Endothiodon

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[1]) 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.[2]

Endothiodon was widespread and is found across the southern region of what was then a single large continent known as Pangaea. 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.[1] 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.[1]

Originally there were thought to be nine species, but this was reduced down to just 3 species.[3] 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.[1] 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.[3] 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.[1] 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.[2] 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.[3] 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.[3] 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.[3]

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.[2] In each Zahnreihe, the anteriormost tooth is the oldest and the posterior most tooth is the youngest. There is active ongoing replacement of these tooth rows. The distal portion of each tooth is compressed from side to side and is somewhat pear shaped in cross section.

The palate shows two distinct regions that are covered in minute foramina. These areas probably had a horny covering in life. The broad groove running along the tooth row on the dentary was probably also covered by a horny layer.[2] It is possible that these regions allowed for occlusion where the upper teeth met the groove lateral to the lower teeth and the lower teeth met one of the regions of the palatine.[3] This is still under scrutiny as the palatine region is short in comparison to the lower tooth row and the second horn covered area on the palatine does not oppose any structure in the lower jaw.[3] Because the palatine region is shortened, effective occlusion for shearing would only be possible when the lower jaw was in a retracted position.

Diet
In adult 'Endothiodons the lower jaw teeth are pear shaped in cross section, compressed distolaterally, and has posterior serrated edges while the upper jaw teeth have anterior serrated edges. In the juveniles, the lower jaw is a lot smaller and more slender. The lower jaw contains one functional tooth row with 5-6 teeth. The teeth are small, conical, and pointed. The distal edge contains serrations that are just starting to appear. The juvenile teeth are much simpler and are more similar to that of a carnivore than an herbivore. It is possible that the different tooth morphology might be due to a change in diet from insectivorous or omnivorous as a juvenile to herbivorous as an adult. This would be achieved as size increases and it is more able to adapt to being herbivorous.