Petrolacosaurus Kansensis
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''Petrolacosaurus'' ("rock lake lizard") is an
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
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 ...
of
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 Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchocephalia ...
from the late
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...
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 Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is the ...
(which survive to the modern day as
birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
) and other famous extinct reptiles such as plesiosaurs,
ichthyosaur Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, alt ...
s, 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 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 period of the Paleo ...
.


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 U.S. state, state in the Midwestern United States, Midwestern United States. Its Capital city, capital is Topeka, Kansas, Topeka, and its largest city is Wichita, Kansas, Wichita. Kansas is a landlocked state bordered by Nebras ...
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 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 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 teeth retained from the common ancestor between true
sauropsid Sauropsida ("lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia. Sauropsida is the sister taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early syn ...
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 In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In ...
, 2 sacrals, and 60
caudal vertebrae 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 i ...
. 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 i ...
, running from the
prezygapophyses 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 ...
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 pelycosaurs. 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 pelycosaurs. Also, there is a large cleithrum 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 (plural, : radii) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', ...
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 ...
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 conn ...
, because 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 conn ...
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, 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 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 Robert Friedrich Wilhelm Mertens (1 December 1894 – 23 August 1975) was a German herpetologist. Several taxa of reptiles are named after him.Beolens, Bo; Watkins, Michael; Grayson, Michael (2011). ''The Eponym Dictionary of Reptiles''. Baltimor ...
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
ligament A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the: * Peritoneal l ...
s. On the most distal
phalanges The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
, ''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.


References


Further reading

* deBraga, M. & Reisz, R. R. (1995) - A new diapsid reptile from the uppermost Carboniferous (Stephanian) of Kansas. ''Palaeontology'' 38: 199-212 * Falconnet, J. & Steyer, J.-S. (2007) - Revision, osteology and locomotion of ''Aphelosaurus'', an enigmatic reptile from the Lower Permian of France. ''Journal of Morphology'' (abstract of the 8th International Congress of Vertebrate Morphology, Paris, July 2007): 38. * {{Taxonbar, from=Q132525 Prehistoric reptile genera Prehistoric diapsids Carboniferous reptiles of North America Carboniferous United States Terrestrial locomotion Fossil taxa described in 1945