Tillodontia

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.[2]

The cranium ranged in length from 5 to 37 cm (2.0 to 14.6 in) 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).[1]

When Marsh first named and described the tillodonts, he explained:[3]

These animals are the 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 (=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 scalpiform 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."[4]

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.[5]

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.[6]

Azygonyx and Esthonyx from North America, Franchaius and Plesiesthonyx from Europe, and possibly Basalina from Pakistan (to the extent anything can be said about it) and an unassigned specimen from India 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 the Indian subcontinent and those from Europe and America adds evidence to a faunal interchange between these continents during the early Eocene.[7]

Suborder Tillodontia[8]

Genus †Azygonyx (Gingerich 1989), dentary, postcranial fragments

Genus †Basalina (Dehm & Oettingen-Spielberg 1958), poorly preserved jaw fragment with incomplete cheek tooth

Genus †Benaius (Wang & Jin 2004), left lower jaw

Genus †Dysnoetodon (Zhang 1980), maxilla and lower jaw

Family †Esthonychidae (Cope 1883) (Syn. Anchippodontidae, Tillotheriidae)

Genus †Adapidium (Young 1937), right lower jaw

Subfamily †Esthonychinae (Zittel & Schlosser 1911)

Genus †Esthonyx (Cope 1874), lower mandibles, teeth

Genus †Megalesthonyx (Rose 1972), left mandible, teeth, feet bones

Subfamily †Trogosinae (Gazin 1953) (Syn. Anchippodus)

Genus †Tillodon (Gazin 1953), skull

Genus †Trogosus (Leidy 1871) (Syn. Tillotherium), skull, lower jaws, teeth, vertebrae, ilium, limb bones, feet bones

Genus †Franchaius (Baudry 1992; synonymized with Plesiesthonyx, Hooker 2010), less than 20 isolated teeth[7]

Genus †Higotherium (Miyata & Tomida 1998),[9] fragmentary right mandible, teeth

Genus †Interogale (Huang & Zheng 1983), well-preserved mandible

Genus †Kuanchuanius (Chow 1963), partial mandible, teeth

Genus †Lofochaius (Chow et al. 1973), poorly preserved skull with few teeth

Genus †Meiostylodon (Wang 1975), three isolated teeth

Genus †Plesiesthonyx (Lemoine 1891), isolated molars

Genus †Plethorodon (Huang & Zheng 1987), partial skull with upper cheek teeth

Genus †Simplodon (Huang & Zheng 2003), right upper jaw with cheek teeth

Family †Yuesthonychidae (Tong, Wang & Fu 2003)

Genus †Yuesthonyx (Tong, Wang & Fu 2003), left mandibles, partial skull, teeth