Pleurostylodon

Pleurostylodon is an extinct genus of notoungulate belonging to the family Isotemnidae. It lived during the Middle Eocene, in what is now Argentina.

Description
This genus is known from numerous remains, mainly cranial, allowing to reconstruct its morphology. It was approximately the size of a dog, with an appearance evocating a tapir or a boar.

Pleurostylodon had a large skull, widening in the orbital arch area, and narrowing in the posterior area of the muzzle, whose terminal part was enlarged and had small incisors ; there was no diastema after the canines. The muzzle was shorter than in more derived and specialized toxodonts such as Adinotherium, and the occipital area was narrower. Several characteristics of its maxilla, of its teeth and of its unspecialized auditory region evocates Homalodotherium.

The third upper incisor was enlarged, and vaguely resembling a canine, while the canine was larger and lanceolate. The premolars and molars had an external edge outside the sinuous protoloph, with a strong parastyle and a fold of the paracon. The protoloph and metaloph were complete, except in the first upper premolar, while the protocon and the hypocon were separated in the non-worn teeth. A sort of hook and several small crests were present in variable numbers in the median valley of the molars.

Classification
The genus Pleurostylodon was first described in 1897 by Florentino Ameghino, based on fossil remains found in Argentina in Middle Eocene terrains. The type species is Pleurostylodon modicus, but Ameghino described numerous other species in subsequent years, including P. complanatus, P. crassiramis, P. recticrista, P. similis.

Pleurostylodon is one of the better known genera of the family Isotemnidae, a group of basal notoungulates comprising numerous relatively unspecialized genera of variable size. The Isotemnidae family is considered by some to be paraphyletic, in which case Pleurostylodon may be closer to the basis of Toxodontia, which includes various derived genera such as Toxodon.