A controversial new analysis of the fossilized remains of six squirrel-sized animals has pushed the lineage of modern mammals back to the Late Triassic — a time when the first dinosaurs emerged. The research also suggests that early mammals didn't just hide in the undergrowth.
Top image: An artist's impression of the mouse-sized Xianshou Songae, a tree-dweller in Jurassic forests. Illustration by Zhao Chuang.
The newly described animals, called haramiyids, lived in Jurassic China around 160 million years ago. They were specialized for life in the trees, featuring hands and feet that could grasp branches and a long prehensile tail not unlike those of monkeys.
"The picture that Mesozoic mammals were shrew-like insectivores that lived in the shadow of the dinosaurs needs to be repainted," noted American Museum of Natural History paleontologist Jin Meng in a National Geographic article. "They walked on the ground; they also swam, dug to burrow, and glided in the forests."
A fossil of Shenshou lui, a shrew-like creature that had long fingers and a tail adapted for life in the trees. Credit: Jin Meng.
In the new study, which now appears in Nature, the researchers report on three new species of haramiyids. As noted by Brian Switek in the NatGeo article:
[The] new haramiyids do more than expand the image of how our ancient mammalian cousins lived. The relationships among these long-enigmatic creatures suggest that the very first mammals originated early.
The traditional view is that the first true mammals evolved sometime during the Jurassic. Haramiyids, while roughly mammal-like, were thought to fall outside the group.
With complete skeletons to work from, however, Meng and colleagues found that haramiyids were true mammals after all. And given the age of the earliest known haramiyid, Meng says that mammals originated "at least in the late Triassic," between 220 million and 201 million years ago.
Wow, that's a long time ago.
Weil fully expects a backlash to this contentious finding, but adds that the study is an important addition to investigations of where mammals came from.
Read the entire article at NatGeo. And check out the study at Nature: "Three new Jurassic euharamiyidan species reinforce early divergence of mammals".