A Baby Mastodon Deathtrap (?)

Detail of a Charles R. Knight mural depicting a family a mastodons.

Fossils often turn up in unexpected places. As people have dug swimming pools, tilled farms, blasted through mountains, and quarried the land for minerals traces of ancient life sometimes come to the surface, from isolated shark teeth to skeletons of our extinct hominin relatives. Even fossil graveyards are found this way every now and then, like the one found in a southern Pennsylvania quarry a little more than a century ago.

In late April 1907 William Jacob Holland, a paleontologist and director of Pittsburgh's Carnegie Museum, received two very similar letters about a quarry near Frankstown, PA. In addition to the limestone they were after workers at the nearby American Lime and Stone Company had been finding fossilized bones, and Holland immediately sent his colleague O.A. Peterson out to investigate. It looked like the site had potential. Based upon the few bones Peterson brought back to the museum Holland thought that many more interesting fossils might be found, and so Peterson was again dispatched to begin a fuller study of the deposit.

It was not long before Peterson found a substantial cache of bones. When workers tried to blast some soil and rock off a hill to get at more limestone they discovered the entrance to a cave that had been closed for thousands of years. Inside it was a mess. The bottom of the 10-foot-high cave was covered in a mat of red soil about two feet in thickness, and over time stalactites and other stones from the ceiling had crashed to the cave floor. As Peterson and his assistants discovered this stone rain had shattered many of the bones tucked beneath the soil, but despite their fragmentary nature the paleontologists were still able to determine what kind of animals many of the fossils represented.

A cross-section of the Frankstown cave, before excavation. From the Annals of the Carnegie Museum.

The assemblage of animals had come together during the later parts of the Pleistocene, within the last 300,00 years or so, and many were starkly different from any of the modern species living in Pennsylvania. A few teeth denoted the presence of the giant ground sloth Megalonyx, while a few more bits of teeth and jaw represented extinct tapirs and peccaries. The remains of birds, bats, bison, deer, snakes, and wolves were also found in abundance, but what most interested Holland (who later wrote a report of the discoveries) was the association between the bones of an enormous bear and many young mastodons.

As reported by Holland the cave contained the fragmentary remains of several "Arctodus haplodon" (Arctodus pristinus to today's paleontologists) skeletons while the American mastodon (Mammut americanum) was represented by elements from one adult and as many as six immature skeletons. Along with the remains of deer the bones of the young mastodons and the predatory bears were among the most prevalent in the cave, and this led Holland to suggest that the site had once been a natural trap used by the bears:

The presence of the remains of numerous infant mastodons and of various species of artiodactyles in the cave, associated with the remains of the huge Arctodus haplodon, suggests that the latter, at a time when what later became a sealed cavern was still an open cleft in the rocks, preyed upon young mastodons, which they may have separated from their mothers and chased over the edge of the cliff-like wall. Falling to the bottom they became an easy prey to the great bears, just as calves to-day fall a prey both to the black bear and to the grizzly. In like manner these bears dragged into this place, which was their lair, in order to feed their young, the carcasses of deer and other hoofed animals. Or they may have driven them over the rocks, as it is possible to imagine that the young mastodons were driven. Arctodus haplodon was a bear somewhat larger than the grizzly and in weight might well have been quite equal, if not superior, to a young mastodon, judging from the size of the jaws of the latter which we possess. A young mastodon could not have been larger than a baby elephant and probably was not more than three and a half feet in height. It is an interesting picture of the life of the Pleistocene period in Pennsylvania which these fragments suggest.

The giant bears of Pennsylvania's past may have used the caves as lairs, just as saber-toothed cats and leopards used similar sites in South Africa during prehistory, but it is difficult to know whether or not Holland was right. Nothing is said of whether coprolites (preserved scat) were found or if any of the mastodon bones showed toothmarks, and so the high incidence of bear and mammoth bones might just be the result of the chance accumulation of fossils. (More infant mastodons than adults may have found their way into the cave because only as babies were they small enough to fall in.) As far as I am aware there has been no investigation on this point or how this mammal graveyard came to be, and if the area was being blasted it is possible that it could have been destroyed after the fossils were removed. If you know anything else about the Frankstown cave, please speak up in the comments.