Odonterpeton is an extinct genus of "microsaur" from the Late Carboniferous of Ohio, containing the lone species Odonterpeton triangulare. It is known from a single partial skeleton preserving the skull, forelimbs, and the front part of the torso. The specimen was found in the abandoned Diamond Coal Mine of Linton, Ohio, a fossiliferous coal deposit dated to the late Moscovian stage (Westphalian D), about 310 million years ago.
Description
Odonterpeton is one of the smallest known microsaurs, with a skull length of only 6.6 millimeters (0.26 inches). The skull is rounded and semi-triangular when seen from above, hence the species name. The snout is short, with moderately large orbits (eye sockets) which are shifted forwards and widely-spaced, facing sideways. The nasal bones (at the top of the snout) are notably small, while the parietal bones (at the back of the skull) are very large. Some older sources reported that Odonterpeton had a large singular postparietal bone at the rear apex of the skull. Later investigations consider this to have been a misidentification of cracks in the parietals, as preserved in the fossil. Restudy of the fossil has also led to the identification of a tabular bone adjacent to each parietal and above the squamosal.
There are around five small teeth in each premaxilla and more than 14 (possibly up to 19) in the maxilla. This is a high tooth count relative to most microsaurs. A high number of teeth are also present in the slender dentary bone of the lower jaw. The teeth mid-way down the maxilla are particularly large, lending their name to the genus ("Odontos" being Greek for "tooth"). The skull texture is mostly smooth, but a series of prominent pits are apparent on several bones, particularly the maxilla, frontal, and jugal. Similar pits are found on the skull of brachystelechids and Brachydectes. They may serve as canals for cranial nerves and blood vessels, or attachment sites for large scales.
The pterygoid bone of the palate (roof of the mouth) has small tooth-like denticles closely packed along the rear edge of a projection which extends outwards and forwards. This may be homologous to a structure found in amniotes: the transverse flange, a tooth-bearing horizontal ridge occupying a sharply angled rim at the back of the pterygoid. The rest of the palate of Odonterpeton is covered in a broad field of less evenly distributed denticles. Odonterpeton has a massive subtemporal fenestra, a hole behind each pterygoid which acts as a window for muscles stretching from the skull to the jaw. The braincase is well-integrated, with many constituent bones fused together. The parasphenoid (lower plate of the braincase) is relatively wide and triangular, tapering forwards to a stout cultriform process.
At least 18 narrow vertebrae are preserved behind the skull, each with a constricted form and a ventral midline keel. The forelimbs are tiny but well-ossified, with individual bones easily distinguishable. The pectoral girdle is also present, with a well-ossified clavicle and possible fragments of an interclavicle. Odonterpeton is similar to an amphiuma regarding the proportions of its forelimbs to the skull. The humerus, ulna, and radius are all very slender, and an olecranon process may be present at the elbow. The hand is diminutive, with only three fingers. The phalangeal formula (number of joints per finger) is 2-4-3. It is uncertain whether hindlimbs were present, though a disarticulated bone on the main slab may represent a femur.
Classification
Like many microsaurs, Odonterpeton was initially considered a small reptile, though most modern sources consider "microsaurs" to be a grade or clade of small reptiliomorph amphibians which branched off the tetrapod family tree prior to amniotes. Microsaur taxonomy was summarized in The Order Microsauria, a 1978 book by Robert Carroll and Pamela Gaskill. This overview proposed a two-fold division of microsaur taxonomy into Tuditanomorpha and Microbrachomorpha. Odonterpeton was assigned as a microbrachomorph based on its simple teeth, three-fingered hands, and skull roof bones which are strongly unequal in size. Other "microbrachomorphs" included aquatic forms like Microbrachis, Hyloplesion, and possibly "Brachystelechus" (Batropetes). Within Microbrachomorpha, Odonterpeton was given its own family, Odonterpetontidae.