Hupehsuchia
Encyclopedia
Hupehsuchia is an order
of diapsid
reptiles
, of which Nanchangosauridae is the only family
. The order was short-lasting, with a temporal range restricted to the Spathian age of the late Olenekian
, 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.
thought to form most of the dorsal and lateral surface while the maxilla
is mostly restricted to the ventral surface beyond the base of the rostrum. An opening between the nasal
and the prefrontal
in one hupehsuchian specimen (known as IVPP V3232) was initially interpreted as an antorbital fenestra
but is now thought to be an artifact caused by the damage of the surrounding bones during preservation. Its position is not indicative of a narial opening either. More likely the naris lies between the nasal and the maxilla in an area anterior to that of the supposed antorbital fenestra, although the preservation of this area in known specimens is too poor to prove definitively that it is the external naris and not an artifact of preservation as is the case for the fenestra.
The neck is relatively elongated and the cervical ribs are short. An unusual feature of the neural spines of the trunk region, from the 11 vertebra to the first caudal, is that each is divided into two distinct units by a suture line. One is proximinal to the rest of the vertebra and the other is distal to it. There is some anteroposterior displacement of these two units along the vertebral column, suggesting that they are not ossified to one another. In vertebrae 8-14, the distal ends of the distal portions of the neural spines are expanded posteriorly. An additional anterior expansion of the neural spine is seen in all vertebrae after the 14th. These expanded distal regions exhibit some sculpturing, and may have penetrated the dermis
. Another unusual characteristic of hupehsuchians is the presence of dermal plates over the neural spines of the approximately 34 presacral vertebrae. A small dermal bone overlies each space between the posterior expansion of one distal neural spine and the anterior expansion of the one behind it. Above these dermal bones lie even larger dermal plates that directly overly even numbered neural spines.
Gastralia are present in some specimens that form a type of ventral armor from the pectoral to the pelvic girdle. The medial row cosists of large, overlapping, V-shaped elements, and lateral rows consist of smaller, cylindrical bones that are widely spaced.
stage of the Middle Triassic
but it is now thought to have formed slightly earlier during the late Olenekian stage of the Early Triassic. The first to be described was Nanchangosaurus
, named in 1959. Fossil
s of the genus have been found from the Daye Limestone in the Hsunjian District of Hubei Province from a single specimen. Another genus, Hupehsuchus
, was named in 1972 from a locality in the Xunjian Commune of Nanzhang County
. The two genera do not differ greatly, and were probably quite similar in appearance. Nanchangosaurus possesses a frontal
that participates in the orbital margin and is quite long, more similar to what is seen in basal diapsids. Additionally, the dorsal plates lack sculpturing. In Hupehsuchus, the zygapophyses of the trunk region are more modified, while those of Nanchangosaurus resemble those of more primitive terrestrial reptiles. Overall, Nanchangosaurus was smaller in size than Hupehsuchus, and although several characteristics of the skeleton single representative specimen of the former genus suggest it belonged to a juvenile individual, other differences between the two genera are clearly non-ontogeneic and dispel the possibility that the single specimen of Nanchangosaurus could be from an immature Hupehsuchus. Different means of preservation can also be taken as evidence for different body forms in the two genera. It is important to note that all specimens of Hupehsuchus are preserved in lateral view while the single specimen of Nanchangosaurus is preserved in dorsal view (except for the neural spines, which were probably too long to be preserved in this manner). It is possible that Hupehsuchus had a more laterally compressed body than Nanchangosaurus, which, as a result, would favor preservation in lateral view.
configuration differing greatly. The overall form of the limbs is more paddle-like than what is seen in Hupehsuchus, although the phalanges still retain a cylindrical configuration. Additional differences include shorter neural spines and neural and haemal arches in the dorsal and caudal vertebrae in V4070, although the cervical neural spines seem to be longer than those of Hupehsuchus. The dermal plates seem to extend down to the caudal vertebrae, unlike specimens of Hupehsuchus in which the dermal plates are absent from the caudal vertebrae. It is possible that caudal dermal plates were present in Hupehsuchus but have been lost during preservation.
. It is most notable for exhibiting polydactyly
, in which there more than the usual maximum of five digits per limb as seen in most advanced tetrapod
s. Polydactyly is also seen in ichthyosaurs. However, in ichthyosaurs, this condition occurs as either bilateral polydactyly in the case of ophthalmosaurids
(extra digits anterior to digit I and posterior to digit V) or interdigital or postaxial phalangeal bifurcation as in non-ophthalmosaurids. Preaxial polydactyly occurs in SSTM 5025, where extra digits only develop anteriorly to digit I. This condition is seen in earlier stem tetrapods from the Devonian
period such as Ichthyostega
and Acanthostega
. SSTM 5025 possessed seven digits on the forelimbs and six on the hindlimbs. The wide manus and pes of the specimen resemble the limb-like fins of extant frogfish
es.
a" in 1972 when it was first thought to be a group of early archosaur
s. 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. 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. It is more likely that the ancestors of hupehsuchians 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). Yet there were some early ichthyosaurs and ichthyosaur relatives such as Chaohusaurus
and Utatsusaurus
that possessed vertebrae that were not deeply heterocoelus and more closely resembled 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 plesiosaur
s, whale
s, 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 to distinguish in hupehsuchians, even though their ancestors may have possessed these characteristics at one point. Three derived characteristics of Neodiapsida are a reduced number of teeth on the pterygoid, an absence of teeth on the parasphenoid, and a lack of caniniform mixillary teeth, but none of these apply to hupehsuchians because they lack teeth altogether. Many derived characteristics that define neodiapsids regard the limbs and girdles, but these characteristics are not seen in marine reptiles because the limbs and girdles are too highly modified. Many characteristics of the skull cannot be seen in hupehsuchians due to poor preservation of remains. Therefore, any placement of Hupehsuchia within Neodiapsida remains tentative until more specimens are found.
Due to the great number of derived characteristics in hupehsuchians that are a result of convergence, computational phylogenetic analyses
using computer programs based on the method of maximum parsimony
do not produce cladogram
s that can accurately establish a relationship between Hupehsuchians and other diapsids. Such programs do not recognize situations where sister group relations are not justified.
The presence of polydactyly in SSTM 5025 may have been an adaptation to moving across underwater substrates in a similar manner to some early tetrapods of the Devonian such as Acanthostega.
The flat, toothless rostrum may have supported an avian-like bill, or perhaps rows of baleen
as seen in cetaceans of the suborder Mysticeti. It seems that hupehsuchians were adapted to continuous ram feeding, a form of mobile suspension feeding. The large skull and lack of a fixed symphysis with the lower jaw is indicative of such a feeding method, but the narrowness of the rostrum and the existence of marine vertebrates with larger skulls that were not completely edentulous (e. g. ichthyosaurs) seem to point against it. The long necks of hupehsuchians would seem to inhibit continuous ram feeding at high speeds through the water, so it is more likely that they practiced intermittent ram feeding at slower speeds.
The purpose of the dorsal dermal plates of hupehsuchians is unknown. These plates as well as the ventral gastralia and the tendency for pachyostosis
would have added considerable weight to hupehsuchians, allowing them to have neutral buoyancy
. However, the position of the dorsal plates are high above the center of gravity, and it seems that this would have made the bodies of hupehsuchians unstable. Hupehsuchians were among the largest marine animals of their time, so there would be no need for dorsal plates as a protective measure. The tendency for the dorsal plates to be more developed anteriorly may have allowed the anterior portion of the vertebral column to remain relatively rigid while the posterior portion could freely undulate.
It is unsure whether the plates were acquired subsequent to an adaptation to the marine environment or were inherited from a terrestrial ancestor, in which such plates would have given rigidity to the spine and protection from predators. In any case, if Nanchangosaurus is seen as representative of an ancestral morphological pattern that led to the more derived Hupehsuchus, then the plates seem to have experienced further development in hupehsuchians and must have had some advantage.
The general shape of hupehsuchians seems to have made any locomotion on land nearly impossible, and, although there is no direct evidence seen in known specimens, these animals were probably viviparous, giving live birth at sea rather than laying eggs on land. Vivipary is also seen in ichthyosaurs.
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
of diapsid
Diapsid
Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...
reptiles
Sauropsida
Sauropsida is a group of amniotes that includes all existing reptiles and birds and their fossil ancestors, including the dinosaurs, the immediate ancestors of birds...
, of which Nanchangosauridae is the only family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
. The order was short-lasting, with a temporal range restricted to the Spathian age of the late Olenekian
Olenekian
In the geologic timescale, the Olenekian is an age in the Early Triassic epoch or a stage in the Lower Triassic series. It spans the time between 249.7 ± 0.7 Ma and 245 ± 0.7 Ma . The Olenekian follows the Induan and is followed by the Anisian.The Olenekian saw the deposition of a large part of the...
, spanning only a few million years of the Early Triassic
Early Triassic
The Early Triassic is the first of three epochs of the Triassic period of the geologic timescale. It spans the time between 251 ± 0.4 Ma and 245 ± 1.5 Ma . Rocks from this epoch are collectively known as the Lower Triassic, which is a unit in chronostratigraphy...
. The order gets its name from Hubei Province
Hubei
' Hupeh) is a province in Central China. The name of the province means "north of the lake", referring to its position north of Lake Dongting...
, China, from which many specimens have been found.
Description
Hupehsuchians display an unusual combination of characteristics. The overall shape of the body is fusiform, with a long tail and large, paddle like limbs. The skull is elongate and the jaws are edentulous. The rostrum is flatteded with the premaxillaPremaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
thought to form most of the dorsal and lateral surface while the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
is mostly restricted to the ventral surface beyond the base of the rostrum. An opening between the nasal
Nasal bone
The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face, and form, by their junction, "the bridge" of the nose.Each has two surfaces and four borders....
and the prefrontal
Prefrontal bone
The prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls. It first evolved in the sarcopterygian clade Rhipidistia, which includes lungfish and the Tetrapodomorpha. The prefrontal is found in most modern and extinct lungfish, amphibians and reptiles...
in one hupehsuchian specimen (known as IVPP V3232) was initially interpreted as an antorbital fenestra
Antorbital fenestra
An antorbital fenestra is an opening in the skull, in front of the eye sockets. This skull formation first appeared in archosaurs during the Triassic Period. Living birds today possess antorbital fenestrae, but the feature has been lost in modern crocodilians...
but is now thought to be an artifact caused by the damage of the surrounding bones during preservation. Its position is not indicative of a narial opening either. More likely the naris lies between the nasal and the maxilla in an area anterior to that of the supposed antorbital fenestra, although the preservation of this area in known specimens is too poor to prove definitively that it is the external naris and not an artifact of preservation as is the case for the fenestra.
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...
. Another unusual characteristic of hupehsuchians is the presence of dermal plates over the neural spines of the approximately 34 presacral vertebrae. A small dermal bone overlies each space between the posterior expansion of one distal neural spine and the anterior expansion of the one behind it. Above these dermal bones lie even larger dermal plates that directly overly even numbered neural spines.
Gastralia are present in some specimens that form a type of ventral armor from the pectoral to the pelvic girdle. The medial row cosists of large, overlapping, V-shaped elements, and lateral rows consist of smaller, cylindrical bones that are widely spaced.
Genera
Only two named genera are known to belong to the order, and both are members of the same family, Nanchangosauridae. They are likely sister taxa. Both have been found from the Jialingjiang Formation in Hubei Province, China. Originally this formation was dated to the AnisianAnisian
In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from 245 million years ago until 237 million years ago, approximately...
stage of the Middle Triassic
Middle Triassic
In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided. It spans the time between 245 ± 1.5 Ma and 228 ± 2 Ma...
but it is now thought to have formed slightly earlier during the late Olenekian stage of the Early Triassic. The first to be described was Nanchangosaurus
Nanchangosaurus
Nanchangosaurus was a large aquatic reptile native to the middle Triassic of China. It was named after the area in China, Nanchang, where it was found. It was about three feet in length, and probably fed on fish or used its long jaws to probe for aquatic invertebrates...
, named in 1959. Fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s of the genus have been found from the Daye Limestone in the Hsunjian District of Hubei Province from a single specimen. Another genus, Hupehsuchus
Hupehsuchus
Hupehsuchus was a small genus of marine reptile, about 1 meter long. It was found in the area of Hubei in China. This marine reptile lived in the Olenekian stage of the Early Triassic period. Hupehsuchus was similar to its close cousin, Nanchangosaurus, but differed from it in a number of ways...
, was named in 1972 from a locality in the Xunjian Commune of Nanzhang County
Nanzhang County
Nanzhang County is a county of Hubei, China. It is under the administration of Xiangfan city....
. The two genera do not differ greatly, and were probably quite similar in appearance. Nanchangosaurus possesses a frontal
Frontal bone
The frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....
that participates in the orbital margin and is quite long, more similar to what is seen in basal diapsids. Additionally, the dorsal plates lack sculpturing. In Hupehsuchus, the zygapophyses of the trunk region are more modified, while those of Nanchangosaurus resemble those of more primitive terrestrial reptiles. Overall, Nanchangosaurus was smaller in size than Hupehsuchus, and although several characteristics of the skeleton single representative specimen of the former genus suggest it belonged to a juvenile individual, other differences between the two genera are clearly non-ontogeneic and dispel the possibility that the single specimen of Nanchangosaurus could be from an immature Hupehsuchus. Different means of preservation can also be taken as evidence for different body forms in the two genera. It is important to note that all specimens of Hupehsuchus are preserved in lateral view while the single specimen of Nanchangosaurus is preserved in dorsal view (except for the neural spines, which were probably too long to be preserved in this manner). It is possible that Hupehsuchus had a more laterally compressed body than Nanchangosaurus, which, as a result, would favor preservation in lateral view.
V4070
One specimen, called V4070, has been found from the same locality as Hupehsuchus and is representative of a different, unnamed genus. The pectoral girdle, neck, and base of the skull are present but are all badly weathered, with impressions present in the underlying matrix of the area that provide little more detail. An impression has been made by much of the postcranial skeleton but that too has experienced much weathering. V4070 is proportionally quite similar to Hupehsuchus, with the tail being around 93% the length of that of Hupehsuchus, but studies of the fore and hindlimbs reveal many differences between the two genera, with carpal and tarsalTarsal
Tarsal could refer to:*tarsus *tarsus *superior tarsal muscle...
configuration differing greatly. The overall form of the limbs is more paddle-like than what is seen in Hupehsuchus, although the phalanges still retain a cylindrical configuration. Additional differences include shorter neural spines and neural and haemal arches in the dorsal and caudal vertebrae in V4070, although the cervical neural spines seem to be longer than those of Hupehsuchus. The dermal plates seem to extend down to the caudal vertebrae, unlike specimens of Hupehsuchus in which the dermal plates are absent from the caudal vertebrae. It is possible that caudal dermal plates were present in Hupehsuchus but have been lost during preservation.
SSTM 5025
In late 2003, a new specimen of hupehsuchian called SSTM 5025, found from the same area as Hupehsuchus and V4070, was briefly mentioned in the journal NatureNature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
. It is most notable for exhibiting polydactyly
Polydactyly
Polydactyly or polydactylism , also known as hyperdactyly, is a congenital physical anomaly in humans, dogs, and cats having supernumerary fingers or toes....
, in which there more than the usual maximum of five digits per limb as seen in most advanced tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
s. Polydactyly is also seen in ichthyosaurs. However, in ichthyosaurs, this condition occurs as either bilateral polydactyly in the case of ophthalmosaurids
Ophthalmosauridae
Ophthalmosauridae is an extinct family of thunnosaur ichthyosaurs from the Middle Jurassic to the early Late Cretaceous of Asia, Australia, Europe, North America and South America. Currently, the oldest known ophthalmosaurid is Mollesaurus from the early Bajocian of Argentina. Named by George H...
(extra digits anterior to digit I and posterior to digit V) or interdigital or postaxial phalangeal bifurcation as in non-ophthalmosaurids. Preaxial polydactyly occurs in SSTM 5025, where extra digits only develop anteriorly to digit I. This condition is seen in earlier stem tetrapods from the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period such as Ichthyostega
Ichthyostega
Ichthyostega is an early tetrapod genus that lived at the end of the Upper Devonian period . It was a labyrinthodont, one of the first fossil record of tetrapods. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps...
and Acanthostega
Acanthostega
Acanthostega is an extinct labyrinthodont genus, among the first vertebrate animals to have recognizable limbs. It appeared in the Upper Devonian about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and the first tetrapods fully capable of coming onto...
. SSTM 5025 possessed seven digits on the forelimbs and six on the hindlimbs. The wide manus and pes of the specimen resemble the limb-like fins of extant frogfish
Frogfish
Frogfishes, family Antennariidae, are a type of anglerfish in the order Lophiiformes. They are known as anglerfishes in Australia, where 'frogfish' refers to a different type of fish...
es.
Taxonomy and phylogenetics
Hupehsuchia was first defined as a suborder of "ThecodontThecodont
Thecodont , now considered an obsolete term, was formerly used to describe a diverse range of early archosaurs that first appeared in the Latest Permian and flourished until the end of the Triassic period...
a" in 1972 when it was first thought to be a group of early archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
s. 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
Lacrimal bone
The lacrimal bone, the smallest and most fragile bone of the face, is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders.-Lateral or orbital surface:...
, not the nasal and prefrontal. 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. It is more likely that the ancestors of hupehsuchians 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). Yet there were some early ichthyosaurs and ichthyosaur relatives such as Chaohusaurus
Chaohusaurus
Chaohusaurus geishanensis was an ichthyosaur, an extinct marine reptile, from the Early Triassic of Chaohu, China. It has also been called Anhuisaurus and Chensaurus. It was described by Young and Dong in 1972...
and Utatsusaurus
Utatsusaurus
Utatsusaurus is the earliest-known form of an ichthyopterygian , which lived in the early Triassic period . Unlike the more advanced ichthyosaurs, Utatsusaurus has no dorsal fin and has a broad skull that tapers slowly toward the snout...
that possessed vertebrae that were not deeply heterocoelus and more closely resembled 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
Sauropterygia
Sauropterygia were a group of very successful aquatic reptiles that flourished during the Mesozoic before they became extinct at the end of the era. They were united by a radical adaptation of their shoulder, designed to support powerful flipper strokes...
. 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 plesiosaur
Plesiosaur
Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...
s, whale
Whale
Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
s, and the early marine bird Hesperornis
Hesperornis
Hesperornis is a genus of flightless aquatic birds that spanned the first half of the Campanian age of the Late Cretaceous period . One of the lesser-known discoveries of the paleontologist O. C. Marsh in the late 19th century Bone Wars, it was an important early find in the history of avian...
, all of which have developed a similar morphology independently
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
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
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
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
Neodiapsida
Neodiapsida is a clade or major branch of the reptilian family tree, that includes all Diapsids apart from some early primitive types known as Araeoscelidia....
, but most of the derived characteristics that define the clade are absent or hard to distinguish in hupehsuchians, even though their ancestors may have possessed these characteristics at one point. Three derived characteristics of Neodiapsida are a reduced number of teeth on the pterygoid, an absence of teeth on the parasphenoid, and a lack of caniniform mixillary teeth, but none of these apply to hupehsuchians because they lack teeth altogether. Many derived characteristics that define neodiapsids regard the limbs and girdles, but these characteristics are not seen in marine reptiles because the limbs and girdles are too highly modified. Many characteristics of the skull cannot be seen in hupehsuchians due to poor preservation of remains. Therefore, any placement of Hupehsuchia within Neodiapsida remains tentative until more specimens are found.
Due to the great number of derived characteristics in hupehsuchians that are a result of convergence, computational phylogenetic analyses
Computational phylogenetics
Computational phylogenetics is the application of computational algorithms, methods and programs to phylogenetic analyses. The goal is to assemble a phylogenetic tree representing a hypothesis about the evolutionary ancestry of a set of genes, species, or other taxa...
using computer programs based on the method of maximum parsimony
Maximum parsimony
Parsimony is a non-parametric statistical method commonly used in computational phylogenetics for estimating phylogenies. Under parsimony, the preferred phylogenetic tree is the tree that requires the least evolutionary change to explain some observed data....
do not produce cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
s that can accurately establish a relationship between Hupehsuchians and other diapsids. Such programs do not recognize situations where sister group relations are not justified.
Paleobiology
Hupehsuchians were clearly well adapted to marine life, as they possessed limbs that were paddle-like in shape and had fusiform bodies. It is likely that the characteristically elongated neural spines were associated with well-developed epaxial muscles (muscles lying above the transverse process of the vertebrae) that facilitated lateral undulation in an axial subundulatory mode. The pattern of articulation in the vertebrae suggest that such undulation was concentrated posteriorly near the pelvic girdle and tail. It is likely that Hupehsuchus was better equipped for lateral undulation as a means of locomotion than Nanchangosaurus was. This is evidenced by the assumed greater degree of lateral compression in the body of the former genus as well as generally more elongate neural spines.The presence of polydactyly in SSTM 5025 may have been an adaptation to moving across underwater substrates in a similar manner to some early tetrapods of the Devonian such as Acanthostega.
The flat, toothless rostrum may have supported an avian-like bill, or perhaps rows of baleen
Baleen
Baleen or whalebone is a filter-feeder system inside the mouths of baleen whales. The baleen system works when a whale opens its mouth underwater and then water pours into the whale's mouth. The whale then pushes the water out, and animals such as krill are filtered by the baleen and remain as food...
as seen in cetaceans of the suborder Mysticeti. It seems that hupehsuchians were adapted to continuous ram feeding, a form of mobile suspension feeding. The large skull and lack of a fixed symphysis with the lower jaw is indicative of such a feeding method, but the narrowness of the rostrum and the existence of marine vertebrates with larger skulls that were not completely edentulous (e. g. ichthyosaurs) seem to point against it. The long necks of hupehsuchians would seem to inhibit continuous ram feeding at high speeds through the water, so it is more likely that they practiced intermittent ram feeding at slower speeds.
The purpose of the dorsal dermal plates of hupehsuchians is unknown. These plates as well as the ventral gastralia and the tendency for pachyostosis
Pachyostosis
Pachyostosis is a condition in vertebrate animals in which the bones experience a thickening, resulting in unusually solid bone structure with little to no marrow. It occurs in both terrestrial and aquatic vertebrates, such as manatees and dugongs, plesiosaurs, and members of the Dinocephalia...
would have added considerable weight to hupehsuchians, allowing them to have neutral buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
. However, the position of the dorsal plates are high above the center of gravity, and it seems that this would have made the bodies of hupehsuchians unstable. Hupehsuchians were among the largest marine animals of their time, so there would be no need for dorsal plates as a protective measure. The tendency for the dorsal plates to be more developed anteriorly may have allowed the anterior portion of the vertebral column to remain relatively rigid while the posterior portion could freely undulate.
It is unsure whether the plates were acquired subsequent to an adaptation to the marine environment or were inherited from a terrestrial ancestor, in which such plates would have given rigidity to the spine and protection from predators. In any case, if Nanchangosaurus is seen as representative of an ancestral morphological pattern that led to the more derived Hupehsuchus, then the plates seem to have experienced further development in hupehsuchians and must have had some advantage.
The general shape of hupehsuchians seems to have made any locomotion on land nearly impossible, and, although there is no direct evidence seen in known specimens, these animals were probably viviparous, giving live birth at sea rather than laying eggs on land. Vivipary is also seen in ichthyosaurs.