Vancleavea

''Vancleavea'' is a genus of extinct, armoured, non- archosaurian archosauriforms from the Late Triassic of western North America. The type and only known species is ''V. campi'', named by Robert Long & Phillip A Murry in 1995. At that time, the genus was only known from fragmentary bones including osteoderms and vertebrae. However, since then many more fossils have been found, including a pair of nearly complete skeletons discovered in 2002. These finds have shown that members of the genus were bizarre semiaquatic reptiles. ''Vancleavea'' individuals had short snouts with large, fang-like teeth, and long bodies with small limbs. They were completely covered with bony plates known as osteoderms, which came in several different varieties distributed around the body. Phylogenetic analyses by professional paleontologists have shown that ''Vancleavea'' was an archosauriform, part of the lineage of reptiles that would lead to archosaurs such as dinosaurs and crocodilians. ''Vancleavea'' lacks certain traits which are present in most other archosauriforms, most notably the antorbital, mandibular and supratemporal fenestrae, which are weight-saving holes in the skulls of other taxa. However, other features clearly support its archosauriform identity, including a lack of intercentra, the presence of osteoderms, an ossified laterosphenoid, and several adaptations of the femur and ankle bones. In 2016, a new genus of archosauriform, ''Litorosuchus'', was described. This genus resembled both ''Vancleavea'' and more typical archosauriforms in different respects, allowing ''Litorosuchus'' to act as a transitional fossil linking ''Vancleavea'' to less aberrant archosauriforms.

History

''Vancleavea'' was first discovered in 1962 from the Petrified Forest Member of the Petrified Forest National Park and initially described by Long and Murry in 1995. At that time the only described specimen was the holotype, PEFO 2427. The genus is named after Phillip Van Cleave, who discovered the first known remains of the genus. Since then, a number of remains have been found. A 2009 reevaluation of the genus by Nesbitt ''et al''. formally described two additional specimens, GR 138 and 139. GR 138 is particularly notable due to being a nearly complete and articulated skeleton preserving a variety of osteoderms in the positions they would have been in during life. It was discovered at the ''Coelophysis'' Quarry in north-central New Mexico ( Ghost Ranch), US, and was prepared at the Ruth Hall Museum of Paleontology in Abiquiú, New Mexico before its formal description. ''Vancleavea'' is a fairly common occurrence in most levels of the Chinle Formation, however, due to the poorly preserved remains, it is difficult to compare specimens across stratigraphic levels.

Description

The most complete specimen of ''Vancleavea'' (GR 138) was around 1.2 m (3.9 ft) in length. However, isolated bones have shown that members of the genus could grow larger than GR 138. In particular, MCCDM 1745 from New Mexico may have reached a total length of . Imbricating osteoderms cover the entire body, the limbs are relatively short, and the skull is highly ossified. The supratemporal fenestra is absent, which may represent a secondary closure rather than a plesiomorphic trait. The nares open dorsally (i.e. nostrils face upward) and the jaw contains enlarged caniniform fangs. Each osteoderm possesses a pronounced central keel and an anterior projection. The ilium of ''Vancleavea'' resembles those of unrelated drepanosaurs. The unique morphology of ''Vancleavea'' differs greatly from any other known basal archosauriform.

Skull

The only known ''Vancleavea'' specimen preserving a complete skull is GR 138, and as a result that specimen is the basis for knowledge of the skull in this taxon. The postorbital region of the skull (behind the eyes) is long and boxy, with a wide and flat skull roof. The preorbital region (in front of the eyes), on the other hand, is short, triangular, and thin. A distinguishing feature of ''Vancleavea'' is the lack of an antorbital fenestra, a hole in front of the eyes which is typical for archosauriforms. The lacrimal bone, which usually forms the rear edge of the antorbital fenestra, has also disappeared. Grooves cover the skull roof while foramina (tiny pores) coat the skull bones near the mouth.

Preorbital region (snout)

The maxilla (the main tooth-bearing bone of the snout) is simple and triangular due to the loss of the antorbital fenestra. The front tip of the bone possesses a toothless notch, known as a diastema, which accepts a large, caniniform (fang-like) tooth of the dentary (main toothed bone of the lower jaw). The second tooth of the maxilla is also a caniniform tooth, approximately as long as that of the dentary. These fangs are flattened from the side, curved backwards, and serrated on their rear edge (and in the case of the maxillary fang, the front edge as well). This contrasts with the rest of the teeth, which are generally cone-shaped. The single tooth in front of the maxillary fang, as well four immediately behind it are very small. These small teeth are followed by six somewhat larger maxillary teeth and a final small tooth.

Five teeth are also present in each premaxilla (a pair of bones at the tip of the snout), with the third tooth being a caniniform tooth similar to that of the maxilla and dentary. The premaxilla also has a pair of bony projections (processes) which connect to other snout bones. The posterodorsal process snakes up the front of the maxilla, separating that bone from the nares (nostril holes). The thin anterodorsal process instead runs along the midline of the snout. The paired nasal bones on the upper edge of the snout are long and rectangular, and at their front edge are the rounded, upwards-pointing nares. The nasals do not contact each other; at the front and middle of the snout the anterodorsal processes of the premaxillae separate each nasal. At the level of the eyes, the nasals are divided by another feature unique to ''Vancleavea'': a single narrow bone likely acquired by a neomorphic mutation. The front edge of the orbit is formed by the narrow prefrontal bone.

Postorbital region

The jugal (cheek bone) is complex. It has a tapering front edge that extends under the eye to contact the prefrontal, thereby excluding the maxilla from the orbit. A triangular upward projection rises up behind the eye and splits the lower part of the postorbital bone (which forms the rear edge of the eye) in half. Both the jugal and postorbital have very long rear extensions which nearly reach the back of the skull. A vast open space (about a third the length of the skull) lies in the area between these two extensions. This hole is known as a lateral temporal fenestra. Reptiles of the group Diapsida are typically characterized by having two temporal fenestrae at the back of the skull: a lower one on the side of the skull (the lateral temporal fenestra) as well as a larger one on the top of the skull (the supratemporal fenestra). ''Vancleavea'' breaks away from this standard, as its supratemporal fenestra has completely closed up during evolution, leaving only the huge lateral temporal fenestra.

The flat and heavily sculptured skull roof is formed by the paired frontal bones above the eyes and the parietal bones above the temporal fenestra. The connection between the frontal pair and the parietal pair is W-shaped, with each parietal having a triangular forward point which penetrates each frontal. A similar connection is also present at the front edge of the frontals, where each frontal is bisected by the rear tip of each nasal bone. Similar to proterochampsians and a few types of archosaurs (crocodylomorphs, dinosaurs, and shuvosaurids), ''Vancleavea'' does not possess a postfrontal, a small wedge-like bone which sometimes occupies the rear upper corner of the orbit.

The rear edge of the skull is somewhat difficult to interpret even in well-preserved specimens such as GR 138. The upper rear corner of the temporal fenestra is formed by the squamosal bone, which connects to the back of the postorbital bone and the parietal bone, as well as the braincase. The front edge of the squamosal possesses a deep 'pocket', while the lower portion of the bone expands into a large downward-pointing 'hood'. A thin structure known as a ventral process stretches downwards, forming the rear edge of the temporal fenestra. This ventral process connects to the quadratojugal bone, which itself contacts the rear branch of the jugal and forms the rear lower corner of the temporal fenestra. The quadrate bone, which forms the cranium's contribution to the jaw joint, is located inwards from the squamosal's ventral process. The quadrate not only contacts the lower jaw, but also connects to the inner face of the quadratojugal and is overlapped by the 'hood' of the squamosal. The appearance of the quadrate is yet another unique aspect of ''Vancleavea''. In most basal archosauriforms, the quadrate is tall and straight, but in ''Vancleavea'' it is short, stout, and arched forwards.

Braincase

As is typical for reptiles, the lower rear portion of the braincase is formed by a bone known as the basioccipital, although this bone is unusually long and low in ''Vancleavea''. The rear portion of the bone has a single large knob, known as an occipital condyle, which attaches the skull to the neck. A pair of bones outwards-angled known as exoccipitals attach to the upper surface of the basioccipital. In most archosauriforms, the exoccipitals form the upper surface of the occipital condyle, but ''Vancleavea'' is unique in lacking this contact. The bases of the exoccipitals are elongated and diverge towards the rear of the basioccipital. They also converge towards the front, although they do not contact each other. In addition, a small ridge runs between the bases on the upper surface of the basioccipital. Thus, when seen from above, the bases of the exoccipitals resemble a forward-pointing arrow with a small gap at the apex. A ridge and a small pit are present on each side of the basioccipital under the contact with the exoccipital. An additional pit believed to be the opening for the hypoglossal canal lies on the outer side of each exoccipital.

A pair of rounded plates known as basitubera jut downwards and slightly outwards from the rear surface of the basioccipital. In front of the basitubera lies the parabasisphenoid, a fused bone which forms the front lower portion of the braincase. Meanwhile, in front of the exoccipitals is an attachment for the opisthotic bone, which forms most of the side of the braincase. A small indentation between the opisthotic and exoccipital attachments may be the lagenar recess. This indentation likely held an organ of the inner ear known as the lagena, which in mammals develops into the spiral-shaped cochlea.

Above the occipital condyle is the foramen magnum, a very large hole where the spinal cord exits the braincase. The upper rear portion of the braincase (as well as potentially the upper edge of the foramen magnum) is formed by the supraoccipital bone. As with other archosauriforms, the rear edge of this bone is acutely angled, terminating in a large keel edged by smooth areas for the attachment of muscles that raise the head. The side of the supraoccipital and a keel on the underside of the parietal bones meet along the side of the braincase, where they contact a downward-pointing spur known as the paroccipital process. The prootic bone, which forms the upper front part of the braincase, possesses a forward-pointing pit for the trigeminal nerve, with the area below the pit being smooth as in advanced archosauriforms. The front of the braincase also has a long and thick forward-pointing bone known as a laterosphenoid. This bone arches outwards but converges along the midline at its front tip.

Lower jaw

The majority of the lower jaw is formed by the dentary, which possess diverse teeth similar to those of the maxilla and premaxilla. The first few are large and conical, and the fourth or fifth tooth is yet another enlarged caniniform tooth. Unlike the maxillary caniniform tooth which is serrated on both the front and rear edges, the dentary caniform is serrated on only the rear edge. Dentary teeth past the caniform are similar to those in the rear part of the maxilla. The outer face of the dentary also has a depression near the tooth row about midway down the length of the bone. This depression would have received the maxillary fang while the mouth was closed, similar to how the diastema at the front of the maxilla would have received the dentary fang. The surangular and angular (a pair of untoothed bones in the rear part of the lower jaw) are deep. The articular bone, which houses the jaw joint at the rear end of the lower jaw, is concave and opens towards the rear, similar to that of phytosaurs. Most archosauriforms possess a hole known as a mandibular fenestra where the dentary, angular, and surangular meet. However, it cannot be determined if ''Vancleavea'' also possessed this hole. It is plausible that it was lost through evolution (as is the case with the antorbital and supratemporal fenestrae), but there is also the possibility that it was too small to be noticeable in the preserved ''Vancleavea'' skull. A small and sometimes overlooked mandibular fenestra is present in '' Proterosuchus'', one of the first archosauriforms to evolve.

Vertebrae

The cervicals (neck vertebrae) are much longer than they are tall, creating a moderately long neck. Seen from below, they are "pinched" midway down the length of the centrum (main body of each vertebra) by large pits. The centra are also slightly procoelous, with a concave front surface and convex rear surface. The underside of the centra has a large, rectangular keel with an unusually straight lower edge. As is typical for reptiles, the vertebrae connect to each other with interlocking plates above the centra known as zygapophyses. More unusually, the rear joint plates (postzygapophyses) also possess additional spurs which do not connect to the following vertebrae. These extra spurs, known as epipophyses, are most common in dinosaurs but now known to exist in other types of extinct archosauromorphs.

The dorsals (back vertebrae) are also long, and also possess "pinched" centra. Unlike the procoelous cervicals, the dorsals are amphicoelous with both the front and rear surface being concave. A pair of large and cylindrical outgrowths extend perpendicular to each centrum, at the front edge of their respective vertebrae. These outgrowths, currently identified as diapophyses, would have connected to ribs. Some dorsals of ''Vancleavea'' are also characteristic in the possession of a shallow groove which extends down the underside of each centrum, bordered by a pair of keels. A large, plate-like structure known as a neural spine juts out of the top of each vertebra. These neural spines are thin seen from the front but broad when seen from the side. They are roughly keystone-shaped, with notches at both the upper front and rear corner, as well as a convex upper portion covered with small grooves. There is some variety between dorsals in different specimens, with some dorsals having only slight keels or additional rib facets (known as parapophyses) to complement the diapophyses. At least some of the ribs of ''Vancleavea'' were thick-walled, strongly curved, and connected to the dorsal vertebrae at two points (hence why some vertebrae had two facets per rib).

The two sacrals (hip vertebrae) are shorter and simpler than the dorsals. Rather than possessing the pinched sides of dorsals, they instead have massive facets for sacral ribs which connect the spinal column to the hip bones. Some specimens retain the characteristic double keels of the dorsal centra, but in other specimens they fuse into a single keel. The numerous caudals (tail vertebrae) are much more slender, and are roughly rectangular when seen from below. Towards the tip of the tail, they become increasingly simple and elongated, losing their rib facets. Nevertheless, they always retain large paired keels along their underside. They possess tall and thin neural spines as well as chevrons, similar structures which extend from the underside of the centra. Both the neural spines and chevrons are angled back, but they are long enough to create a deep tail regardless.

Forelimbs

The scapula (shoulder blade) of ''Vancleavea'' was hourglass shaped, with the front and rear edges being strongly concave and the upper and lower edges being somewhat convex. Most of the bone is thin from side-to-side, with the exception of the glenoid (shoulder socket), which is thickened in an outwards direction. A small bump is present midway down the front edge of the bone. This bump is considered to be the same feature as the acromion process of humans.

The humerus (upper arm bone) is rather simple, with a rounded head that forms the "ball" part of a ball-and-socket joint with the glenoid socket. The front outer edge of the bone has a low and rounded ridge known as a deltopectoral crest. Knob-like structures (condyles

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