Podargosaurus hibbardi Lane, 1945 Petrolacosaurus ("rock lake lizard") is an extinct genus of diapsid reptile from the late Carboniferous period.

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 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.

This length difference is further pronounced because the astragalus has a long-neck and buttressed, oblique platform providing an inflexible, concave articulation with the tibia.

Although originally described as a pelycosaur,[2] Petrolacosaurus is a member of the basal diapsid order Araeoscelidia, along with the aquatic, late Pennsylvanian Spinoaequalis.

[5] 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.

Neodiapsida Spinoaequalis Petrolacosaurus Zarcasaurus Kadaliosaurus Araeoscelis Aphelosaurus Phylogenetic relationships based on De Braga & Reisz (1995) and Falconnet & Steyer (2007).

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 terrestrial nature and coniferous habitat of Petrolacosaurus bring into question whether or not this genus could climb trees.

Among other traits, the main synapomorphies are: The distal tips of the phalanges of Petrolacosaurus has very large and solidly developed lateral tuberosities.