Tillodontia

Tillodontia (Fig. 7.19A) is a relatively small and highly distinctive group of archaic mammals that was widespread across the Northern Hemisphere during the Paleogene. The fifteen or so known genera have a wide range in size, but are dentally relatively conservative. All are assigned to the same family, and most experts have recognized the group as a distinct mammalian order. The family name Tillotheri-idae technically has priority, but Esthonychidae has been almost universally used since Gazin's (1953) revision half a century ago and is therefore used here.

The dentition of tillodonts (Fig. 7.20) is characterized by rodentlike incisors with restricted enamel, the second pair enlarged and in more advanced types ever-growing. These gliriform incisors (especially in the most derived forms) bear a superficial resemblance to the hypertrophied canines of taeniodonts, but they are not homologous. In most forms the dental formula is 2.1.3.3/3.1.3.3. However, a small P1 was present in several of the most primitive genera (Paleo-cene Benaius, Lofochaius, and Yuesthonyx, and Eocene Basalina, all from Asia), and Ij and I3 were lost in middle Eocene Tillodon. The teeth between the enlarged incisors and P3 are typically reduced and may be separated by small gaps, creating a functional diastema resembling that of rodents. The cheek teeth are broad, with the trigonids only a little higher than the talonids, and they often show heavy wear, exposing broad dentinal areas. The fourth premolars above and below are submolariform. The lower cheek teeth are moderately high crowned and columnar (without cingula) on the buccal side, and lower crowned lingually. The crowns of the lower molars are incipiently selenodont and bear a metastylid cusp behind the metaconid. M3 has an extended hypoconulid lobe. The upper molars have a moderate sty-lar shelf, small conules, and a hypocone cusp or lobe that makes M1-2 more or less quadrate. A few genera developed a mesostyle. These features, together with aspects of the postcranial anatomy, suggest that tillodonts were herbivorous or omnivorous, feeding on tough vegetation, fruits, or roots and tubers. Gingerich and Gunnell (1979) observed grooves on the incisors, which may indicate that they were used to pull up roots or coarse vegetation (the name "tillo-dont" comes from Greek roots meaning "teeth that pull out"). They speculated that a diet that inadvertently incorporated considerable grit might explain the dental anatomy and wear.

Esthonychidae is usually considered to include two subfamilies, Esthonychinae and Trogosinae. The middle to late Eocene trogosines are distinguished principally by their

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extremely hypertrophied, gliriform, and continuously growing second incisors, associated with progressive hypsodonty of cheek teeth and variable reduction or loss of premolars. There is no serious challenge to the monophyly of Trogosi-nae. Esthonychines, however, are characterized by their less enlarged, rooted I2 (a primitive trait). They almost certainly include the ancestors of Trogosinae and are thereby para-phyletic. For this reason, McKenna and Bell (1997) placed all these tillodonts in a single subfamily

The skull and skeleton of tillodonts are known from only a few specimens. The skull is best known in trogosines (Fig. 7.21A), which have well-defined temporal fossae, a moderate sagittal crest, flaring zygomae, and a narrow but elongate snout associated with the enlarged incisors. Esthonychines, known from skulls that are less well preserved, appear to be basically similar. The mandibular symphysis is fused and elongate in all but the most primitive tillodonts (e.g., Azy-gonyx, Interogale).

The postcranial skeleton is best known in the dentally derived Bridgerian genus Trogosus, in which it is robust but otherwise rather generalized. The limb bones are heavy and bear strong crests, suggesting powerful musculature that is consistent with digging and perhaps also with climbing. In Esthonyx, Trogosus, and presumably other tillodonts, the carpus still has all nine unfused carpals (the primitive eutherian condition) and the manus is pentadactyl and tipped by un-gual phalanges that were curved, laterally compressed, and unfissured (Gazin, 1953). The manus bears a close overall resemblance to that of the Paleocene arctocyonid Claenodon (quite possibly a primitive resemblance), particularly in the form and arrangement of the carpals and the terminal phalanges. The other phalanges are relatively shorter and broader than in Claenodon. The tibia is noticeably shorter than the femur, which is a tendency of clawed herbivores that load the hind limbs more heavily than the forelimbs (Coombs, 1983). The pes is poorly known.

Only fragmentary postcrania are known for the more primitive esthonychines Azygonyx and Esthonyx. What little exists is not very different from the larger trogosine post-crania, but there is clearer indication of scansorial tendencies, especially in Azygonyx, as reflected by a round radial head; shallow astragalar trochlea; and curved, laterally compressed claws (Gingerich, 1989). Esthonyx was more terrestrially adapted than was Azygonyx, as inferred from the more ovoid proximal radius and well-defined patellar groove on the femur, but it was probably also a capable climber (Rose, 2001a).

Azygonyx, which includes some species formerly referred to Esthonyx (Gingerich and Gunnell, 1979), is the oldest North American tillodont and one of the most primitive members of the family. It first appears at the beginning of

Fig. 7.20. Tillodont dentitions (A-C right upper teeth and D-F right lower teeth): (A) Plethorodon; (B) Simplodon; (C, D) Esthonyx; (E) Trogosus; (F) Higo-therium. (A from Huang and Zheng, 1987; B from Huang and Zheng, 2003; C from Gazin, 1953; D from Simpson, 1937b; E, F from Miyata and Tomida, 1998.)

Fig. 7.21. Tillodonts: (A) Bridgerian Trogosus skull and mandible; (B) Wasatchian Megalesthonyx partial mandible. (A from Gazin, 1953; B from Rose, 1972.)

1 cm the Clarkforkian and is characteristic of this land-mammal age. Azygonyx survived into the early Wasatchian, where it coexisted with the most common tillodont, Esthonyx (Fig. 7.20C,D). The latter, however, persisted through the Wasatchian. The European genus Plesiesthonyx appears to be closely allied with Azygonyx (Baudry, 1992). These and other esthonychines were about the size of a raccoon or smaller.

Older or more primitive forms assigned to Tillodontia are now known from Europe and Asia. These include Fran-chaius, from the lower Eocene of Europe (Baudry, 1992); Benaius, Lofochaius, Meiostylodon, and Huananius, from the early Paleocene of China (e.g., Huang and Zheng, 1999; Wang and Jin, 2004); and Yuesthonyx from the late Paleocene of China (Tong et al., 2003). Late Paleocene Interogale and the poorly known ?middle Paleocene Anchilestes from China, both initially assigned to the order Anagalida, could also be primitive tillodonts (Ting and Zheng, 1989). Like tillodonts, Interogale has an enlarged I2, reduced I3, absent Pp and elongated hypoconulid lobe on M3. However, the incisors lack restricted enamel, a derived characteristic of all other tillo-donts, indicating that it is more primitive in this regard than other tillodonts. Anchilestes resembles tillodonts in having a shelflike ectocingulum on upper teeth, a well-developed hypocone, and relatively low trigonid, but these characters also occur in other mammalian groups. All these Chinese Paleocene forms are much smaller than North American tillodonts, and some are more primitive in having wider sty-lar shelves and lacking a distinct hypocone on upper molars. It may be that some are not tillodonts at all; indeed several authors have noted the difficulty of distinguishing them from pantodonts (e.g., Ting and Zheng, 1989; Wang and Jin, 2004)—which suggests that the two groups are closely allied.

Trogosines almost certainly arose from an esthonychine close to Megalesthonyx (Fig. 7.21B). The latter is a rare late Wasatchian form from Wyoming that is essentially intermediate between other esthonychines and trogosines in size, most dental features, and age (Rose, 1972). Megalesthonyx, like Esthonyx, had rooted second incisors, unlike the ever-growing incisors of trogosines. The presence of a meso-style on the upper molars associates Megalesthonyx with the Asian genera Adapidium and Yuesthonyx (though the latter is more primitive) and precludes the single known species from being directly ancestral to trogosines.

Trogosines (Fig. 7.21A) were relatively large mammals for the Eocene, reaching the size of small bears and weighing at least 150 kg (Lucas and Schoch, 1998a). The most derived forms were the middle Eocene genera Tillodon from North America, and Higotherium (Fig. 7.20F) and Chung-chienia from Asia. In addition to the rootless I2 (which in one specimen of Chungchienia is 26 cm long!), the Asian genera also had remarkable hypsodont to hypselodont (evergrowing) cheek teeth (Chow et al., 1996; Miyata and Tomida, 1998). The enamel of the cheek teeth of Chungchienia is limited to the buccal surfaces only and extends all the way to the open roots. The columnar, very hypsodont condition of these teeth gives them a superficial resemblance to horse

(Equus) teeth in lateral view. Chungchienia seems to have been gnawing and ingesting very abrasive material, but precisely what led to the evolution of such an unusual dentition is unknown. When known from only a few fragments, Chungchienia was believed to be either an aberrant edentate or a taeniodont. Its true affinities only became apparent after the discovery of more complete material.

Based on dental and postcranial anatomy it was long thought that tillodonts were related to, or possibly derived from, the condylarth family Arctocyonidae (e.g., Gregory 1910; Gazin, 1953; Van Valen, 1963; Rose, 1972). This view was undoubtedly influenced by the relative abundance, diversity, and antiquity of North American tillodonts, as well as the presence on this continent of diverse arctocyonids in older strata. However, the discovery of older and more primitive tillodonts in Asia, with dental resemblances to pantodonts, increasingly suggests that these two groups are more closely related (Gazin, 1953; Chow and Wang, 1979; Wang and Jin, 2004). Lucas (1993), while considering many of the resemblances between those two groups to be convergent, nevertheless regarded them as sister taxa. Other authors have emphasized the hypothesized link with pantodonts by including Tillodontia as a suborder of Pantodonta (Chow and Wang, 1979; Marshall and Muizon, 1988). Neither of these phylogenetic hypotheses has been rigorously tested, however; and it remains possible that dental similarities between tillodonts and pantodonts, such as dilamb-dodonty, arose independently (e.g., Gazin, 1953; Ting and Zheng, 1989; Lucas, 1993), perhaps from a common ancestor among the Cimolestidae. This view is consistent with recognition of both Tillodontia and Pantodonta as separate orders or suborders of the Cimolesta, as proposed by McKenna and Bell (1997).

Early Paleocene (Torrejonian) Deltatherium has been considered a pivotal genus variously assigned to arctocyonid condylarths, Pantodonta, or Tillodontia. Its phylogenetic position is still very much in dispute, largely because it exhibits certain derived features of these groups superimposed on a very primitive dental pattern. The upper molars retain a plesiomorphic tritubercular pattern with a moderately wide stylar shelf; in addition there are small conules and a prominent lingual cingulum with a small hypocone (presumably derived). The lowers have sharp cusps and moderately high trigonids (primitive) but are relatively broad and low crowned (derived). The ambiguity in its allocation suggests that it could occupy a phylogenetic position near the divergence of Tillodontia, Pantodonta, or both, linking them to Arcto-cyonia (Van Valen, 1988); however, absence of the first premolar would seem to preclude a position ancestral to either tillodonts or pantodonts. Lucas (1993, 1998) similarly regarded Deltatherium as a sister taxon of both Tillodontia and Pantodonta, perhaps closer to tillodonts than to pantodonts, but concluded that all of them evolved from a "didelphodon-tine" (i.e., cimolestid) rather than an arctocyonid ancestor.

Also potentially pertinent to the origin of Tillodontia is early Paleocene Plethorodon (Fig. 7.20A), based on a snout with complete postcanine dentition from China. Although initially referred tentatively to the Pantodonta (Huang and Zheng, 1987), Plethorodon has less transverse upper cheek teeth, a shallower ectoflexus (buccal indentation), and better developed pre- and especially postcingula (hypocone shelf)—all of which are characteristics of tillodonts. Plethorodon could be a primitive tillodont (e.g., McKenna and Bell, 1997) or another offshoot of a possible pantodont-tillodont common ancestor. The recently described Simplodon (Fig. 7.20B), based on a maxillary dentition from the middle Paleocene of China, may also be relevant to the origin of tillodonts (Huang and Zheng, 2003). Its molars lack conules and a hypocone but have a well-developed posterolingual shelf.

Ting and Zheng (1989) proposed that Anagalida (in their view then, including Zalambdalestidae) is the closest relative of Tillodontia, but this hypothesis appears unlikely. The higher-level relationships of Tillodontia therefore remain unsettled, but existing evidence favors a relationship to pantodonts and an Asian origin from cimolestids, probably very early in the Paleocene.

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