A taxonomic revision of the genus Tanystropheus (Archosauromorpha, Tanystropheidae)

Abstract

Tanystropheus represents one of the most characteristic genera of Triassic reptiles and is typified by easily recognizable, hyperelongate cervical vertebrae. First described in 1852, isolated cervical vertebrae and other remains have been referred to the genus and various species have been erected and rejected based on this material. This has resulted in a complicated and convoluted taxonomic history of the genus and confusion as to the validity of species and the referral of specimens. With the exception of the well-represented T. longobardicus, the five other species of Tanystropheus are known from isolated elements or a single, partial specimen. Here, we provide a complete overview of the taxonomic history and a revision of the genus based on first hand observations of the type material of most of the species. From this, we conclude that T. conspicuus and T. haasi should be considered nomina dubia and that T. meridensis constitutes a junior synonym to T. longobardicus. Furthermore, T. longobardicus can be subdivided into two discrete morphotypes that might represent separate species. However, a more detailed study is required to test this hypothesis. Finally, T. fossai is considered distinctly different from the other Tanystropheus taxa and is therefore referred to a separate genus, Sclerostropheus.

Cranial morphology of the tanystropheid Macrocnemus bassanii unveiled using synchrotron microtomography

“…This is not the case in M. bassanii, making the external naris elongated and slit-like. Protorosaurus speneri and the large morphotype of Tanystropheus longobardicus lack a postnarial process entirely, however it is present to a degree in the small morphotype 7,16,38 . www.nature.com/scientificreports/ Maxilla.…”

“…Palatal teeth are known in Prolacerta broomi, Euparkeria capensis and Azendohsaurus madagaskarensis, but are absent in Eryhthrosuchus africanus and possibly intraspecifically variably present in Garjainia prima [34][35][36][37]65 . In Tanystropheus longobardicus, they are present in small-sized specimens, but absent in large-sized specimens, which might indicate a taxonomic distinction between these two morphotypes 16 . Like M. bassanii, Prolacerta broomi has medially projected teeth posteriorly on the element and teeth on the ventral side more anteriorly, although these anterior teeth are described to be ordered in a single file row 60 .…”

“…Anteriorly, the teeth are sharper and narrow and appear on the medial side of the element, sometimes even pointing directly medially, as in Prolacerta broomi and proterosuchids 44,66,67 . As for the palatine, only the pterygoids of the small-sized specimens of Tanystropheus longobardicus bear teeth on their anterior ramus 16 . It is difficult to determine how many single rows of teeth there are, due to overlapping other bones, but the teeth seem to align in an anterio-posterior direction and not branch out (antero)laterally.…”

The craniomandibular anatomy of the early archosauriformEuparkeria capensisand the dawn of the archosaur skull

“…[ 138 ] on another species, Macrocnemus fuyuanensis , revealed a pair of widely separated laterally oriented rows, the anterior one could be interpreted as a separate portion of T3 that would be scored as state 1 of character 195 [ 36 ]. Furthermore, Tanystropheus appears to show size-related differences in palatal dentition, with dentition being more extensive in smaller individuals [ 140 ]. Given the complexity and variety of different arrangements present, it may be preferable to score the presence of each field (T1–3), and the form of each field as present (i.e.…”

“…Due to its plesiomorphic ossification pattern, it has previously been suggested for these reasons that the taxon could be juvenile [151], but this has seemingly been ruled out by histological analysis [152]. Furthermore, although there is certainly linkage of smaller size to increased palatal dentition in other taxa [140], this may represent intraspecific variation rather than ontogeny. Given that the taxon appears unlikely to be a juvenile, its broadly plesiomorphic morphology and completeness continue to make the taxon an excellent outgroup choice in phylogenetic analysis of early crown archosaurs.…”