Find one mammoth and you have a murder mystery on your hands. “What happened to this beast?” is one of the most immediate questions that comes up with the discovery of any prehistoric pachyderm. Discover and entire bonebed of the beasts, though, and the mystery becomes much more complex. What could have caused so many bulky, wide-ranging mammals to become buried in one place? That’s the head-scratcher at the core of Texas’ Waco Mammoth National Monument, and now paleontologists have a new answer.
The national monument, situated in central Texas, has yielded the remains of a various Ice Age creatures ranging from sabercats to tortoise. The elephants, of course, are the most spectacular of all, and so far the bones of at least 25 Columbian mammoths have been extricated from this one site. At least 16 of those were buried in the same level of 67,000 year old sediment, and that’s led some paleontologists to propose that this great pile of bones came together when a local flood killed a herd, thus preserving the only known Columbian mammoth social group yet found.
Death by flooding certainly paints a dramatic, heartbreaking scene of rushing water and frightened trumpeting. The fact that most of the mammoths found at the site are articulated to greater or lesser degrees – often a sign of rapid burial – and the first 15 skeletons unearthed seemed to indicate that the adults tried to form a defensive circle around the younger animals. But after re-examining the site, paleontologists Logan Wiest, Don Esker, and Steven Driese propose that the ancient bonebed actually represents a different sort of disaster. In fact, it was a lack of water that may have killed the mammoths.
Wiest and colleagues were curious about bioerosion features on the Waco bones. These are furrows, cracks, and other little clues that can indicate whether, say, a bone started to dry out from exposure, or if burrowing insects had a chance to dig into the remains. In short, they can help pin down how long a bone was exposed before finally being covered up. The paleontologists found these traces in abundance. Not only did examined bones show signs of root damage, but it appears that carnivorans, rodents, and even beetles all got a chance to scavenge the mammoth carcasses. The beasts were not killed and buried in the same event. Instead, they died and laid out on the surface for months before a blanket of sediment was laid over them.
So what killed the mammoths? Wiest and colleagues are tentative about their explanation, but they propose that death at the edges of a dwindling watering hole makes more sense than a flood (or any of the other oddball ideas like meteor impact or lightning strike). For one thing, a drying water source better explains the fish, turtle, and alligator fossils found at the site. They were victims of the drought, too. The mammoth demographics of the site fit the scenario, as well. There are no infant mammoths buried at Waco, perhaps a sign that the older animals buried there were traveling long distances to find the very last water source. Finally, Wiest and coauthors note, the death of so many animals would overwhelm the local scavengers. Opportunities could feed on viscera to their heart’s content without having to break apart many of the skeletons.
Rather than closing the case, however, the new study by Wiest and colleagues reopens it. Death by flooding doesn’t fit the scene of the disaster. The task ahead is to test the new hypothesis and look for more clues. Wiest and coauthors suggest testing the Oxygen isotope levels of the mammoth teeth – an indicator of where the animals were getting their water while alive – to see if there was a change in conditions just prior to death. How that information will alter the picture will remain to be seen, but, given that this mystery has stood for so long, turning to chemical clues is simply elementary.
Wiest, L., Esker, D., Driese, S. 2017. The Waco Mammoth National Monument may represent a diminished watering-hole scenario based on preliminary evidence of post-mortem scavenging. PALAIOS. doi: 10.2110/palo.2016.053