1911 Encyclopædia Britannica/Tillodontia

by Richard Lydekker

TILLODONTIA, a group of mammals of uncertain position, typified by Tillotherium from the Middle Eocene of Wyoming, and perhaps including Esthonyx from the Lower Eocene of the same district, and other genera from the same horizon in both North America and Europe. In Tillotherium the skull is decidedly rodent-like, with an elongated cranial and a short facial portion, and a small brain-cavity; the jugal bone occupying the middle of the zygomatic arch. The dentition, of which the formula is i. 2/2, c. 1/1, p. 3/2, m. 3/3, also approximates to the rodent type, the canines being minute and functionless, and the first pair of incisors large and chisel-like. On these and other grounds it has been suggested that Tillotherium (of which the greater part of the skeleton is known) indicates the ancestral form of the Rodentia. Professor Max Weber considers, however, that such a view has but little justification. Relationship with the Ungulata and Carnivora has also been suggested; if there be any with the latter, it must have been with the most primitive forms, as the plantigrade feet are furnished with five toes carrying long pointed claws.

(From Marsh.)

Skull of Tillotherium fodiens. (¼ nat. size.)

Possibly Platychoerops richardsoni, from the Lower Eocene London Clay, belongs to the group.  (R. L.*)