Psephosauriscus

''Psephosauriscus'' is an extinct genus of placodont reptile from the Middle Triassic of Israel and Egypt. It is known from bony armor plates that have been found from Makhtesh Ramon in Israel's Negev desert and Araif en Naqua on Egypt's Sinai Peninsula. The genus was erected in 2002 as a replacement name for several species of the genus ''Psephosaurus'', which was named in 1957. It includes the species ''P. mosis'', ''P. ramonensis'', ''P. sinaiticus'', and a possible fourth species, ''P. rhombifer''. All species, with the exception of ''P. ramonensis'', were once assigned to the genus ''Psephosaurus''. Remains of ''P. mosis'' and ''P. ramonensis'' were found in Makhtesh Ramon, while ''P. sinaiticus'' and ''P. rhombifer'' were found in Araif en Naqua.

History

Armor of ''Psephosauriscus'' is abundant in Middle Triassic limestone in the Sinai and Negev regions, which is comparable to the Muschelkalk of European rock sequences. While the German Muschelkalk contains many nearly complete skeletons of placodonts, the only non-armor skeletal bones from the Middle East are two partial skulls and a fragmentary lower jaw, which cannot be assigned to ''Psephosauriscus'' or any other placodont because of their lack of diagnostic features. Remains of ''Psephosauriscus'' come from two deposits called the ''Beneckeia'' beds and the '' Ceratites'' beds, named after their most common ammonite fossils. The ''Beneckeia'' beds date back to the Anisian stage of the Middle Triassic, and the ''Ceratites'' beds date back to the end of the Anisian and beginning of the Ladinian stage.

Swedish paleontologist F. Brotzen described placodont armor from Makhtesh Ramon in 1957, naming the species ''Psephosaurus mosis'' and ''Psephosaurus picardi'' from the ''Beneckeia'' and ''Ceratites'' beds, respectively. ''P. picardi'' is now considered a ''nomen dubium'' because it was based on an impression of the inner surface of the carapace that did not possess any distinct features. In 1959, Austrian paleontologist Georg Haas named ''P. sinaiticus'' and ''P. rhombifer'' from Araif en Naqua.

In 2002, paleontologist Olivier Rieppel erected the genus ''Psephosauriscus'' to include most of the species named by Brotzen and Haas, which he considered distinct from the type species of ''Psephosaurus'', ''P. suevicus'', named by Eberhard Fraas from the Middle Triassic of Germany. Rieppel noted differences between the armor plates of ''P. suevicus'' and the Middle Eastern species that warranted a new genus. However, ''P. rhombifer'' could not be assigned with certainty to ''Psephosauriscus'' because the holotype specimen described by Haas had since been lost. Rieppel reported additional material from Araif en Naqua that showed similarities to ''P. rhombifer'' and the species of ''Psephosauriscus'', which he tentatively described as a species within ''Psephosauriscus'', ''Psephosauriscus cf. rhombifer''. Rieppel also named ''Psephosauriscus ramonensis'' as an entirely new species.

Species

The type species of ''Psephosauriscus'', ''P. mosis'', is known from a single specimen including portions of the carapace and plastron. The scutes that cover the armor plates do not have as well-defined a shape as the hexagonal osteoderms that lay underneath them. The osteoderms that form the plastron are relatively large. Some osteoderms have a raised keel. Two distinct ridges along either side of the shell separate the carapace, a lateral wall of smaller osteoderms, and the plastron.

''P. ramonensis'' is known from a partial carapace and connected plastron. The osteoderms of the carapace have smoother edges than most other species of ''Psephosauriscus'', and lack the keel of species like ''P. mosis''. The carapace curves into the lateral wall of the body without a separating ridge as in ''P. mosis'', but a lower ridge does separate the lateral wall from the plastron.

''P. sinaiticus'' is known from several armor fragments and larger pieces of the carapace and plastron. The osteoderms of the carapace and plastron are smaller than those of other species. As in ''P. mosis'', two ridges separate run along the side of the shell.

''P. cf. rhombifer'' can be distinguished from the three other species of ''Psephosauriscus'' by the rectangular shape of its scutes. Unlike the smooth shells of other species, the carapace of ''P. cf. rhombifer'' has a bumpy surface; each osteoderm is covered in radiating grooves and has a small depression at its center.

References

{{Taxonbar, from=Q20722690

Middle Triassic reptiles of Africa
Placodonts
Anisian life
Anisian genus first appearances
Ladinian genus extinctions
Sauropterygian genera