromorph from the Middle Triassic Guanling Formation of Guizhou Province, China, which was described in 2003 from a specimen comprising an isolated, well-preserved skull and the first three anterior cervical vertebrae preserved in articulation. Subsequent specimens revealed that Dinocephalosaurus orientalis was of similar size and proportions to the better known Tanystropheus longobardicus, but while both species have extremely elongated necks, exceeding the lengths of their trunks, the neck of Tanystropheus longobardicus has 13 elongated cervical vertebrae, whereas Dinocephalosaurus orientalis has at least 32 vertebrae in its neck. Furthermore, a specimen which cannot confidently be assigned to the species, but which must clearly be closely related, was found to be gravid, with an embryo (not an egg) within its abdominal cavity, indicating the birth of live young, almost certainly an adaptation to a fully marine lifestyle.
In a paper published in journal Earth and Environmental Science Transactions of the Royal Society of Edinburgh on 23 February 2024, Stephan Spiekman of the Staatliches Museum für Naturkunde Stuttgart, Wei Wang of the Institute of Vertebrate Palaeontology and Palaeoanthropology of the Chinese Academy of Sciences, Lijun Zhao of the Zhejiang Museum of Natural History, Oliver Rieppel of the Field Museum of Natural History, Nicholas Frazer of National Museums Scotland, and Chun Li, also of the Institute of Vertebrate Palaeontology and Palaeoanthropology of the Chinese Academy of Sciences, describe five new specimens of Dinocephalosaurus orientalis, and discuss their implications for our understanding of the species.
The first additional specimen described by Spiekman et al., IVPP V13898, has previously been described in 2008, and is the specimen upon which most of our current understanding of the species is based. This specimen comprises the skull, most of the vertebral column, elements of all four limbs and some elements of the pectoral and pelvic girdle. This specimen is more complete than the holotype (first specimen described), however, the skull is relatively poorly preserved, strongly dorsoventrally compressed and exposed in ventral view.
The second next specimen, IVPP V17977, is described for the first time, as are all subsequent specimens. This specimen comprises the skull and mandible preserved in ventral view on one block, followed by the first to the 16th cervical vertebrae, together with associated cervical ribs, preserved in articulation on three additional slabs.
The second new specimen, ZMNH M8727, comprises elements of an exploded skull, a total of 28 cervical vertebrae of which the axis and the following 17 cervical vertebrae are preserved in articulation; two scattered vertebrae located next to the 28th cervical vertebra, one of them exposed in anterior view, a string of six vertebrae lying in front of the remains of the pectoral girdle and forelimbs, plus associated cervical, dorsal and gastral ribs.
The third new specimen, ZMNH M8728, comprises a partially preserved skull exposed in right lateral view, plus the cervical and most of the dorsal vertebral column represented by 56 vertebrae preserved in articulation (i.e. 32 cervical vertebrae and 23 dorsal vertebrae), cervical and dorsal ribs; elements of both pectoral girdles and forelimbs; and the gastral rib basket.
Dinocephalosaurus orientalis, ZMNHM8728 (referred specimen). (a) Photograph. (b) Interpretative drawing. Abbreviations: cl, clavicle; co, coracoid; co-sc, articulated coracoid and scapula; cr, cervical rib; cv, cervical vertebra; hu, humerus; ns, neural spine; ra, radius; ul, ulna. Spiekman et al. (2024).The next new specimen, ZMNH M8752, comprises the skull and mandible, neck, trunk and complete tail, ribs, gastral ribs, pectoral girdle and forelimbs as well as pelvic girdle and hindlimbs, making it one of the most complete specimens known.
The final new specimen, IVPP V20295, however, is the most complete and fully articulated specimen recovered to date, comprising a skull is preserved in dorsal view and is in complete articulation with the neck and the rest of the body, which is essentially exposed on its left side. The total vertebral count is 145, comprising 32 cervical vertebrae, 30 dorsal vertebrae, 2 sacral vertebrae and 81 caudal vertebrae.
Based upon this new material, Spiekman et al. re-interpret Dinocephalosaurus orientalis as a large non-Crocopodan Archosauromorph, reaching as much as 6 m in length, with a neck twice as long as its trunk. It's post-orbital skull is short, and the suborbital fenestra has been obliterated. There is a single fang on each pre-maxillary, as well as further fangs on the front part of the maxilla and dentary. There are 62 pre-sacral vertebrae, of which 32 are cervical, as well as two sacral vertebrae and 81 caudal vertebrae. The limbs are reduced, apparently due to paedomorphosis (the retention of juvenile traits in adults), with a lack of suturing in several places in clearly adult specimens, and a reduction in the number of phalanges.
Spiekman et al. also note that a specimen of an aparently gravid Dinocephalosaurus has been described from a different location, Luoping County in Yunnan Province. This specimen is incomplete making confident assignment to the same species difficult, but has no traits which would justify erecting a second species. Notably, although presumably an adult, this specimen is only about half the size of the largest specimens from the Guanling Formation specimens, suggesting that if it does belong to the same species, then that species must have shown strong sexual dimorphism.
Specimens of Dinocephalosaurus orientalis are quite abundant in the Guanling Formation, a coastal deposit laid down in the eastern Tethys Ocean, but no similar specimens have to date been found in deposits from the western Tethys, which outcrop in Europe and the Middle East, despite many of these sites having been sampled extensively for over three centuries. However, based upon their new material, Spiekman et al. not that two maxillae from the Lower Muschelkalk of Krapkowice and Gogolin in Upper Silesia, Poland, and a dentary from the Lower Muschelkalk of Winterswijk in the Netherlands, currently assigned to 'Lamprosauroides goepperti', show strong similarities to Dinocephalosaurus orientalis, although the extremely fragmentary nature of this European material prevents a proper re-evaluation at this time.
Extremely elongate necks appear to have been achieved at least twice in non-Crocopodan Archosauromorphs, with Dinocephalosaurus orientalis increasing the number of vertebrae in its neck, while Tanystropheus spp. increased the length of the individual cervical vertebrae. While this might seem likely to have given Dinocephalosaurus orientalis a more flexible neck than Tanystropheus spp., both taxa had elongated cervical ribs which would have served to stiffen the neck, something widespread in non-Crocopodan Archosauromorphs.
It is possible thar the arrangement of cervical ribs and an elongate neck would have facilitated suction feeding, however, both taxa have 'fish-trap' dentition, suggesting that this is unlikely, as the long teeth would tend to prevent prey being drawn into the buccal cavity. Instead Spiekman et al. theorise that both taxa probably caught prey with a lateral snapping motion of the head, something which has also been proposed for piscivorous Triassic Sauropterygians.
The largest specimens of Dinocephalosaurus orientalis are considered to be adults with some confidence, showing full fusion of the bones of the skull and trunk region. Notably, a specimen from the Triassic of Yunnan, which appears very similar to Dinocephalosaurus orientalis, but is not complete enough to confidently assign to the species, is gravid, suggesting either that Dinocephalosaurus orientalis was sexually dimorphic, or that a second, much smaller species of Dinocephalosaurus was also present. The limbs of Dinocephalosaurus orientalis are underdeveloped, with little development of articular surfaces and poorly ossified carpus and tarsus, even compared to the functionally similar Tanystropheus spp., suggesting that Dinocephalosaurus orientalis had a greater degree of adaption to a marine environment, and was probably not capable of leaving the water and crawling onto land.
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