Petrolacosaurus kansensis

Petrolacosaurus
 is an extinct genus of diapsid reptile from the late Carboniferous
period. It was a small, 40-centimetre long reptile, and the earliest
known reptile with two temporal fenestrae. This means that it was at the
 base of Diapsida, the largest and most successful radiation of reptiles
 that would eventually include all modern reptile groups, as well as
dinosaurs and other famous extinct reptiles such as plesiosaurs,
ichthyosaurs, and pterosaurs. However, Petrolacosaurus itself was part
of Araeoscelida, a short-lived early branch of the diapsid family tree
which went extinct in the mid-Permian.

The first Petrolacosaurus fossil was found in 1932 in Garnett, Kansas by a field expedition from the University of Kansas
 Natural History Museum. The party consisted of Henry H. Lane, Claude
Hibbard, David Dunkle, Wallace Lane, Louis Coghill, and Curtis Hesse.
Unfortunately, no field notes or documentation of their discovery are
available.

Petrolacosaurus fossils were found preserved within a
 layer of laminated shale that was also plant bearing. The strata that
the remains were found in was of Upper Pennsylvanian age (approximately 323-298 ma). The fossil itself is estimated to be approximately 302 million years old.

Specimens reveal that Petrolacosaurus had a slightly elongated skull with two temporal fenestrae. The upper temporal fenestra is located posteriorly to an enlarged orbit. This is a distinctly diapsid character.

The largest teeth in the jaw were at the front of the snout, erupting from the premaxilla bone. There were also a few larger-than average teeth further back on the maxilla, which were probably homologous with caniform teeth retained from the common ancestor between true sauropsid reptiles and synapsids (mammal ancestors). On the dentary,
 there are marginal teeth, displaying a primitive type of shallow
implantation. The palatal arrangement bears close resemblance to Youngoides.

Axial Skeleton
There are seven elongated cervical vertebrae, 2 sacrals, and 60 caudal vertebrae.
 The number and placement of the vertebrae show that Petrolacosaurus was
 a rather long-necked reptile with a shorter torso. The number and
spool-like shape of the vertebrae, in addition to the poses that the
reptiles died in, show that they were also very flexible creatures.

The
 vertebral centra were amphicoelous (concave from the front and behind)
and had large ventrolateral depressions. A similar depression can be
seen on the massive neural arches, running from the prezygapophyses to the postzygapophysis. These depressions allowed the vertebral column to be lighter while still retaining a sturdy build.

Petrolacosaurus is a member of the basal diapsid order Araeoscelidia, along with the aquatic, late Pennsylvanian Spinoaequalis.

The
 specific taxonomic placement of Petrolacosaurus is a highly debated
topic. Because of the unusual osteological characters and how old the
reptile is, scientists go back and forth between what position makes the
 most sense.

Some paleontologists are skeptical of
Petrolacosaurus' current placement because of large lapses in the fossil
 record, specifically between the earliest know neodiapsid fossils
(appearing in the Lower Permian ) and their proliferation in the Late
Permian.

Current phylogenetic placement indicates that
Petrolacosaurus is derived from a sister to Spinoaequalis, a second
species of early diapsid. In contrast to Spinoaequalis, Petrolacosaurus
exhibits many distinct cranial and post-cranial features, characteristic
 of the most primitive diapsids.

The fact that Petrolacosaurus has features that are distinctly similar to other romeriidans
 and others which were very primitive, leads scientists to believe that
the species is old enough to be an ancestor of neodiapsids. Because
Petrolacosaurus does not have more developed features that the
neodiapsids have, it has been placed in a group separate from
Neodiapsida.

Petrolacosaurus teeth were of moderate length, slightly recurved, and
possessed no lateral compression. Located on the premaxilla of the upper
 jaw are two teeth, reminiscent of fangs. On the dentary were around 25
smaller teeth, all of different lengths. Placement in the jaw reveals
that the teeth on the upper and lower jaw do not interlock but rather
meet along a medial plane. The haphazard tooth lengths is consistent
with a primitive method of regular tooth replacement. The teeth located
 on the transverse flange of the pterygoid were significantly larger
than the others, indicating that these teeth were specialized for
grabbing smaller, hard shelled insects.

The jaws of
Petrolacosaurus were long and slender. Based on the skeletal muscle
attachment points and the shortened temporal region, adductor muscles
would have had limited amounts of leverage, resulting in a quick but
weak bite. This is further support for the hypothesis that
Petrolacosaurus was insectivorous.

Environment
The
 mud that the reptile was fossilized in is known to create well
preserved insect and plant prints, allowing extrapolation about the
surrounding environment. Because of fossil evidence of winged insects,
the fruits of conifers, and other pteridosperms, Petrolacosaurus is
thought to be a completely terrestrial reptile that lived in a
conifer-fern forest.

The front limb is 135mm and the back limb is 165mm, giving the reptile a length ratio of almost 1x as lon…