Tillodontia is an extinct order or suborder of eutherian mammals known from the Early Paleocene to Late Eocene. They were most diverse and long-lasting in Asia, though several species also lived in North America from the Late Paleocene to Middle Eocene, and a few inhabited Europe during the Early Eocene. Leaving no descendants, their exact affinities are unclear. They may be most closely related to the pantodonts, another extinct group of early Paleogene herbivores. The tillodonts were medium- to large-sized animals that probably fed on roots and tubers in temperate to subtropical habitats.
Description
Skull
The cranium ranged in length from . The rostrum (snout) is rather long and narrow from side-to-side. The lower jaw has an elongated and fused mandibular symphysis (the contact point between each side of the lower jaw). The basicranial region (behind the eyes) is short (from front-to-back) but broad. It probably attached to large jaw muscles, since it has deep temporal fossae, flared zygomatic arches, and a sagittal crest.
Teeth Tillodonts have distinctively large and chisel-shaped incisors with restricted enamel, convergently similar to rodents and rabbits. The second upper and lower incisors are the largest teeth in the skull, while the first and third incisors are small or absent. Behind the incisors, the canines and first two premolars are also reduced or absent, leaving a wide diastema (gap) between the incisors and the cheek teeth.Rose 2006, p. 110
The upper molars are quadrate (square-shaped), having evolved a fourth major cusp, the hypocone. Many tillodont species have dilambdodont upper molars, with a W-shaped crest connecting between the stylar shelf, paracone, and metacone. The lower molars have a trigonid (front area with a trio of large cusps) which is low, only slightly taller than the talonid (rear crushing "heel"). The fourth upper and lower premolars are molariform (molar-like).
In later tillodonts of the subfamily Trogosinae, the teeth develop to further extremes. The incisors become ever-growing teeth without distinct roots, while the cheek teeth become increasingly hypsodont (high-crowned, similar to horses, as an adaptation to a gritty diet).
Postcrania
Tillodont skeletons are stocky but generalized, with clawed five-fingered paws and muscular limbs. Early tillodonts may have had scansorial habits, comfortable both climbing and foraging on the ground. Later members of the group were almost certainly fully terrestrial. Tillodonts were mostly medium-sized animals, although the largest of them (such as Trogosus) could reach the size of a large bear.
Classification
Affinities Tillodonts have long been considered enigmatic and difficult to compare to living mammals. When Marsh (1875) first named and described the tillodonts, he explained:
When naming his new "pachyderm" species Trogosus castoridens ("beaver-toothed gnawing-hog"), Leidy (1871) 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."
For much of their history, tillodonts were compared to "condylarths", a broad category of Paleocene-Eocene mammals potentially ancestral to hoofed mammals or carnivorans. The omnivorous arctocyonids were particularly frequent comparisons among "condylarths". Another perspective considers tillodonts to be closely related to pantodonts, another group of Paleocene-Eocene mammals with similar habits and distribution. Deltatherium, an odd mammal from Early Paleocene North America, has some similarities to tillodonts. Tillodonts show a combination of uniquely derived adaptations and very generalized traits, complicating comparisons to other mammals.
Subgroups 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 Esthonychinae 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 Retrieved July 2013.
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
Notes
References