''Petrolacosaurus'' ("rock lake lizard") is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years a ...

reptile from the late Carboniferous period. It was a small, long reptile, and the earliest known reptile with two temporal fenestrae (holes at the rear part of the skull). 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 (which survive to the modern day as birds) and other famous extinct reptiles such as

plesiosaurs The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared i ...

, ichthyosaurs, and

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...

s. 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 Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...

Discovery

The first ''Petrolacosaurus'' fossil was found in 1932 in

Garnett, Kansas Garnett is a city in and the county seat of Anderson County, Kansas, United States. As of the 2020 census, the population of the city was 3,242. History Garnett was platted in 1857. Garnett is named for W. A. Garnett, a native of Louisville, ...

, by a field expedition from the University of

Kansas Kansas () is a state in the Midwestern United States. Its capital is Topeka, and its largest city is Wichita. Kansas is a landlocked state bordered by Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to th ...

Natural History Museum. The party consisted of Henry H. Lane, Claude Hibbard,

David Dunkle David Hosbrook Dunkle (September 9, 1911 – January 3, 1984) was an American paleontologist. Dunkle was curator of vertebrate paleontology for the Cleveland Museum of Natural History and later associate curator for the Smithsonian Museum of Natu ...

, 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.

Description

Skull

Specimens reveal that ''Petrolacosaurus'' had a slightly elongated skull with two temporal fenestrae. The upper

temporal fenestra An infratemporal fenestra, also called the lateral temporal fenestra or simply temporal fenestra, is an opening in the skull behind the orbit in some animals. It is ventrally bordered by a zygomatic arch. An opening in front of the eye sockets ...

is located

posteriorly Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

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 The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

bone. There were also a few larger-than average teeth further back on the

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates 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. T ...

, which were probably homologous with

caniform Caniformia is a suborder within the order Carnivora consisting of "dog-like" carnivorans. They include dogs (wolves, foxes, etc.), bears, raccoons, and mustelids. The Pinnipedia (seals, walruses and sea lions) are also assigned to this group. ...

teeth retained from the common ancestor between true sauropsid reptiles and

synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...

s (mammal ancestors). On the

dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

, 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 The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

, running from the prezygapophyses to the

postzygapophysis The articular processes or zygapophyses (Greek ζυγον = "yoke" (because it links two vertebrae) + απο = "away" + φυσις = "process") of a vertebra are projections of the vertebra that serve the purpose of fitting with an adjacent vertebr ...

. These depressions allowed the vertebral column to be lighter while still retaining a sturdy build.

Pectoral girdle

The pectoral girdle in ''Petrolacosaurus'' is light in comparison to the massive girdles found in taxa such as

pelycosaur Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term ''mammal-like reptile'' had been used, and pelycosaur was considered an order, but this is ...

s. Petrolacosaurus specimens had a

clavicle The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the r ...

with an expanded ventral blade, yet shorter than that of C''aptorhinus'' or

s. Also, there is a large

cleithrum The cleithrum (plural cleithra) is a membrane bone which first appears as part of the skeleton in primitive bony fish, where it runs vertically along the scapula. Its name is derived from Greek κλειθρον = "key (lock)", by analogy with "cla ...

with a broad, rounded end.

Limbs

The limbs of ''Petrolacosaurus'' were long and slender in comparison to other primitive diapsids such as '' Araeoscelis''. The

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

and

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

in ''Petrolacosaurus'' are of relatively equal length. Unlike these two bones, the

fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity i ...

is significantly shorter than the

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

, because the

has distal articulation with the astragalus. 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. This articulation creates a virtually immobile joint between the two. ''Petrolacosaurus'' has a phalangeal formula for the manus of 2-3-4-5-3. For the pes, the formula is 2-3-4-5-4. The digits increase in length from digit I-digit V. Metatarsal IV is 3.5 times as long as metatarsal I. Many lines of post-Paleozoic reptiles have a reduction in digit IV in the manus and pes, indicating that ''Petrolacosaurus'' is more primitive than those reptiles.

Classification

''Petrolacosaurus'' is a member of the basal diapsid order

Araeoscelidia Araeoscelidia or Araeoscelida is a clade of extinct diapsid reptiles superficially resembling lizards, extending from the Late Carboniferous to the Early Permian. The group contains the genera ''Araeoscelis'', ''Petrolacosaurus'', the possi ...

, 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 known 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.

Phylogeny

Phylogenetic relationships based on De Braga & Reisz (1995) and Falconnet & Steyer (2007).

Paleobiology

Diet

''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 135 mm and the back limb is 165 mm, giving the reptile a length ratio of almost 1x as long as the dorsal-sacral region. ''Petrolacosaurus'' also has a zeugopodium in both its front and hind limbs that is longer than the stylopodium. In aquatic reptile species, a shorter zeugopodium and longer stylopodium are characteristic in fins. Both this length relationship and a wide sacral rib in ''Petrolacosaurus'' are indicative of a sprawled,

quadrupedal Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor ...

posture. The elongation of limbs were also adaptations for faster locomotion. The terrestrial nature and coniferous habitat of ''Petrolacosaurus'' bring into question whether or not this genus could climb trees. In 1942, Robert Mertens published an analysis of reptiles that had the morphological traits that conferred with the ability to climb. Among other traits, the main synapomorphies are: # Long digits # Strongly decurved digits on the front and hind limbs.Mertens, R. (1950). Notes on some Indo-Australian monitors (Sauria, Varanidae). ''American Museum Novitates''; no. 1456. The distal tips of the phalanges of ''Petrolacosaurus'' has very large and solidly developed lateral tuberosities. These processes on these bones served as attachments for ligaments. On the most distal phalanges, ''Petrolacosaurus'' had moderately decurved areas where moderate claws would have attached. The elongated digits and moderate decurved points with claws point to ''Petrolacosaurus'' being a moderate climber, perhaps only when being cornered by a predator.