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 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 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. 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. 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. The lower portion of the ilium is mostly formed by the
acetabulum (hip socket), and has a triangular lower edge which wedges between the other two bones of the hip. The pubis is incomplete, but possesses a knob-shaped rear branch (posterior process) which extends under the acetabulum. This branch contacts the ischium along a straight edge, although part of their connection is left open as a notch which partially or completely enclosed the
obturator foramen. The ischium is fan-shaped and points backwards and downwards. When seen from the front, the two ischia (on either side of the body) converge at the rear edges of each. Although the majority of the acetabulum is formed by the lower portion of the ilium, a noticeable portion is also formed by the upper portion of the ischium and a tiny sliver of the socket is formed by the upper portion of the pubis.
Leg The top part of the S-shaped
femur (thigh bone) is paddle-like and oval-shaped in cross-section. It includes a noticeable, but not strongly offset '
head'. The upper portion of the wide rear/inner face of the femur sometimes has a small ridge. This ridge, which is not present in every femur referred to
Vancleavea, is believed to be an area for the attachment of the powerful
caudofemoralis muscle which helps to pull the leg back. This ridge is believed to be synonymous with the
fourth trochanter of archosaurs as well as the internal trochanter of basal archosauriforms. The midshaft of the femur is circular in cross-section. The lower portion of the femur includes both a
medial condyle (which connects to the tibia) and a
lateral condyle (which connects to the fibula), although neither of these knobs are particularly strongly developed. In general, the femur of Vancleavea is intermediate in structure between basal archosauriforms and archosaurs. The
tibia and
fibula (lower leg bones) are both slightly curved away from the center of the leg. They are also thinnest at their respective midshafts. The front edge of the tibia possesses a sharp ridge known as a
cnemial crest, which extends about two thirds the way down the shaft. The fibula is thinner and somewhat twisted compared to the tibia.
Ankle and foot view (i.e. with the
tibial and
fibular joints facing the viewer), with the calcaneum in blue and the astragalus in pink. Like most other archosauromorphs, the ankle is formed by two notable bones: the
calcaneum, which connects to the fibula on the outer side of the heel, and the
astragalus, which connects to both the tibia and the fibula on the inner part of the heel. The astragalus has facets for the tibia and fibula as well as a gap between the two facets, known as a non-articular notch. The facet for the tibia stretches down the inner edge of the astragalus, but not along the rear edge. This is similar to the situation in basal archosauriforms but contrasts with that of more advanced taxa which have a more sprawling tibial facet. The outer edge of the astragalus (which connects to the calcaneum) is also convex like basal archosauriforms. The calcaneum is small and triangular. The outer edge has a spur which points outwards and slightly downwards. Although this spur (formally known as a calcaneal tuber) is common in many archosauriforms, that of
Vancleavea is unique in having a 'squashed' shape, with the upper/front and lower/rear surfaces meeting each other at a rounded outer apex. Similar to advanced archosauriforms,
Vancleavea only possessed two distal
tarsal bones in the ankle (apart from the calcaneum and astragalus), rather than four as in more basal members of the group. The main portion of the foot is formed by four thin
metatarsal bones and a much shorter fifth metatarsal, each of which probably connect to a short toe. However, the bones of the foot are jumbled even in complete specimens such as GR 138, making it difficult to assign a specific location to any metatarsal or toe bone.
Osteoderms The body is heavily armored, coated in various bony plates known as
osteoderms. There are five to six distinct osteoderm morphotypes (i.e. variants) which are expressed at different parts of the body. Hunt, Lucas, & Spielmann (2005) were the first paleontologists to use morphotypes to describe
Vancleavea osteoderms, but their designations differed from those in later studies such as Nesbitt
et al. (2009), which are used here. The throat region, between the shoulder and jaws, possesses "morphotype A" osteoderms, which are teardrop-shaped and sometimes keeled, with a pointed front tip and rounded rear edge. "Morphotype B" osteoderms cover the back and sides of the body. They all tightly overlap and possess front spurs and low keels, but the osteoderms that form several rows on the back are more symmetrical and diamond-shaped. The osteoderms on the sides, on the other hand, are asymmetrical, with a smaller lower portion (below the keel) that slants forwards, a front spur that dips slightly downwards, and a rear tip that is more pointed than those of the back osteoderms. The osteoderms that cover the belly, known as "morphotype C" osteoderms, are of a type unique to
Vancleavea. They are large, boxy, and rounded, with large keels and heavily serrated front edges. This morphotype has been compared to the armor of
ankylosaurs. The upper side of the tail also possesses very characteristic and unusual osteoderms, designated as "morphotype D". These are elongated plates that stick up from the tail, with one plate per vertebra. They are wide at their base but flattened (from the side) elsewhere, and also acquire a swept-back orientation. In life, they would probably have been connected by soft tissue to form a tail fin to assist swimming. The morphotype D osteoderms are largest towards the front of the tail and gradually decrease in height and overall size towards the tip. The sides of the tail are covered with morphotype B osteoderms. The limbs are covered with "morphotype E" osteoderms. These plates are smaller, rounder and thinner than the osteoderms of the body. A sixth type of osteoderm, which may or may not belong to
Vancleavea, was also reported in 1995. This osteoderm, of which only the base is known, is comparable to the tail spike of a
Stegosaurus. Large, spike-like osteoderms were not present in GR 138, which was otherwise a complete skeleton with all of the other osteoderms in the same positions they were in life. It is conceivable that the "
Stegosaurus spike" osteoderm was actually misplaced from the skeleton of an
aetosaur, although the spikes of aetosaurs known from
Vancleavea-bearing fossil sites differed from the "
Stegosaurus spike". ==Classification==