page_title: Hupehsuchia
section_title: Taxonomy and phylogenetics
hierarchical_section_title: Hupehsuchia / Taxonomy and phylogenetics
caption_attribution_description: Skeleton of Nanchangosaurus suni Wang, 1959, holotype (GMC V636). Scale bar is 1 cm.
context_page_description: Hupehsuchia is an order of diapsid reptiles closely related to ichthyosaurs. The group was short-lasting, with a temporal range restricted to the late Olenekian age, spanning only a few million years of the Early Triassic. The order gets its name from Hubei Province, China, from which many specimens have been found. They are probable members of the newly defined clade Ichthyosauromorpha.
context_section_description: Hupehsuchia was first defined as a suborder of "Thecodontia" in 1972, when it was first thought to be a group of early archosaurs. It was also thought to be related to several other groups of Triassic reptiles previously thought to be clearly distinct. The presence of the supposed antorbital fenestra described above was seen as evidence for grouping hupesuchians within Archosauria, but the antorbital fenestra characteristic of archosaurs is surrounded by the maxilla and lacrimal, not the nasal and prefrontal bones. The dermal plates were also seen as evidence for terrestrial archosaur ancestors, and comparisons were made with some early armored forms. However, a later study in 1976 could find no similarities between the dermal plates of hupehsuchians and the armor of early archosaurs. The ancestors of hupehsuchians more likely were earlier, more basal terrestrial diapsids, as suggested by several synapomorphies they share with such primitive ancestors. In 1991, Hupehsuchia was recognized as a distinct order after better methods of specimen preparation allowed new features to be revealed that distinguished Hupehsuchia from all other diapsid orders.
  The assumed position of the naris in hupehsuchians as explained above can be taken as evidence for possible ichthyosaurian affinities, as it is in the same general area as those of ichthyosaurs. Hupehsuchians do resemble earlier ichthyosaurs in outward appearance with slightly fusiform bodies and long, straight, non-lunate tails. Other features shared with ichthyosaurs include a supraoccipital similar to what is seen in early forms, a relatively long antorbital region, and a short transverse process for the ribs. Many more differences exist between hupehsuchians and ichthyosaurs, however. In hupehsuchians, the surfaces of the vertebral centra that articulate with one another are distinctively flat, or acelous, while in ichthyosaurs they are noticeably heterocelous (it is also important to note that the surfaces of centra in the possible early diapsid ancestors of hupehsuchians were amphicelous). However, there are some early ichthyosaurs and ichthyosaur relatives such as Chaohusaurus and Utatsusaurus that possess vertebrae that are not deeply heterocoelus and more closely resemble those of hupehsuchians. Unlike more derived ichthyosaurs, the centra of these two genera are about as long as they are high. In hupehsuchians, where the neural arches dominate the vertebral column, the height of the centra is reduced and the height to length ratio of the centra is smaller, meaning that they are also about as long as they are high. This may suggest that hupehsuchians may have evolved from an ichthyosaur relative. It has also been suggested that hupehsuchians were related to or members of Sauropterygia. Indeed, Nanchangosaurus was classified as a sauropterygian upon its initial description.
  Many of the features seen in known hupehsuchian specimens that are comparable to those of more well known diapsids may not necessarily be evidence of ancestry or relationship. The lower jaw and rostrum of hupehsuchians have been compared to many other secondarily aquatic tetrapods such as plesiosaurs, whales, and the early marine bird Hesperornis, all of which have developed a similar morphology independently in response to the need for adaptation to a marine environment.   
   
  Classification of Hupehsuchia remains difficult because most of the derived characters exhibited in the clade that can be helpful in phylogenetic analyses are also present in other unrelated groups of secondarily aquatic reptiles, and the overall record of diapsids during the Late Permian-Early Triassic is relatively poor, making it difficult to find any closely related or ancestral taxa. Even higher level classification is difficult because many of the plesiomorphies that characterize such groups are absent in the highly derived, marine adapted hupehsuchians. For example, several characteristics suggest that Hupehsuchia belongs within Neodiapsida, but most of the derived characteristics that define the clade are absent or hard
caption: Nanchangosaurus holotype
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