After larger terrestial animals had become extinct at the end of the Cretaceous, pantodonts, uintatheres and xenungulates were the first mammals to evolve to larger size. These animals were once united in an order Amblypoda (meaning "blunt foot"), but are today assigned to separate orders Pantodonta, Dinocerata and Xenungulata.
Pantodonts are well known from the Paleocene of North America and Asia. Their earliest representatives are still comparably small animals and may have been omnivores. Surprisingly, one of these, Alcidedorbignya, has recently turned up in the South American early Paleocene, which proves that migrations between South America and Asia via North America were possible at that time. Later pantodonts were all clearly herbivorous animals. In comparison to other groups of Paleocene mammals their skeleton is remarkably well known.
Paleocene pantodonts of Asia were relatively lightly built, like Archaeolambda, which had claws and may have been living in trees. North American pantodonts were generally larger. Paleocene representatives include both generalized forms like Pantolambda or Caenolambda and more specialized members like Titanoides and Barylambda.
Barylambda was one of the largest Paleocene mammals with an estimated mass of 650 kg. Its skeleton suggests that this cumbersome animal was browsing by resting on its massive hind limbs and its heavy tail, which allowed to raise the front of the body and to reach higher branches. This adaptation is similar to that of the extinct ground sloths like Megatherium.
Figure 1: Restorations of some pantodonts of the North American Paleocene. A. Coryphodon. B. Barylambda. C Titanoides primaevus. D. Caenolambda. E. Pantolambda cavirictus. E. Pantolambda bathmodon. After Simons (1960).
Titanoides, an animal of approximately 150 kg, had saberlike upper canines, large front limbs and strong claws at its feet. No mammal of today has a comparable anatomy of the limbs, but several extinct groups like the Eocene to Pleistocene chalicotheres show similar adaptations. Titanoides may have utilized its claws to dig for food or to tear tough plants.
Figure 2: Skeleton of the pantodont Titanoides primaevus from the Late Paleocene of North America, showing the fearsome canines and unusual proportions of this animal. Copyright by Kelly Taylor.
Figure 3: Life restoration of Titanoides primaevus. With its claws the animal may have laid bare underground roots and tubers, which were then pulled up by the hooklike lower canines and sliced off by the sabrelike upper canines. Copyright by Kelly Taylor.
The most succesfull branch of pantodonts is represented by Coryphodon and its descendants. Coryphodon was a heavily built pantodont (up to 300 kg). Especially the males had large canines similar to those of a hippo. Like this animal Coryphodon may have been amphibious and may have used its tusks to dig for plants. Coryphodon appeared first in the latest Paleocene and occured all over the northern hemisphere in the early Eocene. This longest lived branch of pantodonts survived into the Oligocene of Asia, when they were probably replaced by a group of rhinos with similar specializations. Although pantodonts became extinct at that time without leaving descendants, this archaic radiation had anticipated the adaptations of several groups of mammals that did not evolve until many millions of years later.
Figure 4: Life restoration of the pantodont Coryphodon, a large hippo-like animal that probably spent much time in the water. From Savage & Long (1986).
Uintatheres (order Dinocerata) are best known to the public by grotesque Eocene forms like Uintatherium. With an estimated body mass of up to 4500 kg, these were the first really gigantic mammals. Eocene uintatheres sported several blunt pairs of horns that were probably covered by skin. The upper canines were enlarged in a saberlike form, and the lower jaws had strong flanges to protect these teeth. These animals were browsers and may have been ecologically similar to todays rhinoceroces.
The first members of the order Dinocerata appear in the late Paleocene of Asia (Prodinoceras) and North America (Probathyopsis). The Asian and North American forms are closely related, so they are important evidence for the relative dating of faunas of the two continents. Fossil jaws of these early uintatheres occur in two classes of different size and form, which are interpreted as male (larger) and female (smaller) of the same species. Males of Probathyopsis had a body mass of approximately 300 kg, similar to the weight of the recent tapir Tapirus bairdi. Although this is small in comparison to their gigantic descendants, Paleocene uintatheres were among the largest mammals of their time. They had already started to enlarge the upper canines and to develop the corresponding flange of the lower jaw, both more pronounced in the males, but they do not yet show the development of horns. Their cheek teeth have the distinctive pattern that also characterizes later members of the order, with two crests per tooth, an adaptation to the efficient processing of plants.
Figure 5: Reconstructed skeleton of the primitive uintathere Prodinoceras martyr (= P. plantigradum) from the late Paleocene of Mongolia. Note the initial development of enlarged canines and an associated lower jaw flange. Length approximately 2.9 m. From Piveteau (1961), after Flerov (1952).
An enigmatic group of South American mammals, the order Xenungulata, may be related to the uintatheres. This poorly known order is mainly represented by the middle Paleocene genus Carodnia (meaning 'thunder' in a South American native language), the largest mammal yet discovered in the Paleocene of South American. Carodnia is known from fossils, including a partial skeleton, found in a limestone quarry in Brazil, as well as from parts of the dentition from Patagonia. Like the uintatheres, Carodnia was heavily built for its time, had large canines and cheek teeth with a crested pattern. This suggests that uintatheres and xenungulates may have had a common ancestor, which migrated either from Asia to South America or vice versa during the Paleocene. There is some evidence that this ancestor was related to small Asiatic mammals called anagalids, which in turn are related to rodents and rabbits. Consequently, it has been proposed that uintatheres could be considered as giant horned bunnies - but this hypothesis still has to be demonstrated!
Possible South American descendants of the xenungulates are the extinct pyrotheres. They are best represented by Oligocene Pyrotherium, a remotely elephant-like but also poorly known animal.
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Much of this page is based on the articles on Pantodonta and Dinocerata in Janis et al. (1998). Special thanks to Kelly Taylor for permission to use the skeleton drawing and life restoration of Titanoides.
Flerov, K. K. 1952: Novye Dinocerata iz Mongolii (New Dinocerata from Mongolia). Doklady Akademii Nauk SSSR 86, 1029-32
Janis, C. M. (ed.) 1998: Evolution of Tertiary Mammals of North America. Cambridge University Press
Piveteau, J. (ed.) 1961: Traité de Paléontologie. Tome VI: L'origine des mammifères et les aspects fondamentaux de leur évolution
Savage, R. J. G. & Long, M. R. 1986: Mammal evolution. An illustrated guide. British Museum (Natural History)
Simons, E. L. 1960: The Paleocene Pantodonta. Transactions of the American Philosophical Society, new series, 50, 1-99