Enigmatic basal archosauromorph from the Late Triassic of Poland

Abstract: Choristodera, a lineage of basal archosauromorphs (Reptilia: Diapsida), first appeared in Early/Middle Jurassic (possibly Late Triassic; approximately 201 million years ago) and extended all the way into early Miocene (approximately 23 million years ago). Choristoderans are the only group of more basal archosauromorphs that survived after the Jurassic period, along with Archosauriformes (a more derived group of Archosauromorphs). The time of origin of the lineage is still speculative and preceded with long ghost lineages, a timespan when the animals were known to be alive, but are not represented in the fossil record. Unresolved phylogenetic position and inter-relationships, along with limited information about such an important group of early semi-aquatic reptiles, set choristodera in the focus of tetrapod evolution. In order to add information to gaps in the fossil records during the long temporal range of the group, new discoveries and descriptions of early choristoderan taxa are needed. Here, a description of long bones of a choristodere-like animal from the Late Triassic of Poland is presented adding information to the basal archosauromorpha and possibly shifting back the time of choristodera origin. Furthermore, bone histology analysis was conducted for the first time for choristodera, adding new information to the group. The other description of postcranial material of a possible early choristodere from Storrs et al. (1996) sets the origin of the group to Latest Triassic (Rhaetian, approximately 201 Mya). Description of newly discovered fossil material along with the histology sections of such an important group of tetrapods that are choristoders is of great significance, not only for the Triassic tetrapod communities, but for tetrapod evolution, development, ecology and life history in general.