The Tooth of the Matter

A Tyrannosaurus rex tooth pulled from the ground in eastern Montana is roughly 66 million years old, which places it at the very end of the Cretaceous — the last few flickering millennia before the asteroid arrived and ended the whole reptilian project. The animal it came from was, in geological terms, one of the last of its kind.

Eastern Montana is the right place to be looking. The region sits atop the Hell Creek Formation, a band of mudstone and sandstone that has produced more good T. rex material than anywhere else on Earth. The first T. rex ever described, in 1902, came out of these badlands, dug up by Barnum Brown, a fossil hunter who wore a full-length fur coat into the field and once used dynamite to loosen a skeleton. The rock here erodes fast and cleanly, which means bones that spent tens of millions of years buried keep resurfacing, weathering out of hillsides a little more with every rainstorm.

A single tooth is not a modest find. T. rex teeth were serrated and thick — "lethal bananas," as one paleontologist has described them — built to crush bone rather than merely slice meat. The largest run to about a foot long including the root, and the animal grew new ones throughout its life, shedding old teeth the way a shark does. That constant turnover is precisely why isolated teeth turn up more often than intact jaws.

There is one small correction worth making, and pedantry here is entirely earned. A tooth that has weathered out to the surface and is simply picked up has not been unearthed. Unearthing implies digging, excavation, the removal of overlying material. A great many spectacular fossils are found lying in plain sight, having done the hard work of emerging on their own over a few thousand years of erosion. The finder mostly has to be walking in the right dry gully and looking down at the right moment.

Which is its own quiet marvel: a predator that died before there were flowers as we know them, before the Rocky Mountains finished rising, leaves behind a single serrated tooth. It waits out the extinction of its species, the drift of continents, and the entire span of human history — and then someone in Montana happens to glance at the ground.