Tillodontia

Tillodontia is an extinct suborder of eutherian mammals known from the Early Paleocene to Late Eocene of China, the Late Paleocene to Middle Eocene of North America where they display their maximum species diversity, the Middle Eocene of Pakistan, and the Early Eocene of Europe. Leaving no descendants, they are most closely related to the pantodonts, another extinct group. The tillodonts were medium- to large-sized animals that probably fed on roots and tubers in temperate to subtropical habitats.

Description

Tillodonts had rodent-like incisors, clawed feet and blunt, cusped teeth. They were mostly medium-sized animals, although the largest of them (such as Trogosus) could reach the size of a large bear.

The cranium ranged in length from and had a characteristic elongated rostrum, an elongated mandibular symphysis, and a shortened basicranial region. The second upper and lower incisors are large in most species, the first upper and lower premolars are small or absent, the fourth upper and lower premolars are molariform (molar-like).

When Marsh first named and described the tillodonts, he explained:

These animals are among the most remarkable yet discovered in American strata, and seem to combine characters of several distinct groups, viz: Carnivores, Ungulates, and Rodents. In Tillotherium Marsh [=Trogosus], the type [specimen] of the order, the skull has the same general form as in the Bears, but its structure resembles that of Ungulates. The molar teeth are of the ungulate type, the canines are small, and in each jaw there is a pair of large scalpriform incisors faced with enamel, and growing from persistent pulps, as in Rodents.

When naming his new "pachyderm" species Trogosus castoridens ("beaver-toothed gnawing-hog"), Leidy added that it was a fossil "which would appear to have pertained to the stock from which diverged the Rhinoceros and Mastodon, the Peccary, and perhaps the Beaver."

Classification

Franchaius from the early Eocene of Europe, Benaius, Lofochaius, Meiostylodon, and Huananius from the early Paleocene of China, and Yuesthonyx from the late Paleocene of China are primitive forms. Interogale from the late Paleocene of China, and Anchilestes probably from the middle Paleocene of China, were once assigned to Anagalida, but may also be primitive tillodonts.

The monophyly of the subfamily Trogosinae is unchallenged, but Esthonychines most likely includes the ancestors of Trogosinae and therefore is probably paraphyletic. Tillodontia is mostly known from dentaries and teeth. The cranium is best known from Trogosinae and the postcranium from Trogosus.

Azygonyx and Esthonyx from North America, Franchaius and Plesiesthonyx from Europe, and Basalina from Pakistan are all morphologically closely related but obviously geographically quite widespread. In contrast, Asian tillodonts tend to be smaller and less derived. This possible link between specimens from Pakistan and Europe with those from North America adds evidence to a faunal interchange between these continents during the early Eocene.

Order Tillodontia

Genus †Azygonyx (), dentary, postcranial fragments

Genus †Basalina (), poorly preserved jaw fragment with incomplete cheek tooth

Genus †Benaius (), left lower jaw

Genus †Chungchienia (),

Genus †Dysnoetodon (), maxilla and lower jaw

Family †Esthonychidae () (Syn. Anchippodontidae, Tillotheriidae)

Genus †Adapidium (), right lower jaw

Subfamily †Esthonychinae ()

Genus †Esthonyx (), lower mandibles, teeth

Genus †Megalesthonyx (), left mandible, teeth, feet bones

Subfamily †Trogosinae () (Syn. Anchippodus)

Genus †Tillodon (), skull

Genus †Trogosus () (Syn. Tillotherium), skull, lower jaws, teeth, vertebrae, ilium, limb bones, feet bones

Genus †Franchaius (; synonymized with Plesiesthonyx, ), less than 20 isolated teeth

Genus †Higotherium (), fragmentary right mandible, teeth

Genus †Interogale (), well-preserved mandible

Genus †Kuanchuanius (), partial mandible, teeth

Genus †Lofochaius (), poorly preserved skull with few teeth

Genus †Meiostylodon (), three isolated teeth

Genus †Plesiesthonyx (), isolated molars

Genus †Plethorodon (), partial skull with upper cheek teeth

Genus †Simplodon (), right upper jaw with cheek teeth

Family †Yuesthonychidae ()

Genus †Yuesthonyx (), left mandibles, partial skull, teeth