Taeniodonts and tillodonts:

Strange rooters and diggers

The Early Cenozoic is well known for its archaic, yet sometimes bizarrely adapted groups of mammals, which often lack any direct living counterparts. Prime examples for this are the taeniodonts and the tillodonts, two extinct orders of mammals that evolved into a roughly similar direction: Both were medium-sized to large animals that developed their front teeth into great tusks and had claws at their feet (1, 2, 3). Once regarded as closely related, it is now clear that both groups had a completely separate history.

The order Taeniodonta is a thoroughly North American group. Although fossil finds of taeniodonts are widely distributed across North America, these animals are rare in all known fossil assemblages. The first taeniodonts appear very early in the Paleocene, perhaps not more than 500000 years after the end of the Cretaceous. They probably evolved from insectivore-like animals such as Cimolestes and Procerberus that occur in the Latest Cretaceous and Early Paleocene of the Rocky Mountain region. Possible transitional forms have been reported from the Earliest Paleocene of the same area but still remain to be described in detail (1, 4).

The most primitive taeniodonts were still quite generalized animals. Onychodectes from the early Paleocene of New Mexico is their best known representative (1, 4). With the size of a large house cat or a small dog, Onychodectes was significantly larger than its insectivorous ancestors, yet its skeleton is still comparable to primitive mammals like the opossum. The skull of Onychodectes was long and narrow, quite insectivore-like and very different from the shortened, wide skulls of advanced taeniodonts. The canines that would later become great tusks were still moderate in size. Onychodectes had robust limbs, in particular a powerful forearm, and a mobile hand, which suggests a certain propensity for digging. Hands and feet each carried five digits with claws, and the tail was extremely long and heavy. It is reasonable to suspect that Onychodectes was a good climber, and the tail may have been prehensile to a certain degree. According to its dentition the animal was an omnivore. The sharp incisors and canines and the cutting premolars that Onychodectes had inherited from its ancestors were probably utilized to feed on insects, eggs and small vertebrates. As the well developed grinding capabilities of its posterior cheek teeth show, the animal also took a large amount of plant matter. Onychodectes may have used its relatively heavy forelimbs and the claws to dig up vegetables hidden in the ground. The large amount of grit that is inevitably mixed with such food would explain the heavy abrasion which can be observed on teeth of Onychodectes.

Figure 1: The primitive taeniodont Onychodectes climbing up a tree in pursuit of food. Skull and body length about 60 cm (4). From (1).

Generalized taeniodonts like Onychodectes only enjoyed a short-lived success, persisting no longer than until the Middle Paleocene. By this time better adapted herbivores and predators were narrowing the ecospace that had previously been available to such unspecialized animals. More progressive taeniodonts had already firmly occupied the niche of specialized rooters and diggers and were thus preventing their generalized cousins from evolving successfully into this direction (1, 4).

The more progressive taeniodonts, or stylinodontids, were in fact among the animals that evolved most rapidly after the disappearance of the dinosaurs - a classical example of so-called quantum evolution. While their Cretaceous ancestors did not exceed rat size, the Early Paleocene stylinodontid Wortmania already attained a body mass of about 20 kg, perhaps as little as 500000 years after the K/T boundary. All important adaptations of the stylinodontids are at least foreshadowed in Wortmania. About two million years later, the stylinodont body plan was essentially established with Psittacotherium, a robust animal of about 50 kg. Wortmania and Psittacotherium were each among the largest mammals of their time. Later stylinodontids only grew little in size. Ectoganus of the Late Paleocene to Early Eocene includes species both larger and smaller than Psittacotherium. The last of the taeniodonts, Early to Middle Eocene Stylinodon, reached a body mass of up to 80 kg and was roughly pig-sized, though there was also a rare dwarfed variant. Several much larger animals had evolved by this time, including the gigantic uintatheres (1, 4, 5).

Figure 2: Partly preserved skull of the Middle to Late Paleocene taeniodont Psittacotherium, showing huge canines and massive jaws. Total skull length about 25 cm (4). From (13).

Psittacotherium, the "parrot beast", exemplifies well the striking specializations that the stylinodontids evolved for food processing. Its skull was heavily built, with a shortened face and very robust jaws, trends that were further elaborated in later stylinodontids. As areas of muscle attachment show, the jaw musculature was powerful and the tongue was large and well-developed. In the dentition emphasis was clearly on the front teeth, which formed an impressive cutting apparatus. The canines were greatly enlarged, as were the upper incisors, though to lesser extent. Like in the gnawing teeth of rodents the hard enamel was limited to the front of these teeth, while the softer dentine was exposed at the rear, which ensured that the chisel-like teeth always had sharp cutting edges. The surface on the back served for crushing and grinding. In Ectoganus and Stylinodon the canines became ever-growing. The cheek teeth of these advanced stylinodontids were reduced to simple grinding pegs. They lost their enamel covering early in the life of an individual because of heavy wear. The full set of teeth was ever-growing in Stylinodon (1, 4).

Figure 3: Reconstructed skulls of Early Paleocene Onychodectes (left) and Wortmania (middle) and Late Paleocene to Early Eocene Ectoganus (right), scale bar 2 cm. Although not necessarily forming a single ancestor-descendant lineage, these taeniodonts illustrate how inconspicuous insectivore-like animals evolved into some of the most bizarre creatures of the Early Cenozoic. After (4).

As their skeleton testifies, the stylinodontids were heavily built animals. Their neck was short and stout. The limbs were robust and powerful, especially the forelimbs with their prominent crests and processes for the attachment of muscles. Both hands and feet had five digits, the first and fifth one much reduced on the forelimb, all equipped with large claws. The claws of the hand are particularly striking in being recurved and laterally compressed - a feature already present in the earliest stylinodontid Wortmania. Together with the powerful musculature of the forelimb, these claws must have rendered the stylinodontids excellent diggers. On the foot the claws are broader, and the digits are arranged in an arc. They provided firm support during digging, together with the long and heavy tail (1, 4).

Figure 4: Skeleton of the taeniodont Psittacotherium. Though this restoration from the 1930s is not completely up to date, it still illustrates well the digging adaptations of the forelimb, including large claws and a prominent posterior process of the ulna. Strong muscles attached to the latter for the retraction of the forelimb. Skull and body length about 110 cm (4). From (13).

In terms of their skeleton, advanced taeniodonts such as Ectoganus or Stylinodon can be compared to the aardvark Orycteropus, an active digger. Like this recent mammal, the stylinodontids may have dug burrows to live in. Yet their powerful masticatory apparatus indicates that they fed on more resistant things than the ants and termites which the aardvark prefers, presumably some kind of tough plant matter that was ripped and torn with claws and teeth. Using their powerful forelimbs and claws, the stylinodontids may have dug for tubers, roots and other underground vegetables. Like toady's pigs and peccaries, they may also have utilized their powerful snouts and enlarged canines for digging and rooting. In analogy to these recent mammals, it seems reasonable that stylinodontids did not refrain from eating carrion or other animal matter if they happened to stray across it (1, 4).

The stylinodontids seem to have been relatively slow-moving animals, adapted for strength at the expense of speed. Judging from their small brain size, they were not particularly bright. While the stylinodontids of the Eocene may have been hunted by advanced carnivorous mammals (4), earlier forms like Psittacotherium were probably not seriously threatened by the primitive predators of their time thanks to their large size, except at the waterside where crocodiles lurked. Even encounters with these dangerous reptiles may have been rare, since it has been suggested that the taeniodonts were mainly open-country and upland forms (1, 4). Roots and tubers contain large amounts of water and nutrients, and access to these natural storages may have enabled the taeniodonts to invade drier regions than it was possible for animals depending on vegetation above the ground. If this hypothesis is correct, taeniodonts could be rare in the fossil record simply because most Early Tertiary deposits in North America represent animal communities close to rivers. In their preferred upland habitat, taeniodonts could have been much more common than the fossil record reflects. In analogy to the aardvark, we may imagine them as solitary animals, combing the open landscape in search of food and returning to their burrows for shelter at the end of their daily trips.

Figure 5: Restoration of the advanced taeniodont Ectoganus, together with two large herbivores of the genus Pantolambda that belong to another extinct order of mammals, the Pantodonta. The scene is from the Middle Paleocene of the Rockies according to (17), but in fact Ectoganus was somewhat later in age.

Members of the second early Tertiary order of large herbivorous mammals with claws, the Tillodontia, were in many ways similar to the taeniodonts (2, 4, 6). Like the latter, tillodonts evolved a pair of enlarged, chisel-shaped front teeth. This was once interpreted as evidence of close relationships between taeniodonts and tillodonts. However, the rodent-like teeth represent different tooth positions in the two groups (the canines in taeniodonts and the second incisors in tillodonts) and must therefore have evolved independently. Although this had already been recognized by the end of the nineteenth century, finding more convincing links between tillodonts and other groups of mammals proved to be a difficult task.

Identifying relatives of the tillodonts was long hindered by the fact that the only substantial remains of the group came from North America. Typical tillodonts appear abruptly on this continent with Esthonyx (2, 6, 7), a genus ranging from the latest Paleocene to the Middle Eocene, although the early species are sometimes referred to a separate genus Azygonyx (8). Esthonyx already foreshadows the chisel-like second incisors of the advanced tillodonts, with the enamel becoming restricted to the front side of the teeth, but the canines and the third upper incisor are large as well in this genus. Later tillodonts like Trogosus and Tillodon further emphasized their second incisors, which became ever-growing, while other teeth in the front of the jaw were either reduced or lost (2, 6). Together with a lengthening of the snout this led to the development of gaps in the jaws between the enlarged tusks and the cheek tooth battery - an arrangement vaguely similar to that in rodents but very different from that in the short-snouted taeniodonts. Like the latter group, the tillodonts disposed of powerful jaw muscles, which were already well-developed in Esthonyx, and their teeth are often heavily worn. This suggests that the animals ingested a lot of soil and sand with their food, which may have consisted largely of roots and tubers. Grooved wear patterns on the enlarged incisors of Esthonyx provide further clues to the feeding habits of the tillodonts. These marks may have been produced as the animal was pulling roots out of the earth or stripping off leaves or buds with its tusks (2, 7).

Figure 6: Dentition of the Latest Paleocene to Early Eocene tillodont Esthonyx. Incisors (I1, I2, I3) and canines (C) are labeled in the drawing. As is typical of tillodonts, the chisel-like second incisors are particularly well-developed, although the third upper incisor and the canines are still prominent, too. Preserved jaw length about 10 cm. After (6).

The first North American tillodonts were still medium-sized animals, with body masses of approximately 10 to 30 kg in the different species of Esthonyx (8). Later members of the order, like the Early to Middle Eocene Trogosus, became much larger, weighing about 150 kg or more. These tillodonts were stout, compact animals that may have been almost bear-like in appearance. Their hands bore moderately large and recurved claws, and the musculature of their forelimb was powerful. This may indicate well developed digging capabilities like in taeniodonts, though the evidence is less conclusive than in the latter case. Perhaps the advanced tillodonts used a combination of digging and tearing with their claws and pulling with their front teeth to obtain food (2, 3). Our knowledge of the anatomy of Esthonyx is still very limited, despite the fact that this genus was moderately common in the latest Paleocene to Early Eocene of the Western Interior. What is known of its skeleton suggests rather generalized capabilities of locomotion. Like in later tillodonts, the feet of Esthonyx were equipped with laterally compressed, recurved claws which were perhaps also utilized for climbing (8, 9, 10). Again, the very small brain of Esthonyx suggests that these archaic mammals were not particularly intelligent (7).

Apart from Esthonyx, the only pre-Eocene mammal from North America that could possibly be a tillodont is the enigmatic Deltatherium from the Middle Paleocene of New Mexico (11, 12) (see here for life restoration). Although the cheek teeth of Deltatherium somehow approach those of the tillodonts, its anterior dentition looks very different, with small incisors but large saber-like upper canines that almost foreshadow those of the saber-toothed cats (13). If actually related to the tillodonts, Deltatherium must represent an early offshoot that branched away into a different, more carnivorous direction and was not involved in the ancestry of the typical tillodonts.

Figure 7: Skull of Deltatherium fundaminis, a possible Middle Paleocene relative of the tillodonts that evolved saber-like upper canines. Skull length about 14 cm. From (12).

The sudden appearance of Esthonyx in the Latest Paleocene of North America thus apparently represents an immigration event, just like the incursion of tillodonts into Europe at the beginning of the Eocene (this time probably from North America, which was then connected to Europe via Greenland). During the last decades, discoveries of primitive tillodonts in the Paleocene of South China have made it possible to identify Asia as the probable place of origin of the order (2, 14, 15). Among the Chinese forms, Meiostylodon is closest to the first North American tillodonts of the genus Esthonyx. The Chinese genus is still smaller in size, yet it definitely shows the characteristic chisel-shaped incisors of the group. Meiostylodon was found in sediments from the earlier part of the Paleocene and is therefore much older than Esthonyx. It may be closely related to the ancestor of the latter which migrated into the New World across a land bridge in the region of today's Bering Strait, probably as climatic conditions became milder in these high latitudes at the close of the Paleocene. Another probable tillodont from China, Interogale, comes from late Paleocene rocks but is nevertheless the smallest known member of the order, about half the size of small species of Esthonyx. Finally, an extremely primitive tillodont-like form, called Lofochaius, is known from the earlier Paleocene of China. Lofochaius may be close to the base of the tillodont radiation if it represents in fact a member of this order. These and other enigmatic forms from China are still poorly known, and more fossils from this region are required to get a clearer picture of the early evolution of the Tillodontia.

The Chinese discoveries also stimulated new discussions on the relationships between tillodonts and other mammals. Based in part on the new finds, the tillodonts are now usually regarded as the sister group of another now extinct order of herbivorous mammals that arose in Asia, the pantodonts (2, 11) (see also restoration above). Some advocates of this idea have even gone so far to include the tillodonts in the order Pantodonta (16). In contrast to earlier ideas, tillodonts and pantodonts do not seem to belong to the hoofed mammals or Ungulata, a group to which most large placental herbivores tended to be assigned in the past. Instead, they may be direct descendants of primitive insectivore-like mammals, just like the taeniodonts.

Both taeniodonts and tillodonts are part of the first wave of archaic mammals which dominated the Paleocene faunas but succumbed to competition with more modern mammals in the Eocene. Advanced members of the Taeniodonta and Tillodontia were probably ecologically similar to a certain degree, so they may also have competed with each other in North America where both groups occurred. This may have contributed to the disappearance of the tillodonts from this continent in the early Middle Eocene, while the order persisted into the Late Eocene in Asia. The last taeniodonts of the genus Stylinodon managed to survive until the late Middle Eocene in North America. By this time, they had to face increasing competition with the even-toed ungulates or Artiodactyla, in particular with pig-like forms that probably had similar digging and rooting habits but were larger-brained. These modern hoofed mammals were apparently capable of utilizing similar resources in a more efficient way. Though the impressive food processing apparatus of the taeniodonts had served its purpose well for a long time in the absence of more efficient competitors, it still seems to have been a rather crude solution, and highly wasteful in energy. Extremely specialized but poor in diversity as the last taeniodonts were, they were unable to produce new adaptive patterns as the old ones proved to be no longer successful. They were trapped in their narrow ecological niche which they had occupied for more than 20 million years but were finally unable to maintain (1, 2, 4).

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References:

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