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''Xilousuchus'' is an
extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of poposauroid from lower
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
(
Olenekian In the geologic timescale, the Olenekian is an age (geology), age in the Early Triassic epoch (geology), epoch; in chronostratigraphy, it is a stage (stratigraphy), stage in the Lower Triassic series (stratigraphy), series. It spans the time betw ...
stage) deposits of Fugu County of northeastern
Shanxi Province Shanxi; formerly romanised as Shansi is a province in North China. Its capital and largest city of the province is Taiyuan, while its next most populated prefecture-level cities are Changzhi and Datong. Its one-character abbreviation is ( ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
. Xilousuchus is one of the oldest
archosaurs Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...
known to date.


Discovery and naming

It is known from the
holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
, IVPP V 6026, a single well-preserved partial
skeleton A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal fra ...
including the
skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...
. It was found from the Heshanggou Formation of the Ordos Basin, Hazhen commune. It was first named by Xiao-Chun Wu in
1981 Events January * January 1 ** Greece enters the European Economic Community, predecessor of the European Union. ** Palau becomes a self-governing territory. * January 6 – A funeral service is held in West Germany for Nazi Grand Admiral ...
and the
type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
is ''Xilousuchus sapingensis''. Wu (1981) referred ''Xilousuchus'' to the
Proterosuchia Proterosuchia is one of the suborders of the paraphyletic group Thecodontia, containing the most primitive and ancestral forms. These were primitive, vaguely crocodile-like, archosauriforms that mostly lived during the Early Triassic epoch. Th ...
. Gower and Sennikov (1996) found it to be an
erythrosuchia Erythrosuchidae (meaning "red crocodiles" in Greek language, Greek) are a family of large Basal (phylogenetics), basal archosauriform carnivores that lived from the later Early Triassic (Olenekian) to the early Middle Triassic (Anisian). Naming T ...
n based strictly on the
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
. A more detailed re-description of the genus was provided by Nesbitt ''et al.'' (2010) and found poposauroid affinities. In his massive revision of archosaurs which included a large
cladistic Cladistics ( ; from Ancient Greek 'branch') is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is ...
analysis, Sterling J. Nesbitt (2011) found ''Xilousuchus'' to be a poposauroid which is most closely related to '' Arizonasaurus''. ''Xilousuchus'' is the oldest
archosaur Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...
to date, although ''
Ctenosauriscus ''Ctenosauriscus'' is an extinct genus of sail-backed Poposauroidea, poposauroid archosaur from Early Triassic deposits of Lower Saxony in northern Germany. It gives its name to the family Ctenosauriscidae, which includes other sail-backed poposa ...
'' and ''
Vytshegdosuchus ''Vytshegdosuchus'' is an extinct genus of paracrocodylomorph archosaur known from the Early Triassic (latest Olenekian stage) Yarenskian Gorizont of the Komi Republic of the European section of Russia. It contains a single species, ''Vytshegdo ...
'' might be even older by less than one million year. Since ''Xilousuchus'' is a
suchia Suchia (from Ancient Greek σούχος (''soúkhos''), meaning "crocodile") is a clade of archosaurs containing the majority of pseudosuchians (crocodilians and their extinct relatives). It was defined as the least inclusive clade containing ''A ...
n archosaur, its early age suggests that most of the major groups of archosaurs (
ornithodira Avemetatarsalia (meaning "bird metatarsals") is a clade of diapsid reptiles containing all archosaurs more closely related to birds than to crocodilians. The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs. Di ...
ns, ornithosuchids,
aetosaur Aetosaurs () are heavily armored reptiles belonging to the extinct order (biology), order Aetosauria (; from Ancient Greek, Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized Omnivore, omnivorous or Herbivore, herbivor ...
s, and
paracrocodylomorphs Paracrocodylomorpha is a clade of pseudosuchian archosaurs. The clade includes the diverse and unusual group Poposauroidea as well as the generally carnivorous and quadrupedal members of Loricata, including modern crocodylians. Paracrocodylomor ...
) developed by the Early Triassic, soon after the appearance of the first archosaur.


Features

''Xilousuchus'' was probably quite a large animal when alive, possibly between three and four metres long. It had a small head, and a
sail A sail is a tensile structure, which is made from fabric or other membrane materials, that uses wind power to propel sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and even sail-powered land vehicles. Sails may b ...
along its neck and back. It probably had long and quite powerful legs, and would have been an active hunter. Only the partial
skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...
,
cervical vertebrae In tetrapods, cervical vertebrae (: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In saurop ...
, and a few other fragments are preserved. ''Xilousuchus'' has a relatively small head and long neck, with a skull length of approximately 25 cm and neck of 45 cm. The skull is fragmentary, but much of the snout and
maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
are present. Only one maxillary tooth has been fossilised. The maxilla has a partially developed palatal process, and the angle of the dorsal process indicates that ''Xilousuchus'' had a large
antorbital fenestra An antorbital fenestra (plural: fenestrae) is an opening in the skull that is in front of the eye sockets. This skull character is largely associated with Archosauriformes, archosauriforms, first appearing during the Triassic Period. Among Extant ...
. The single preserved maxillary tooth is large, almost 15 mm long, and in other dental alveoli where the teeth are missing replacement teeth are sometimes visible, indicating that it was a
polyphyodont A polyphyodont is any animal whose tooth (animal), teeth are continually replaced. In contrast, diphyodonts are characterized by having only two successive sets of teeth. Polyphyodonts include most toothed fishes (most notably sharks), many repti ...
. The tooth has small serrations, is curved backwards, and is laterally compressed, indicating that it could have punctured flesh or possibly sliced off chunks. The lacrimal and
nasal Nasal is an adjective referring to the nose, part of human or animal anatomy. It may also be shorthand for the following uses in combination: * With reference to the human nose: ** Nasal administration, a method of pharmaceutical drug delivery * ...
bones are large but quite thin and fragile. The skull has a reasonably well-preserved
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
with thick walls and a small volume, and this shows that the brain was not large. It closely resembles that of ''Arizonasaurus,'' indicating that the two species are close relatives. The
dentary bone In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...
is partially preserved for both left and right, and would have held thirteen teeth on each side, although only the nutrient foramina remain. Like that of most archosaurs from the Triassic, the dentary is unspecialised. The cervical vertebrae have large, flattened
neural spines Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...
which make up most of their height and would have formed a sail-like structure in life, similar to other ctenosauriscids. Unusually, however, they curve slightly forwards. There is little space between the neural spines, indicating that ''Xilousuchus'' would probably not have had a very flexible neck. The spines gradually increase in height as they move backwards - that on the axis vertebra is 42 mm high, whereas that on the tenth cervical vertebra is 92 mm high. The height of the sail along the back is not known, as none of the dorsal vertebrae have been preserved, but it was probably similar to that in ''Arizonasaurus'' or '' Hypselorhachis.'' One
sacral vertebra The sacrum (: sacra or sacrums), in human anatomy, is a triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part of the pelvic cavity, ...
is present, and it shows a clear suture where it was joined to the sacral ribs and pelvis. The orientation of the sacral rib suggests that the ilium was downturned, although this is not certain. Two
caudal vertebrae Caudal vertebrae are the vertebrae of the tail in many vertebrates. In birds, the last few caudal vertebrae fuse into the pygostyle, and in apes, including humans, the caudal vertebrae are fused into the coccyx. In many reptiles, some of the caud ...
are present as well, and these are poorly preserved with missing neural spines, but show facets where chevrons would have been attached. The proximal ends of two cervical ribs have also been fossilised, and these have two heads - a tuberculum and capitulum. They are slender, unlike those of phytosaurs and
crocodylomorphs Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. Extinct crocodylomorphs were considerably more ...
. One
clavicle The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately long that serves as a strut between the scapula, shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavic ...
and one
ungual An ungual (from Latin ''unguis'', i.e. ''nail'') is a highly modified distal toe bone which ends in a hoof, claw, or nail. Elephants and ungulates have ungual phalanges, as did the sauropod Sauropoda (), whose members are known as sauropods (; ...
are also known, but it is not known where the ungual is from. They have few distinguishing features and resemble those of other ctenosauriscids very closely.


Phylogeny

Cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
after Nesbitt, 2010:


References

{{Taxonbar, from=Q948654 Early Triassic reptiles of Asia Poposauroidea Fossil taxa described in 1981 Prehistoric pseudosuchian genera