Comments on the taxonomic diversity and distribution of non-mammaliaform prozostrodontian cynodonts during the Late Triassic

AGUSTIN MARTINELLI; MARINA BENTO SOARES

Gramado

Congreso; XXIII Congresso Brasileiro de Paleontologia; 2013

The clade Prozostrodontia (Therapsida, Cynodontia) was recently created to include Therioherpeton, Prozostrodon, ictidosaurs, tritylodontids, brasilodontids and other poorly known species plus Mammaliaformes. At this clade several mammalian features (e.g., slender dentary bone, lack of prefrontal and postorbital bones, unfused dentary symphysis) first evolved, occasionally exhibiting a mosaic distribution. The distribution and relationships of the non-mammaliaform members of this clade are discussed. In our analyses, Prozostrodon is placed closer to ?brasilodontids? than in a basal position, differing from previous analyses. Although based upon scarce material, Microconodon is positioned within Prozostrodontia, closer to Prozostrodon and ?brasilodontids?, but lacking dental features (e.g., position and relative size of the main crown cusps similar to morganucodontids with the presence of cusps e and g ?kuhnecone? on middle postcanines, and lower postcanines with a tongue ?by cusp d? and groove-like ?by cusps b and e? system) which are synapomorphic of ?brasilodontids? plus mammaliaforms. Several other taxa with sectorial multi-cuspidate postcanines and constricted roots from the Late Triassic of Central and Western Europe, and India (such as Tricuspes, Pseudotriconodon and Rewaconodon), are likely representative of this radiation that, together with Therioherpeton, did not reach the dental grade observed in ?brasilodontids? but patently depart from basal non-prozostrodontians. Other taxa from the Late Triassic nested into mammaliaforms, such as Adelobasileus and Brachyzostrodon, could likely represent non-mammaliaform prozostrodontians taking into account the amount of apomorphies achieved by the well documented ?brasilodontids?. Moreover, this issue point out that further studies are needed for review the original definition of Mammaliaformes in light of the relevant interpretations on ?brasilodontid? morphology. Another point to consider is that these prozostrodontians show a Pangean distribution along the Norian but, notoriously, by the latest Triassic and Early Jurassic the record of prozostrodontians and early mammaliaforms becomes meager in South America. This latter scenario is possible affected by deficient sample in the fossil record.