Hyloplesion
Introduction '''''Hyloplesion''''' is an extinct genus of microbrachomorph microsaur. It is the type and only genus within the family '''Hyloplesiontidae'''. Fossils have been found from the Czech Republic near the towns of Plzeň, Nýřany, and Třemošná, and date back to the Middle Pennsylvanian. The type species is ''H. longicostatum'', named in 1883.Two species belonging to different genera, ''Seeleya pusilla'' and ''Orthocosta microscopica'', have been synonymized with ''H. longicostatum'' and are thought to represent very immature individuals.
Description Illustration of an immature ''Hyloplesion'', originally called ''Seeleya pusilla'', by Antonin Fritsch. ''Hyloplesion'' was about as large as a medium-sized salamander, with the length of known specimens ranging from 17-77mm.The skull is triangular in shape. Unlike many other microsaurs, the palate of ''Hyloplesion'' contains large vacuities, or openings. The fifth maxillary tooth is enlarged and resembles a canine. The skull of ''Hyloplesion'' superficially resembles that of the unrelated romeriid reptile ''Romeria'' in lateral view due to similarities in the pattern of the dermal bones and the hooked shape of the premaxilla. However, the skull differs from that of ''Romeria'' in dorsal view, as the parietals are much wider in ''Hyloplesion''.The atlas arch also resembles those of romeriids and, unlike other microsaurs, is unswollen. The trunk is elongate, with thin ribs extending from each vertebra. The presence of a large scapulocoracoid in ''Hyloplesion'' distinguishes it from ''Microbrachis''. The limb bones are small and robust, with the hindlimbs being quite larger than the forelimbs . Like other microbrachomorphs such as ''Microbrachis'', ''Hyloplesion'' has only three digits in the manus, a condition known as tridactyly.
Paleobiology A range of morphological characteristics makes it difficult to determine whether ''Hyloplesion'' was primarily terrestrial or aquatic. Although the digits are well ossified, the reduced number of toes in ''Hyloplesion'' is seen as an adaptation for an aquatic lifestyle. There is no evidence for lateral-line canals in the skull, although they were most likely present, separated from the skull by a layer of connective tissue.
References
External links * ''Hyloplesion'' in the Paleobiology Database