Beaver
Fossil incisor and partial jaw of castor
Zebra, llama, mastodon, ground sloth, sabertooth cat; whenever we think about the different types of fossil animals found at Hagerman Fossil Beds it is usually the exotic species that come to mind. Yet, many of the animals that are part of the monument's diverse fossil fauna are very similar or identical to animals living today in the Hagerman Valley and would be quite recognizable. One of those animals is the beaver.
The sediments that form the bluffs of the Monument above the Snake River were deposited in a floodplain of a meandering river that was dotted with ponds. Just as beavers today flourish in such wetland habitats so it was 3.5 million years ago. Beaver bones are often encountered while conducting paleontological field work on the monument, and of the few skeletons found on the monument outside of the Horse Quarry, most have been beaver. An almost complete skeleton of beaver was discovered during the summer of 1994.
The beaver that lives in the Hagerman Valley to day has the scientific name Castor canadensis. Our fossil beaver is also the genus Castor but it is placed in a separate species, californicus, in reference to where the first fossil specimen was found, California. There is a second species of living beaver Castor fiber which today lives in Europe and Asia.
One of the features that distinguishes the living beavers Castor from all other rodents is their large flat scaly tail. Because the bones of the tail, or caudal vertebrae, of the fossil beaver from Hagerman are similar in shape to the tail bones of the living beavers, it is safe to speculate that its tail looked similar to today's beaver.
In fact the skeleton and teeth of all of these species are very similar and can only be distinguished by subtle details. For example the front gnawing teeth or incisors are chisel-like and the area of the skull and jaw that anchors the jaw muscles that permit both gnawing and chewing are similar in both modem and fossil species of Castor. The resemblance of the skull and teeth in fossil and modern beavers indicates that like its modem relatives, the Hagerman beaver was capable of cutting down trees. It may be that some of the carboniferous paper shales found in the sediments at the Monument are the remnants of old beaver ponds, but we haven't found any fossil beaver dams yet.
Although there is only one genus and two species of beaver living today, there are may different kinds of fossil beavers. In North America the fossil record of beavers extends back to the Oligocene about 30 million years ago. However, these fossil beavers were not aquatic like the living beaver but instead lived on land. At Agate Fossil Beds National Monument in Nebraska, skeletons of a fossil beaver have been found preserved in the burrows that they dug. The preserved burrows showed that this particular fossil beaver had a life style similar to that of today's prairie dogs. The modern genus Castor which we know is aquatic, does not show up in the fossil record until much later in the Pliocene. The first appearance of Castor is in both North America (Hagerman) and Europe (France) in rocks of almost identical age, so at the moment we don't know if our living beaver originated in North America and then dispersed into Europe. or if an earlier beaver ancestor in Europe evolved into the living beaver which then entered North America. This is one of the many interesting problems that we hope the fossils preserved at the monument will help resolve.
Since the sediments at the Monument preserve about half a million years of earth's history. the location of each of the beaver fossils found will be important. Those beaver fossils found at lower levels in the sediments may prove to be older than those in Europe. especially if found at a level below the Peter's Gulch ash which current research considers to be 3.7 million years old. If this is the case then we may be able to safely say that the ancestors of the modern beaver first originated in North America. unless an older example is found in Europe. In either case, the question can only be resolved by carefully recording the location and level of the specimen when it is found and before it is collected. since with that information we can place the fossil in the proper time frame. Eventually comparisons will need to be made between the Hagerman and European material to see how similar or different these animals are and what their relationship to each other might be. No matter what the final results turn out to be , you can be sure that Hagerman fossils will play an important role in resolving the question -- and probably in generating some new questions.