An article published in the journal “Scientific Reports” reports the identification of a new species of cynodont that lived about 220 million years ago, in the Triassic period, in today’s Argentina. A team of researchers named it Tessellatia bonapartei after examining a partial skull using an unusual neutron tomography technique after a classic X-ray tomography gave limited results due to the high presence of iron in the rock. The researchers’ effort paid off because some characteristics suggest a rather close relationship with mammals, which are likely descended from a single group of cynodonts.

The Los Colorados Formation is a fossil deposit that formed between the provinces of San Juan and La Rioja in today’s Argentina. These are fossils from the Triassic period, to be precise in the Norian age. In particular, vertebrate fossils discovered in that area are well known, and among them are cynodonts, a group that has almost completely gone extinct, leaving only mammals.

The first cynodonts discovered in the Los Colorados formation were studied by Dr. José F. Bonaparte. the authors of this new study honored him by naming Tessellatia bonapartei the species discovered after the surprising results of a complex examination of the partial skull discovered.

The first attempt to examine the fossil was conducted using X-ray tomography but the rock around it contains a lot of iron, which makes the bone almost indistinguishable from the petrified earth around it. To obtain better results while avoiding the risk of damaging the fossil, the researchers subjected it to neutron tomography in a facility equipped for this examination at the Comisión Nacional de Energía Atómica. The images obtained were decidedly better because the iron creates no interference with this type of examination but still with little spatial resolution. For this reason, a new neutron tomography was conducted in Germany with the ANTARES instrument at the University of Munich. This time, the result was excellent offering a detailed 3D reconstruction of the fossil.

The image (Courtesy Gaetano et al. 2022 Scientific Reports. All rights reserved) shows on the left the fossil and on the right a 3D image of its lateral view constructed thanks to neutron tomography.

The researchers were able to examine the anatomical features of the cynodont and realized that it’s a species that was unknown, which they named Tessellatia bonapartei. The teeth are similar to those of Triassic and Jurassic cynodonts that show various changes compared to previous species and this suggests a rather close relationship with mammals. However, the palate and bones of the orbit are more similar to those of primitive cynodonts.

One interesting discovery is that of a canal in the upper jaw that was traversed by nerves when the animal was alive. This suggests that it had whiskers, just like many mammals. Generally, the teeth are the parts mostly used to try to reconstruct the relationships between cynodonts but the whiskers could be useful as well in the reconstruction of the family tree of this group, including mammals.

The set of primitive and evolved characteristics of Tessellatia bonapartei makes it difficult to establish relationships with the other cynodonts with precision. Having other bones, even better finding more specimens besides the one cataloged as PULR-V121, could offer more crucial information to put it in the right place in the cynodont family tree.

Despite the limitations given by the availability of the skull only, this small animal offers interesting information on the evolution of cynodonts. The phylogenetic and paleobiogeographic analysis of the group of cynodonts in which Tessellatia bonapartei was inserted suggests certain relationships between the various species and according to the researchers confirms the theory that mammals emerged in today’s South America, perhaps in Brazil.