Cauaht in Time 



by Richard H, Tedford 



The scene — a chase — is one that has 

 been enacted, throughout the history of 

 mammals. The time is some fourteen and 

 a half milUon years ago, and the place is a 

 mud flat in what is today northeastern Col- 

 orado. In a burst of power, a huge carni- 

 vore known as a bear-dog lunges at a tiny 

 pronghom antelope, which leaps in an at- 

 tempt to elude its pursuer In an evolution- 

 ary sense, the two main players are at op- 

 posite ends of their destiny — the bear-dog 

 being on the verge of extinction, the 

 pronghom near the beginning of its kind's 

 history. The scene, presented in the Amer- 

 ican Museum's new Hall of Mammals, 

 with mounted skeletons of predator and 

 prey, has a timeless quality. While the 

 scene is red in tooth and claw, the predator 

 and prey are caught, like the figures on 

 Keats's Grecian urn, in an action just be- 

 fore closure. 



This depiction for the new exhibition 

 came about as a result of serendipitous 

 discoveries of bones in Colorado and a 

 dramatic set of fossihzed tracks in Califor- 

 nia. It is not an exact reconstruction; the 

 predator and prey did not drop dead and 

 fossilize in tandem, but we have good rea- 

 son to beheve the scene is plausible. Both 

 the bear-dog and the pronghom skeletons 

 were discovered in northeastern Colorado 

 in successive fossil deposits laid down a 

 few hundred thousand years apart. Be- 

 cause this is a short span by geological 

 standards, we believe that the two kinds of 

 animals very likely coexisted for a time in 

 this part of North America. 



The bear-dog skeleton, the most com- 

 plete recovered in North America, was 

 collected in the 1930s by a team from the 

 University of California at Berkeley. The 

 new mount consists of a cast of this mate- 

 rial, combined with a more complete skull. 



jaws, and a few limb bones from the 

 American Museum's collection, which 

 were found in western Nebraska. The 

 pronghom mount is the actual skeleton of 

 a single individual collected by the Amer- 

 ican Museum in 1901. A third element in 

 the scene, a cast of the trackway that we 

 have placed beneath the bear-dog, was 

 collected in the early 1960s from the Mo- 

 have Desert by Raymond Alf and his stu- 

 dents at die Webb School of Califomia. It 

 coincides with the age of the bear-dog 

 skeleton. The bear-dog was the largest ter- 

 restrial predator of its time, and the paw 

 prints on the trackway fit those of the large 

 male animal represented by the skeleton. 



The bear-dog, Amphicyon ingens, was 

 neither a bear nor a dog, but a member of 

 a separate, now extinct family of carni- 

 vores, the Amphicyonidae. The evolution- 

 ary position of this family lies between 

 that of dogs, the Canidae, and bears, the 

 Ursidae, but is not ancestral to either of 

 them. Between about thirty-seven million 

 and nine million years ago, bear-dogs in- 

 habited Eurasia and North America. A 

 species closely related to A. ingens has 

 been found in contemporaneous French 

 deposits, indicating that the geographic 

 range of the giant species of Amphicyon 

 was comparable to those of present-day 

 brown and grizzly bears. 



Not counting its long, doglike tail, A. in- 

 gens was the size of a northern brown 

 bear, and the relative length of its hmbs 

 and feet was comparable to that of a griz- 

 zly. However, its body was more slenderly 

 buih, suggesting that Amphicyon, weigh- 

 ing a httle less than the 560 pounds of the 

 average grizzly, could mn faster than the 

 grizzly's thirty miles per hour. The track- 

 way shows a stride length about equal to 

 the total length of the body (less the tail) 



and also indicates that as it moved at this 

 clip, Amphicyon was pacing — moving the 

 two left legs and the two right alternately, 

 as bears are known to do at a slower stride. 

 Still, an animal the size and weight of a 

 bear-dog would not have been capable of 

 sustained pursuit. It was built for explosive 

 power rather than stamina. Like a hon, it 

 would have ambushed prey, pressing its 

 attack with a short burst of speed. 



The pronghom Ramoceros osbomi was 

 a member of the earliest group of prong- 

 homs, known as the merycodonts. These 

 homed animals appear to have Uved only 

 in North America, as do their descendants, 

 the antilocaprines, the group to which the 

 living pronghom antelope of the Ameri- 

 can West belongs. In both groups, the 



Leaving its tracks in the impressionable mud, a bear-dog 

 (Amphicyon), in one burst of ferocity, isolates a pronghom 



(Ramoceros) /ram its herd and lunges in pursuit. The 



pronghom veers in an attempt to evade the predator's teeth 



and claws. In the background, two members of the pronghom 



herd dash across the mud flat to safety. 



Painting by Marianne Collins 



90 Natural History 4/94 



