The fossil here is the jaw of Arundelconodon hottoni, the oldest known mammal from the eastern United States. As far as we know, Arundelconodon was a fairly typical Cretaceous mammal. It probably spent its days hiding in the forest undergrowth, only emerging at night to hunt for insects. The jawbone was discovered at Dinosaur Park in the 1990s by paleontologist Tom Lipka. The jaw itself is just over a centimeter long, and the entire animal would have been no bigger than a mouse or a shrew. In this case, however, size doesn’t matter: this is arguably the single most important fossil ever found at Dinosaur Park.

Mammals are warm-blooded, backboned animals with fur and the ability to feed their young with milk. Mammals also have unique inner ear bones and specialized teeth. Horses, dogs, kangaroos, whales, and humans are all examples of mammals. The first mammals appeared approximately 225 million years ago, around the same time as the first dinosaurs. But while dinosaurs rapidly diversified and evolved into a broad range of shapes and sizes, mammals stayed small. Our earliest ancestors were mostly tiny, secretive insect-eaters that lived in the shadow of the dinosaurs. It was only when an asteroid impact wiped out the large dinosaurs 66 million years ago that mammals were able to show their true potential and take over the world.

Mammal fossils from the Age of Dinosaurs are prized because they are extremely rare. Not only are these tiny bones hard to find, but early mammals tended to live in places like forests, where their remains were less likely to be fossilized. Nevertheless, these miniscule fossils are important because they are part of our own family tree. We quite literally owe our own existence to creatures like Arundelconodon!