Dinocephalosaurus (meaning "terrible-headed reptile") is a genus of long necked, aquatic protorosaur that inhabited the Triassic seas of China. The genus contains the type and only known species, D. orientalis, which was named by Chun Li in 2003. Unlike other long-necked protorosaurs (which form a group known as the tanystropheids), Dinocephalosaurus convergently evolved a long neck not through elongation of individual neck vertebrae, but through the addition of neck
vertebrae that each had a moderate length. As indicated by phylogenetic analyses, it belonged in a separate lineage that also included at least its closest relative Pectodens, which was named the Dinocephalosauridae in 2021. Like tanystropheids, however, Dinocephalosaurus probably used its long neck to hunt, utilizing the fang-like teeth of its jaws to ensnare prey; proposals that it employed suction feeding have not been universally accepted. It was probably a marine animal by necessity, as suggested by the poorly-ossified and paddle-like limbs which would have prevented it from going ashore.
Specimens belonging to the genus were first discovered in a locality near Xinmin in Guizhou, China in 2002. At the same locality, which dates to 244 million years ago, other marine reptiles such as Mixosaurus, Keichousaurus, and Wumengosaurus have also been found. While the type specimen consisted only of a skull and the very front of the neck, additional specimens soon revealed the complete form of the body. Further discoveries of Dinocephalosaurus specimens were made in Luoping, Yunnan, China, starting in 2008. At this locality, Dinocephalosaurus would have lived alongside Mixosaurus, Dianopachysaurus, and Sinosaurosphargis. One specimen discovered at the Luoping locality preserves an embryo within its abdomen, indicating that Dinocephalosaurus gave birth to live young like many other marine reptiles. Dinocephalosaurus is the only known member of the Archosauromorpha to give live birth, with the possible exception of the metriorhynchids, a group of marine crocodylomorphs.
Discovery and naming
Panxian
thumb|Location of Luoping in Yunnan, China; Panxian is located in the neighboring Guizhou, to the east
The type specimen of Dinocephalosaurus was first discovered in 2002, during fieldwork conducted in Yangjuan Village, Xinmin District, Panxian County, Guizhou, China. It consists of a nearly-complete skull missing the left side of the jaw, as well as several associated cervical vertebrae. It was subsequently stored at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing, China under the collection number IVPP V13767, and a research paper describing the specimen was authored by IVPP paleontologist Chun Li and published by Acta Geologica Sinica in December 2003.
A second specimen discovered at the same locality represents a partially articulated skeleton that is only lacking the tail. Likewise stored at the IVPP, the specimen has the collection number IVPP V13898. The specimen was described in a brief correspondence authored by Li, Olivier Rieppel, and Michael LaBarbera that was published by Science in September 2004; a more detailed description was subsequently published by Rieppel, Li, Nicholas Fraser in a 2008 Journal of Vertebrate Paleontology paper. but has most recently been dated to 244 ± 1.3 million years old based on uranium–lead dating. Predominant deposits at this locality are composed of grey to dark grey marly limestone, as well as cherty limestone containing dolomite and bentonite beds. Dinocephalosaurus was specifically found in layer 90 of the Panxian deposits, a thin limestone layer which is traditionally assigned to the Upper Reptile Horizon (layers 87–90). Further below are the Middle Reptile (layers 81–85) and Lower Reptile (77–79) Horizons.
Luoping
thumb|left|Map showing the location of Panxian and Luoping during the Anisian (a), with stratigraphic position of Luoping Dinocephalosaurus (b)
Subsequently, additional Dinocephalosaurus specimens were discovered from the slightly older Luoping locality, which has been dated to 245–244 million years old based on conodont biostratigraphy as well as preliminary radiometric dating.
Naming
In his 2003 description, Li combined the roots of din- ("terrible"), kephal- ("head"), and saur ("lizard") to create the genus name Dinocephalosaurus. According to Li, this name refers to the "ghastful skull" of the holotype. Meanwhile, the species name is derived from Latin orientalis ("eastern"), in reference to D. orientalis representing the only known record at the time of the Tanystropheidae in what would have been the eastern portion of the Tethys Ocean. The known specimens were probably mature, given that they have fused skull bones and lack the multi-cusped teeth seen in juvenile Tanystropheus. Like Pectodens, the thickness of the premaxilla meant that the nostrils were retracted from the tip of the snout. whereas the neck of Dinocephalosaurus was composed of at least 27 neck vertebrae that were not as elongated. Among the 27 vertebrae of Dinocephalosaurus, the longest was the nineteenth, which measured approximately long. By comparison, the longest vertebrae in a Chinese specimen of Tanystropheus were the ninth and tenth, which measured long. Additionally, the neck vertebrae of Dinocephalosaurus were not hollow, unlike those of Tanystropheus.
Additional features of the neck vertebrae which distinguished Dinocephalosaurus from other protorosaurs included the low and keel-like neural spines with concave top edges, and the front and rear articular surfaces of the vertebrae both being concave (amphicoelous). In the first ten neck vertebrae, the bottom margin was also concave. The long, slender neck ribs bore frontal projections free of the vertebral bodies, which was a rare feature otherwise seen only in Pectodens, Czatkowiella, Sclerostropheus, and Tanytrachelos. Whereas most protorosaurs, such as Tanystropheus, Macrocnemus, and Langobardisaurus, had relatively ossified limbs adapted for terrestrial life, the stout limbs of Dinocephalosaurus were poorly ossified and resembled those of nothosaurs. Out of the carpal bones, only six were ossified; similarly, only three of the tarsal bones were ossified. Additionally, the astragalus and calcaneum also did not articulate with each other in the ankle, instead forming simple and rounded ossifications. On the third digit of the foot of Dinocephalosaurus, there were four phalanges, but none of them appear to have been the terminal claw. This suggests that Dinocephalosaurus had a higher-than-average count of at least five phalanges in the third digit. These characteristics were shared with Tanystropheus, Macrocnemus, and other protorosaurs. but phylogenetic analysis has subsequently verified that they were in fact non-archosaur archosauromorphs. As originally defined, the Protorosauria referred to the group containing Protorosaurus and Prolacerta, and included the "Prolacertiformes" as a subdivision. In 1997, Nour-Eddine Jalil conducted an analysis of the "Prolacertiformes"; this analysis expanded its definition to include 14 genera, including the Tanystropheidae. making the "Prolacertiformes" non-monophyletic (i.e., composed of several groups that were not sister groups). This research also suggested that the remaining protorosaurs – including Protorosaurus, tanystropheids, and drepanosaurs in some studies – did not necessarily form a monophyletic group. An increasing number of analyses have found a non-monophyletic Protorosauria with Protorosaurus as being more basal (less specialised) than the Tanystropheidae, and drepanosaurs outside Archosauromorpha altogether.
In the original description of Dinocephalosaurus, Li recognized the postorbital region and the elongated cervical centra as being indicative of a close relationship with Tanystropheus, which led him to assign it to the Tanystropheidae.
Topology A: Rieppel et al. (2008) In 2020, Tiane De-Oliveira and colleagues added Dinocephalosaurus, Jesairosaurus, and Elessaurus to another dataset published by Adam Pritchard and colleagues in 2018, while also introducing modifications based on Ezcurra and Butler's dataset. Although their phylogenetic analysis recovered a poorly-resolved tree with a large polytomy, they found that Dinocephalosaurus and Jesairosaurus formed a group that was basal to Tanystropheidae and other archosauromorphs.
thumb|The terrestrial Pectodens was the closest relative of Dinocephalosaurus in the Dinocephalosauridae
In 2021, Stephan Spiekman and colleagues introduced yet another dataset specifically to test the phylogenetic relationships of protorosaurs. Different analyses were performed using datasets that incorporated different species and anatomical characteristics. They found that Dinocephalosaurus and Pectodens consistently formed a group, which they named as the Dinocephalosauridae. Some analyses (specifically, those excluding characters based on ratios or with orderings) also found that either Sclerostropheus and "Tanystropheus" antiquus fell inside this group. Either Fuyuansaurus or Jesairosaurus were found to be the closest relative of the dinocephalosaurids in some analyses. With the exception of Jesairosaurus, these genera of uncertain placement were found to be deeply nested within the Tanystropheidae by other analyses. Providing formal support for the original hypothesis of Rieppel and colleagues, Spiekman and colleagues found that placing Dinocephalosaurus as the sister group of Tanystropheus required a tree that was six steps longer, and therefore less likely.
In 2024, Spiekman and colleagues published a redescription of Dinocephalosaurus, including a description of five additional new specimens. Using an updated version of Spiekman's previous dataset, they recovered similar phylogenetic relationships, with Dinocephalosaurus forming a clade with Pectodens outside of the Tanystropheidae. The following cladogram shows the results of their analysis that included ratio and ordered characters, excluding "T. antiquus", T. "conspicuus", and Czatkowiella due to their instability:
Paleobiology
Neck and feeding
thumb|left|Life reconstruction of LPV 30280, shown eating a perleidid fish and bearing an embryo
The long neck of Dinocephalosaurus probably served a functional role. In particular, the length of the neck places a long distance between the head and the remainder of the body. This would have allowed Dinocephalosaurus to approach potential prey without the majority of its bulk being detected, which would have been effective in the murky waters of its habitat. Nevertheless, like other aquatic amniotes, Dinocephalosaurus would have swallowed and digested its prey head-first, as evidenced by the preservation of a perleidid fish in the abdominal region of LPV 30280 from Luoping.
It is not likely that Dinocephalosaurus used its long neck for breathing by extending it vertically. If it tried to do so, the difference in pressure between the surface and its torso would be sufficiently extreme such that its lungs would not have inflated. Thus, in order to breathe, Dinocephalosaurus would need to approach the surface with a nearly horizontal neck. The horizontal posture of the neck would also have facilitated locomotion at the surface, due to its long profile increasing its "hull length" and reducing the effect of resistance from waves. One consequence of these adaptations is that Dinocephalosaurus would have been incapable of building nests on land. This would also have prevented it from possessing hard-shelled reptilian eggs; such eggs necessitate the exchange of gases with the outside environment, and this process is significantly slower in water than it is in air. Thus, Dinocephalosaurus could not have been oviparous. At the same time, however, archosaurs are known for the total absence of viviparous, or live birth, among its living members.
thumb|Aside from Dinocephalosaurus, metriorhynchids such as Metriorhynchus represent the only viviparous archosauromorphs
The embryonic individual of Dinocephalosaurus preserved inside LPV 30280 from Luoping can be identified as such for several reasons. First, it is enclosed entirely within the body cavity of the adult. Its cervical ribs - which are long, like that of the adult - face the same direction as the dorsal vertebrae of the adult, which is in contradiction to the typical head-first method of swallowing prey among amniotes. It is also preserved with its neck curling towards its forelimbs, a posture which is seen among vertebrate embryos but not adult Dinocephalosaurus specimens, which generally have the neck bent upwards. Furthermore, the relative proportions of the humerus and the fibula in the embryo, compared with the maternal individual and IVPP V13898 from Panxian, indicate that the embryo is around 12% of the size of its mother. Combined with its ossified bones, this suggests that the embryo was at an advanced developmental stage, whereas crocodilians, birds, turtles, and tuataras lay eggs at very early developmental stages.
Overall, the evidence provided by the embryo suggests that Dinocephalosaurus was viviparous, making it the first viviparous archosauromorph This is consistent with the separation of its sacral ribs from its sacrum, While the otherwise absence of viviparous archosaurs has been historically attributed to common attributes inherited from the archosaurian stem-lineage, the discovery that Dinocephalosaurus was viviparous suggests that this phenomenon is due to lineage-specific lifestyle restrictions. Although the sex-determination systems among living archosauromorphs are diverse, with crocodilians and turtles using temperature-dependent sex determination, phylogenetic modelling suggests that Dinocephalosaurus retained the basal condition of genotypic sex determination from early diapsids, and that this system facilitated its transition to an obligately marine lifestyle alongside viviparity.
Paleoecology
Until at least the end of the Middle Triassic, high sea levels enabled shallow water to cover much of the South China Block, a tectonic plate that today consists of the stable Yangtze Craton and the less stable South China Fold Belt. A mountain-building event known as the Indosinian orogeny uplifted Precambrian rocks to form four major landmasses on the South China Block: Khamdian to the west, Jiangnan in the centre, Yunkai to the south, and Cathaysia to the east. Island chains also stretched between Yunkai and Cathaysia in the east. Located between Khamdian and Jiangnan was a deep oceanic basin known as the Nanpanjiang Basin. Along the western edge of this basin, fossil-bearing sediments were laid down to become what are now the Lagerstätten (sedimentary deposits characterized by exemplary preservation) of Panxian, Luoping, and other localities. From lower in the Upper Reptile Horizon, fossils have also been found of the ubiquitous mixosaurian ichthyosaur Mixosaurus panxianensis (which occurs in all layers), the pachypleurosaurs Keichousaurus sp. and Wumengosaurus delicatomandibularis, and fish, although fossils of the latter are fragmentary. The lower Middle and Lower Reptile Horizons also include the mixosaurian Phalarodon cf. fraasi, the primitive ichthyosaur Xinminosaurus catactes, the placodont Placodus inexpectatus, the nothosaurs Lariosaurus hongguoensis and Nothosaurus yangjuanensis, and the archosaur Qianosuchus mixtus, alongside bivalves and saurichthyid fish.
The fauna of Luoping appears to have been preserved in a small intraplatform basin instead of the surrounding open water, judging by the anoxic sediments present at the site. Otherwise, fish including saurichthyids, palaeoniscids, birgeriids, perleidids, eugnathids, semionotids, pholidopleurids, peltopleurids, and coelacanths have been found at Luoping, forming 3.66% of fossils with 25 taxa in 9 families. Molluscs such as bivalves and gastropods, along with ammonoids and belemnoids, account for 1.69% of specimens. Rare and probably non-endemic fossils also include echinoderms such as crinoids, starfish, and sea urchins; branchiopods; and relatively complete conifer branches and leaves, which probably originated from coastal forests less than away.