Fossil Friday – “Then the rats got him”

Bison are among the most common large animals in the Pleistocene Diamond Valley Lake fauna, but like almost all the remains from these deposits they are usually fragmentary. But even fragmentary fossils can provide a lot of information, including the bison right lower jaw fragment shown here.

Sometimes bonebeds or other rich deposits of fossils are formed during catastrophic events, with large numbers of organisms buried together very rapidly. That’s not what we see in Diamond and Domenigoni Valleys, however; these deposits are primarily the result of the gradual accumulation of skeletons over thousands of years. That means that an individual skeleton may have laid exposed at the surface for a significant period of time before it was eventually buried. This exposes the bones to sun, wind, and rain, all of which can cause the bone to deteriorate and the skeleton to fall apart, and to scavengers, which may damage or eat the bones, or carry pieces away. All these things contribute to the fragmentary nature of the Diamond Valley Lake specimens.

Evidence of the specific types postmortem trauma experienced by these bones is sometimes preserved. If we zoom in on the bison jaw shown above, we can see a series of grooves cut into the bone:

 These grooves are bite marks, most likely made by rodents. Bone gnawing has been documented in lots of different rodents, including rats, mice, and squirrels. It’s generally attributed to the need for rodents to keep their ever-growing incisors from becoming too long, and perhaps to provide an additional source of calcium. But the presence of these marks immediately tells us quite a bit of additional information; the bison died in a setting that didn’t result in immediate burial, and rodents were present in the environment.

Of course, from the bison’s point of view, maybe this is only adding insult to injury!


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