Endothiodon bathystoma

Endothiodon (/ɛndoʊθiːoʊdɔːn/ "inner tooth" from Greek:endothi (ἔνδοϑι), "within", and Greek:odon (odōn), "tooth", most likely named for the characteristic of the teeth being placed internally to the maxilla ) is an extinct genus of large dicynodont from the Late Permian. Like other dicynodonts, Endothiodon was an herbivore, but it lacked the two tusks that characterized most other dicynodonts. The anterior portion of the upper and lower jaw are curved upward, creating a distinct beak that is thought to have allowed them to be specialized grazers. Endothiodon was widespread and is found across the southern region of what was then a single large continent known as Pangea. It was originally only found in southern Africa but has now also been found in India and Brazil, which were both close to Africa at the time. The finding in Brazil marks the first dicynodont to be reported for the Permian of South America. This finding also shows that part of the Rio do Rasto Formation in Brazil can now be correlated with deposits in India, Malawi, Mozambique, South Africa, Tanzania, Zambia, and Zimbabwe. Originally there were thought to be nine species, but this was reduced down to just 3 species. A fourth species was later discovered. The four currently known species of Endothiodon are E. bathystoma, E. uniseries, E. whaitsi, and E. mahalanobsi. However, the basic distinction among the four species is size, which leads some to believe that E. uniseries, E. bathystoma, and E. whaitsi may actually represent an ontogenetic series rather than three distinct taxa. E. mahalanobsi, on the other hand, is most likely a separate, truly smaller species based on the size of both juvenile and adult forms that have been found. Apart from size, E. mahalanobsi also has a single longitudinal ridge on the snout (compared to three on the other species), a lower position of the pineal boss, and a swollen prefrontal bone.

Skull The skull of Endothiodon is most quickly recognized by the prominent upturned beak. The premaxilla and palate of the upper jaw are vaulted and allows for the upturned and pointed lower jaw to fit into this region. On the lower jaw, lateral to the teeth, is a broad groove. Endothiodon lacks a lateral dentary shelf but has a bulbous swelling of the dentary. The function of this swelling is not yet known. The pineal foramen is situated on a boss, which is high in three of the species and low in one of them (E. mahalanobsi). There is also a boss situated on the ventral margin of the jugal. The anteroventral process is an anteroposteriorly short triangular bone, while in most other dicynodonts it is long and pointed. Teeth The teeth in the upper and lower jaw differ both in morphology as well as in tooth replacement. The teeth of the upper jaw tend to be larger (5-9mm) than those of the lower jaw (<5mm) and are serrated on the anterior edge while the lower jaw has serrations on the posterior edge. Although it was originally thought that E. bathystoma had several rows of teeth on the upper jaw, it was later discovered that the tips of the teeth from the lower jaw had been left behind in the upper jaw. Now it is known that the upper teeth are roughly positioned into a single row. The entire row is moved posteriorly so that the anterior portion of the premaxilla contains no teeth but the most posterior portion still holds two teeth. The teeth are also situated internally to the edge of the maxilla. It was first thought that the dentary contained three parallel rows of teeth. Instead of arranging the teeth in longitudinal rows, they are now known to fall into obliquely arranged Zahnreihen. In each Zahnreihe, the anteriormost tooth is the oldest and the posterior most tooth is the youngest.