Pantodonta

The Pantodonta were a group of heavily built omnivorous and herbivorous mammals that were moderately common and diverse in the Northern Hemisphere during the Paleocene and Eocene. One genus is known from South America. About two dozen genera in 10 families are currently recognized (McKenna and Bell, 1997). Most of the families are restricted to the Paleocene and are known from either North America or Asia, but not both. Pantolambdo-dontidae and Coryphodontidae extended into the Eocene as well, and the latter family is the only group of pantodonts to disperse across the three Holarctic continents. Pantodonts were long grouped with ungulates, either together with uintatheres as amblypods, as paenungulates (Simpson, 1945), or linked with arctocyonids via Deltatherium (e.g., Van Valen, 1988). In recent years there has been a growing consensus that they are closely allied with tillodonts and derived from Cimolestidae (e.g., Muizon and Marshall, 1992; Lucas, 1993; McKenna and Bell, 1997). Pantodonts tended toward ponderous size, some probably exceeding 500 kg and having skulls half a meter or more in length. They include some of the largest mammals of their time. Nevertheless, as in many clades, some of the most primitive members weighed less than 10 kg.

Pantodonts are characterized mainly by dental traits (Simons, 1960; Lucas, 1998; Figs. 7.22, 7.23). Their most important synapomorphy is the distinctive V-shaped ectoloph of the posterior upper premolars (P3-4). The upper molars are usually dilambdodont (with a W-shaped ectoloph), although the latter tendency was poorly developed in the most primitive members. In Asian families, the paracone and meta-cone tend to be closer together and more lingually situated than in the North American families (Ting et al., 1982). Nearly all pantodonts lack a hypocone, and the conules are typically small. P3-M3 usually have a moderate to deep in dentation, or ectoflexus, of the buccal margin. The lower cheek teeth are also dilambdodont and have broad, high metalophids with tall metaconids and much lower para-cristids with reduced paraconids. The dental formula is primitive, 3.1.4.3/3.1.4.3, with no significant diastemata. The incisors are small and the canines large and sometimes saberlike.

The postcranial skeleton is known in several genera and is plesiomorphic (with no elements lost or fused) and robust. The feet are pentadactyl and usually hoofed. In addition to the presence of hoofs in most genera, evidence for possible ungulate ties has come from the overall similarity of the foot skeleton, especially the tarsus, to that of arcto-cyonid "condylarths" such as Protungulatum (Szalay, 1977), but these similarities are quite possibly primitive (Lucas, 1993). At the same time, Szalay (1977) has pointed out that basal pantodonts lack the tarsal specializations present in cimolestids, an argument against their relationship. Proposed pantodont-cimolestid relationship is based primarily on dental resemblances between the oldest pantodonts and Wasatchian Didelphodus (Muizon and Marshall, 1992; Lucas, 1993).

The classification and interrelationships of pantodonts (Fig. 7.19A) are controversial, but there is general agreement that the most primitive pantodonts are the Paleo-cene genera Harpyodus and Alcidedorbignya and the family Bemalambdidae.

Bemalambdidae (Figs. 7.22B, 7.23A; see also Fig. 7.26) are represented by several small to medium-sized early Paleocene species from China (Zhou et al., 1977). They have the hallmark upper premolars of pantodonts, but the upper molars are very transverse, almost zalambdodont (not at all dilambdodont), with closely appressed or connate paracone and metacone. The stylar shelf of P3-M3 is very wide, and the ectoflexus is deeply incised. The lower cheek teeth have the typical pantodont morphology. Bemalambdids had low, rather short skulls with a broad snout, flaring zygomatic processes, and a very small braincase. Deep temporal fossae, a prominent sagittal crest, and a high mandibular coro-noid process suggest relatively better-developed temporal musculature than in later pantodonts. The postcranial skeleton was robust. One species had a particularly massive humerus, suggesting a propensity for digging.

Harpyodus (Fig. 7.22A) was a very small pantodont from the early and late Paleocene of China (Wang, 1979). Like Bemalambda, it has typical pantodont premolars and upper molars with closely appressed or connate paracone and metacone and a very wide stylar shelf. It differs from Bemalambda and most other pantodonts, however, in having a distinct hypocone, presumably an autapomorphy of this genus.

Alcidedorbignya (Figs. 7.22C, 7.23B), from the early Paleocene Tiupampa local fauna of Bolivia, may be older than the pantodonts from China (Muizon and Marshall, 1992). It is small, about the same size as Harpyodus, but differs from the Chinese forms in having the paracone and metacone of the upper molars separate rather than connate as in Harpyodus and Bemalambda. Although this feature has been interpreted as a derived trait of Alcidedorbignya, its polarity is uncertain. The stylar shelf is wide and there is a deep notch, or ecto-flexus, in the buccal margin; there is no mesostyle. Alcide-dorbignya has a shelflike postcingulum, sometimes bearing a small hypocone. The upper molars of all three genera lack the inverted V-shaped centrocrista and mesostyle, which contribute to the W-shaped ectoloph characteristic of other pantodonts. It is not clear which of these three genera has the most primitive upper molars—that is, whether separate or connate paracone-metacone is the plesiomorphic condition in pantodonts.

The Paleocene Pantolambdidae are usually considered the most primitive North American pantodonts. Pantolambda (Figs. 7.22-7.26), from the early Paleocene (Torrejonian), was typical. It was a medium-sized animal, about the size of the wolverine Gulo, characterized by typical pantodont upper premolars, dilambdodont upper and lower molars, and a robust skeleton (Matthew, 1937). Several late Paleocene

Paleocene Skull

Fig. 7.22. Upper dentition of pantodonts (anterior to left): (A) Harpyodus skull; (B) Bemalambda; (C) Alcidedorbignya; (D) Pantolambda; (E) Cyriacotherium; (F) Coryphodon. (B)-(F) show left dentition. (A from Wang, 1979; B from Zhou et al., 1977; C from Muizon and Marshall, 1992; D from Matthew, 1937; E from Rose and Krause, 1982; F modified from Osborn and Granger, 1931.)

Fig. 7.22. Upper dentition of pantodonts (anterior to left): (A) Harpyodus skull; (B) Bemalambda; (C) Alcidedorbignya; (D) Pantolambda; (E) Cyriacotherium; (F) Coryphodon. (B)-(F) show left dentition. (A from Wang, 1979; B from Zhou et al., 1977; C from Muizon and Marshall, 1992; D from Matthew, 1937; E from Rose and Krause, 1982; F modified from Osborn and Granger, 1931.)

1 cm

Fig. 7.23. Left lower dentition of pantodonts: (A) Bemalambda; (B) Alcidedorbignya; (C) Pantolambda; (D) Pantolambdodon; (E) Cyriacotherium; (F) Coryphodon. (A from Zhou et al., 1977; B from Muizon and Marshall, 1992; C from Matthew, 1937; D from Granger and Gregory, 1934; E from Rose and Krause, 1982; F modified from Osborn and Granger, 1931.)

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Fig. 7.23. Left lower dentition of pantodonts: (A) Bemalambda; (B) Alcidedorbignya; (C) Pantolambda; (D) Pantolambdodon; (E) Cyriacotherium; (F) Coryphodon. (A from Zhou et al., 1977; B from Muizon and Marshall, 1992; C from Matthew, 1937; D from Granger and Gregory, 1934; E from Rose and Krause, 1982; F modified from Osborn and Granger, 1931.)

Fig. 7.24. Skulls of North American pan-todonts. (A) Coryphodon; (B) Barylambda; (C) Titanoides; (D) Haplolambda; (E) Caenolambda; (F, G) Pantolambda. (From Simons, 1960.)

Fig. 7.24. Skulls of North American pan-todonts. (A) Coryphodon; (B) Barylambda; (C) Titanoides; (D) Haplolambda; (E) Caenolambda; (F, G) Pantolambda. (From Simons, 1960.)

Pantodonta

pantodonts seem to be closely related to, or perhaps derived from, pantolambdids, based on dental similarities. Barylambda (Figs. 7.24-7.26), best known of the Barylambdidae, was a large (650 kg), lumbering pantodont from the late Paleocene of Wyoming and Colorado. It had a small head, a graviportal pelvis and hindlimb skeleton, and an especially robust tail, suggesting an ability for bipedal browsing, as in extinct giant ground sloths (e.g., Simons, 1960; Coombs, 1983; Lucas, 1998). Barylambdids were apparently supplanted by Coryphodon in the latest Paleocene (Clarkforkian)

of western North America (Gingerich and Childress, 1983), though Coryphodon was surely quadrupedal. Other probable relatives of pantolambdids were the families Titanoideidae and Cyriacotheriidae.

Late Paleocene Titanoides (Figs. 7.24, 7.25B, 7.26) of western North America, the sole genus of Titanoideidae, differs from other North American pantodonts in having claws rather than hoofs. Based on the widespread presence of hooflike terminal phalanges in all other pantodonts, the presence of claws clearly must be a derived condition in

Pantolambda
Fig. 7.25. Skeletons of pantodonts: (A) Pantolambda; (B) Titanoides; (C) Barylambda; (D) Coryphodon. (A, B from Simons, 1960; C from Lavocat, 1955, after Patterson; D from Osborn, 1898b.)

Titanoides. The cheek teeth of Titanoides are basically similar to those of pantolambdids. Titanoides had large, saberlike upper canines, typically unworn, whose function is therefore difficult to decipher (Coombs, 1983). Apart from the claws, the skeleton is similar to that in other pantodonts. Titanoides was evidently somewhat adapted for digging but, unlike Barylambda, had no adaptations for bipedal browsing (Coombs, 1983).

Based partly on the presence of claws, Lucas (1993, 1998) allied Titanoides with the Asian pantolambdodontid Archae-olambda, the only other pantodont known to have had claws (Huang, 1977; Lucas, 1982). But Archaeolambda was much smaller (about 7 kg). Perhaps it was the combination of small body size and a relatively gracile, clawed skeleton that led Lucas (1998) to suggest that Archaeolambda was arboreal. No detailed analysis of the skeleton has been presented, however, so this novel hypothesis cannot yet be evaluated.

Pantolambdodontids (Fig. 7.23D) were mainly late Paleocene-Eocene pantodonts further characterized by shallow jaws, small canines and premolars, lower molar trigo-nids that are larger than the talonids and bear high para-conids, and especially pronounced W-shaped ectolophs on M1-2 (Lucas, 1993; Huang, 1995). One species of Pantolamb-dodon had an elongate, low rostrum with a large, high nasal opening, indicating the presence of a tapirlike proboscis (Ding et al., 1987). Another Asian group, Pastoralodontidae, had a greatly expanded mandibular angle, resembling that of a hippopotamus (Chow and Qi, 1978). Huang (1995) relegated pastoralodonts to a subfamily of Pantolambdodontidae.

Cyriacotherium ("Sunday beast") differs from all other pantodonts in having molariform premolars (Figs. 7.22E,

7.23E). P3-4 have W-shaped ectolophs rather than the simpler V-shape characteristic of other pantodonts, and the lower premolars have larger talonids. This striking contrast with other pantodonts was the basis for assigning Cyriacotherium to its own family, Cyriacotheriidae (Rose and Krause, 1982). Some subsequent researchers, however, cited this feature as grounds for excluding Cyriacotherium from the Pantodonta and instead proposed that it may be a pla-giomenid or a mixodectoid (Van Valen, 1988; Muizon and Marshall, 1992; Lucas, 1993). The matter has not been resolved, and the phylogenetic position of Cyriacotherium remains controversial; but either way there has been remarkable dental convergence to one group. Cyriacotheriids resemble pantolambdids in several derived features, including lower molar dilambdodonty, lingually placed hypo-conulids, and reduced or absent entoconids, as well as in having small incisors that enlarge slightly from Ij to I3. They lack certain dental synapomorphies of plagiomenids, such as accessory stylar cusps, skewed cheek teeth, and an enlarged Ir Cyriacotherium was somewhat smaller than Pantolambda but larger than Harpyodus and is known only from the late Paleocene of the northern Rocky Mountain region.

Coryphodontidae were large, derived pantodonts of the late Paleocene and Eocene, probably also related to panto-lambdids. However, intermediate stages leading to the unusual upper molars of Coryphodon are unknown. Named in 1845 by Richard Owen, Coryphodon has been well known from Europe and North America for more than a century It first appeared in the late Paleocene (Clarkforkian) of North America and is particularly abundant in early Eocene (Wasatchian) strata; it is also known from early

Fig. 7.26. Forefeet (top two rows) and hind feet of pantodonts. Key: B, Bary-lambda; Be, Bemalambda; C, Coryphodon; L, Leptolambda; P, Pantolambda; T, Titan-oides. (From Lavocat, 1955; Simons, 1960; Zhou et al., 1977.)

Pantolambda Foot

Fig. 7.26. Forefeet (top two rows) and hind feet of pantodonts. Key: B, Bary-lambda; Be, Bemalambda; C, Coryphodon; L, Leptolambda; P, Pantolambda; T, Titan-oides. (From Lavocat, 1955; Simons, 1960; Zhou et al., 1977.)

Eocene beds of Europe and Asia. Several other genera of coryphodontids are found in middle and upper Eocene deposits of Asia.

Coryphodon is known from numerous skulls and skeletons (Figs. 7.24-7.26, Plate 3.3). It was graviportal, with a massive skeleton and rather short limbs. As in other pantodonts, the radius and ulna are separate, as are the tibia and fibula. The distal limb segments are distinctly shortened and the feet are broad and spreading and bore short, wide hoofs. The lophodont teeth and large canines are reminiscent of the arrangement in the hippopotamus and, together with the skeleton, suggest that Coryphodon was semiaquatic, feeding largely on aquatic vegetation. Simons (1960) described grooves on the lower canines that may have been caused by pulling up tough vegetation.

Coryphodon was sexually dimorphic in body size and canine size. Large individuals approximated the size of a steer. Body mass estimates are highly variable, depending on whether they are based on teeth, particular postcranial elements, or overall body length, and which regression is employed. Estimates range from 90 to 800 kg for various species (Uhen and Gingerich, 1995). Even assuming it only reached the middle of this range, Coryphodon was without doubt one of the biggest early Eocene mammals. Its brain, however, was relatively among the smallest known for an animal of its size, about 90 g in an animal of 500 kg (Savage and Long, 1986)!

The antiquity, diversity, and plesiomorphic state of Asian members of both Pantodonta and Tillodontia suggest that continent as a center of origin of these clades.

Creodonta

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  • luis feemster
    Do wombat have claws ir hoofs?
    6 years ago

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