The first mammals
Numerous fossil finds reveal an amazing diversity of early mammals and their ancestors.
Night is falling. Kayentatherium, a cat-sized creature, tends to its newly hatched brood. Heavy rain drums on the earth wall above its burrow as the animal surveys its numerous tiny cubs. Kayentatherium could easily pass as a mammal, but its powerful jaws, distinctive teeth, and lack of external ears betray its true identity: it is a member of the cynodontia, the canid-toothed dinosaurs from which mammals are thought to evolve. Suddenly the completely sodden embankment collapses and buries the mother and her offspring in the mud.
They stayed like that for 185 million years. Finally, in the summer of 2000, a group of fossil hunters led by Timothy Rowe of the University of Texas, digging for Jurassic-era remains in the strata of the Kayenta Formation in northern Arizona, came across the scattered bones.
These fossils did not impress paleontologists at first. The researchers dug up the boulder and shipped it to the laboratory. It wasn't until nine years later that a specialist who was preparing the fossil for further analysis noticed something amazing: embedded in the rock were tiny teeth and jaws measuring just one centimetre. "They immediately stopped dissecting and started thinking about how to examine the babies non-destructively," says Eva Hoffman, Rowe's then colleague at the University of Texas, who is now a paleontologist at the American Museum of Natural History in New York. Instead of breaking up the rock and exposing the bones, the scientists extracted the fossils virtually using a micro-computed tomograph, which uses X-rays to produce high-resolution three-dimensional images.
What the researchers found in the boulder were the first known offspring of mammals, or their Lower Jurassic ancestors…