Majoria - Description - Skull

Skull

There are four known skulls (three of P. madagascariensis and one of P. germainepetterae), each of which is damaged. All are missing the front (rostral) part, and three are broken at about the same place (at the paranasal cavities, at the front of the braincase), suggesting that the front part of the skull was thinner and more fragile than the back part, which consists of thick bones. MacPhee estimated maximum skull length in P. madagascariensis at 101 millimetres (4.0 in). The length of the frontal bone averages 35.4 millimetres (1.39 in) in P. madagascariensis and is 29.4 millimetres (1.16 in) in P. germainepetterae.

The robust nasal bones, preserved in a single specimen, are widest at the front, a feature unusual among placentals that is also seen in armadillos, and are also unusually flat. The ethmoid labyrinth, in the nasal cavity, was large, suggesting that Plesiorycteropus had a good sense of smell. A much larger part of the nasal septum, which separates the left and right nasal cavities, is ossified than usual in other mammals; MacPhee could find a similar condition only in sloths, which have a very short nose. The lacrimal bone is relatively large. At it is a single lacrimal canal, which opens near the suture between the frontal and lacrimal bones, like in lipotyphlans. There is a small tubercle (absent in aardvarks) near this opening. The orbital cavity, which houses the eyes, is relatively short, similar to the situation in pangolins and armadillos. A distinct tubercle is present on the suture between the frontal and parietal bones in P. germainepetterae, but not P. madagascariensis. P. madagascariensis has a more expansive braincase and a less pronounced narrowing between the orbits. The foramen rotundum, an opening in the bone of the orbit, is present. The optic canal, which houses the nerves leading to the eyes, is narrow, suggesting that the eyes were small. As in pangolins and xenarthrans, little of the squamosal bone can be seen from above. The temporal lines on the braincase, which anchor muscles, are located lower in P. germainepetterae. Like in aardvarks, the parietals are relatively large. An interparietal bone is present. Unlike in anteaters and pangolins, the occiput (the back of the skull) is flat and vertical. Plesiorycteropus lacks notches above the foramen magnum (the opening that connects the brain to the spinal cord), which are present in aardvarks. The nuchal crest, a projection on the occiput, is straight in P. madagascariensis, but in P. germainepetterae it is interrupted in the middle, similar to the situation in armadillos and hyraxes.

In their descriptions of Plesiorycteropus, Lamberton and Patterson posited different interpretations of the location of the mandibular fossa, where the mandible (lower jaw) articulates with the cranium. MacPhee found problems with either interpretation and suggested that the true mandibular fossa was part of the area Lamberton identified as such, at the side of the braincase. The fossa is small and low, suggesting that the animal was not capable of powerful biting. At the back of this fossa is a pseudoglenoid proces, which is more prominent in P. germainepetterae. In P. germainepetterae but not P. madagascariensis, a small opening, perhaps the vascular foramen, is present next to the foramen ovale. The petrosal bone forms a relatively large portion of the roof of the tympanic cavity, which houses the middle ear; parts of the petrosal are more developed in P. madagascariensis. Endocasts (casts of the inside of the skull) indicate that the neopallium part of the brain was relatively small.