In the last article, we visited the Agate Fossil Beds National Monument. Staying in Nebraska, we travel 300 miles eastward, through the Niobrara Valley and 8 million years forward in time. Here, not far from where the Niobrara meets the Missouri River, we find the Ashfall Fossil Beds. This is an amazing trove of complete, articulated skeletons of the animals that populated the Nebraskan savannah 12 million years ago.
The Ashfall Fossil Beds are a State Historical Park and a National Natural Landmark. As the name suggests, the animals preserved here met their end in a volcanic eruption. But this wasn’t just any old volcano. The eruption took place in what is now Idaho and the volcano exploded with 2000 times the power of the Mount St. Helens explosion. This titanic blast spewed out immense amounts of fine, silicate ash. Transported by the prevailing winds, the ash blanketed the land eastward for a thousand miles.
The animals at Ashfall were quickly overwhelmed. They struggled to breathe air filled with ash particles – actually tiny shards of glass – that caused swift and lethal lung damage. The birds were the first to die, probably within hours. Their remains are at the lowest level of the ash layer. Larger, hardier animals lasted longer – the rhinoceroses might have held on for weeks – but once the ash began to fall, the fate of every creature in its path was sealed. They gathered at the waterhole, seeking whatever comfort it could provide, and there they died. The wind-blown ash covered their bodies and filled the waterhole, forming a layer 12 feet deep.
The site was discovered in the 1970’s and it preserves a broad diversity of fauna, from salamanders to saber-tooth deer. However, the signature fossil is the barrel-bodied rhinoceros. As the extent of the fossil bed became clear, the “Rhino Barn” was built over it. Inside the barn, there are dozens of preserved rhinoceros skeletons. Visitors can watch the paleontologists at work, ask them questions, and even see the close-up work on video screens. Along with interactive displays at the visitors center, this makes for a good visitor experience.
The barrel-bodied rhinoceros, Teleoceras major, is short and stocky, with a circumference just about equal to its length. An entire herd of them perished at Ashfall and they are the most abundant fossil there. With over a hundred individuals unearthed, paleontologists have been able to make a systematic study of age characteristics, gender differences, and even the social structure of the herd.
To determine T. major’s diet, researchers look at the teeth. The high crowns of the molars suggest that they were grazers and fossilized grass seeds in their teeth (they rarely flossed) confirm it.
The teeth can tell us more than just the diet, though. The young rhinos seem to fall into three distinct age groups on the basis of tooth wear. This suggests that birthing was seasonal. Teeth are also the easiest way to distinguish males and females: The females have a smaller lower tusk. A population study of the young adults shows that most of them were females. The probable explanation for this is that the dominant males had harems and drove the subordinate males from the herd.
The T. major remains are the uppermost ones in the ash layer, meaning that they were the last to succumb to the ash. The cataclysm that laid them low must give one pause: Could such a thing happen again? Actually, it has. The volcano was caused by a subterranean “hot spot”, which periodically wells up lava through Earth’s crust. The most recent eruption formed the caldera of Yellowstone National Park. So the question isn’t whether there will be another big eruption, but when. If you must dwell on it, just think of all the wonderful fossils it will produce for paleontologists millions of years hence.
Comments