They nearly went the way of the dinosaurs, but after barely surviving the brimstone, they rapidly inflated their bodies from rat-sized to cow-sized, diversified their diets and behaviors-and eventually expanded their brains-and rang in a new Age of Mammals. Finally, over the past two decades, new discoveries and research techniques have unmasked these placental pioneers. Scientists have debated these questions since the 1870s, when the first fossils of Paleocene placental mammals emerged in New Mexico. How were they able to persist when 75 percent of species died, and how did they set the foundation for the more than 6,000 species of placental mammals that thrive today, from the aerial bats to the aquatic whales to humans? Yet this account has glossed over a troubling reality: we actually know very little about the mammals that endured the extinction and persevered during the next 10 million years, during the Paleocene epoch. Textbooks often tell a simple tale: the dinosaurs died, but mammals survived and quickly took over. It lived a mere 380,000 years after the worst day in Earth history, when a six-mile-wide asteroid ended the Age of Dinosaurs in fire and fury, ushering in a new world. This fossil mammal, Ectoconus, was a revolutionary. We could tell from its pelvis that it gave birth to live, well-developed young. My colleague Tom Williamson had found a skeleton-one belonging to a big animal, weighing around 100 kilograms. On one trip, in 2014, I followed their trail into a dry creek bed sacred to the Navajo called Kimbeto-the “sparrowhawk spring.” From the other end of the wash, I heard a victory yelp. rex, but teeth with complex cusps and valleys. And then, suddenly, the bones disappear.Īs we continue walking up through the rock layers, we begin to notice a new type of fossil. The ground is littered with busted Tyrannosaurus rex limbs and chunks of vertebrae that anchored the lofty necks of sauropods some 66.9 million years ago during the Cretaceous period. As we hike across the pastel-striped badlands, we can’t help but tread on dinosaur bones.
Every spring I bring my students to the desert of northwestern New Mexico, just north of Chaco Canyon, where the ancestral Pueblo people built a great city out of rocks a millennium ago.
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