Odonterpeton

fossil as photographed by Moodie (1909) Odonterpeton is one of the smallest known microsaurs, with a skull length of only 6.6 millimeters (0.26 inches). == Classification ==

Earlier interpretations Like many microsaurs, Odonterpeton was at first considered a small reptile upon its brief initial description by R.L. Moodie (1909). The holotype and single known specimen is a slab (USNM PAL 4465) and counterslab (USNM PAL 4467) stored at the National Museum of Natural History (Smithsonian) in Washington, D.C. or a grade. Apart from its roughly contemporaneous time and place, Odonterpeton shares many traits with Joermungandr. These include a rounded skull, short snout, tiny forelimbs, large parietals, a pineal foramen which is shifted forward far enough to contact the frontals, and thin, distinctively textured scales. Unlike Joermungandr, Odonterpeton retains a higher tooth count, a tabular bone next to the parietal, and better-ossified limb and shoulder bones. According to the 2022 redescription, odonterpetids belong within the clade Recumbirostra, which encompasses many other long-bodied "microsaurs" with burrowing or semiaquatic adaptations. Recumbirostrans are classified among reptiles, inheriting the results of Pardo et al. (2017). Odonterpetids are particularly closely related to the Early Permian hapsidopareiids, represented by Llistrofus. Below is a cladogram showing the results of the analysis: == Paleobiology ==

With its small size, small limbs, and presumed long body, Odonterpeton has been compared to fully aquatic salamanders such as Amphiuma. Supporting bones for external gills were tentatively identified in the neck region in 1978. However, these bones were later reinterpreted as cervical ribs. The skull of Odonterpeton has large eye sockets (suggesting good vision and an active lifestyle) and voluminous subtemporal fenestrae (suggesting a strong bite), though the unspecialized teeth indicate that most of its diet was likely soft invertebrates. Odonterpeton preserves scale impressions very similar to its sister taxon Joermungandr. In Joermungandr, the ridged, slightly overlapping scales may combine both dermal bone (like many extinct amphibians) and a keratin covering (like many living reptiles). The scales could be an adaptation to help shed dirt while burrowing, an interpretation consistent with its flexible skeleton and long, streamlined body. == Paleoecology ==

Odonterpeton was one of many vertebrates preserved in the Diamond Coal Mine of Linton, Ohio. Fossils are concentrated in a thick cannel coal bed contemporaneous with the Lower Freeport Coal of the Allegheny Group. The most common fossils are fish, particularly coelacanths (Rhabdoderma), small haplolepid 'palaeoniscoids' (Microhaplolepis, Parahaplolepis), and xenacanthid shark teeth (Orthacanthus). Lungfish teeth (Conchopoma, Sagenodus) and xenacanthid spines are less common but still far from rare. Legitimate rhizodont, megalichthyid, and acanthodian fossils are unknown from the deposit. Linton preserves at least 6 temnospondyl species, including common freshwater dvinosaurians (Isodectes obtusus, Erpetosaurus radiatus) and rare terrestrial dissorophoids (Platyrhinops lyelli, Stegops newberryi) and edopoids (Macrerpeton huxleyi). During the Pennsylvanian, Linton was a tropical fluviodeltaic environment, with a large meandering river flowing northwest towards a sandy delta. The fossil-bearing cannel coal deposit is encased in a thick but geographically narrow sequence of river sandstone. Due to a fault just north of Linton, the river valley passing through the area would have been deeper and more isolated relative to the shallow rivers common in the muddy floodplains of the surrounding region. The river meanders were prone to sudden cutoffs, creating abandoned channels and broad oxbow lakes. Peat was likely deposited as organic residue accumulating in a deep, stagnant channel with deoxygenated bottom waters hostile to decomposing organisms. Freshwater and riparian animals would have continued to inhabit shallower water until the channel was fully filled in by collapsed levee debris and encroaching land plants. Linton's vertebrate faunas may have been tolerant of mild salinity levels, which may explain the absence of fully freshwater specialist amphibians like microbrachids, micromelerpetontids, and branchiosaurids. ==References==