When we think of mammals, we imagine a post-dinosaur world with new species thriving where dinosaurs could not. Two new discoveries in China, however indicate that not only did mammals and dinosaurs coexist as far back as the Jurassic period, but the mammals of the time were more evolved and diversified than previously believed.
Found in remarkably good condition in 2011 by a farmer in a Mongolian lake bed, Agilodocodon scansorius is the earliest known example of a tree-dwelling mammal. Its remains show evidence of long claws, spade-shaped teeth and articulated wrists and elbows, all indicating that it would have been a strong climber.
What it wasn’t, however, was a strong swimmer: It would have lived on the then-supercontinent Laurasia, amid ample flora and insect meals on the hilly shores of a lake. In all likelihood, Agilodocodon fell from its perch (perhaps evading a predator) into the lake, where it drowned. The lake bed, known as the Daohugou Formation, has proved to be a treasure trove of fossils. Past finds include a beaver-like swimming mammal and flying dinosaur species.
Fossil hunters also found Agilodocodon‘s cousin, Docofossor brachydactylus, in yet another lake bed in 2012. In many ways, Docofossor was the opposite of its tree-dwelling cousin. The earliest-known underground mammal, it was similar to modern day moles with stubby, wide paws for digging.
The two discoveries have one thing in common: They were tiny. Agilodocodon, unless unreasonably muscular, would have weighed just 40 grams. Docofossor was even smaller, clocking in at 17 grams and just 3.5 inches tall.
While scientists were aware of these early mammals, their only information came from skeletal fragments, such as skull pieces and teeth. Now, they have a much clearer picture of the state of mammals 160+ million years ago.
“What’s new with this discovery of two additional docodonts is that one of them turns out to be a subterranean mammal with highly specialized digit patterns; the other is a bona fide excellent tree climber,” Zhe-Xi Luo, a professor of organismal biology and anatomy at the University of Chicago, told LiveScience. “From their locomotory functions, we can safely infer that docodonts’ ecological diversity had a tremendous range — far more so than we previously anticipated.”
The new findings open up some exciting possibilities: Mammals either developed earlier than anyone previously thought, or they diversified with unprecedented speed.
Image: April I. Neander, the University of Chicago