A new look at the mammals who lived with dinosaurs

Nature rounds up a “rash of fossil finds” revealing the lives of the first mammals, shedding a little more light on how our forebears survived the extinction of the dinosaurs:

They dug up the block and shipped it back to the laboratory for safekeeping. It wasn’t until nine years later that a specialist preparing the fossil for study noticed something startling: embedded in the block were tiny teeth, and jawbones just 1 centimetre in length. “Immediately they stopped the preparation and thought about ways of non-destructively examining the babies,” says Eva Hoffman, at Texas with Rowe at the time and now a palaeontologist at the American Museum of Natural History in New York City. Instead of breaking into the rock, Hoffman and Rowe digitally extracted the bones with a microcomputed tomography (microCT) scanner, which uses X-rays to create fine-grained 3D images.

What they found inside the rock were the first known babies of mammals or their relatives from the Jurassic — and not just one, but 38 of them, placing this among the most significant discoveries related to mammal origins made in the past decade1. Kayentatherium is at the cusp of mammalhood — and researchers say that it provides crucial insights into which traits define mammals and which were present in their earlier relatives.

Kayentatherium’s skeleton is mammal-like in many ways, but the fossil suggested that it still reproduced very much like a reptile, giving birth to large litters of small-brained offspring.

The find is among a mass of discoveries in the past 10–20 years that are illuminating milestones in mammalian evolution. Although major finds are emerging all over the world, the largest number are coming out of China; together, they have overturned the now dated belief that dinosaur-era mammals were small, unremarkable insectivores, eking out a life in the shadows of the giant reptiles.

The fossils have revealed that early mammals were ecologically diverse and experimenting in gliding, swimming, burrowing and climbing. The discoveries are also starting to reveal the evolutionary origins of many of the key traits of mammals — such as lactation, large brains and superbly keen senses.

Most of China’s mammal fossils were formed when volcanoes buried the animals in ash — and they are exquisitely detailed. Typical mammal fossils from the Mesozoic era (252 million to 66 million years ago) are little more than teeth and jaw fragments, but Chinese specimens often have entire skeletons, with fur, skin and internal organs.

Among the first innovations that researchers began to find in fossil form were those to do with locomotion. In 2006, [American Museum of Natural History paleontologist Jin] Meng’s team reported the first gliding mammal, 164-million-year-old Volaticotherium, which had wing membranes made of furry skin, like today’s flying squirrels. In 2017, [University of Chicago palaeontologist Zhe-Xi] Luo’s team added Vilevolodon and Maiopatagaium, which lived at around the same time and belonged to a group called the haramiyids. These animals swooped between the trees alongside some of the first flying dinosaurs, taking advantage of previously unexploitable food resources.

Researchers found other specializations that they assumed had evolved only later: Agilodocodon could climb trees and gnawed into bark to feast on sap; the platypus-sized river-dweller Castorocauda had webbed feet and a beaver-like tail for swimming; and Docofossor had paws and claws for digging, and looked like a modern mole.

And a few dinosaur-era mammals were much bigger than suspected, too. Repenomamus was 12–14 kilograms, and the racoon-sized Vintana weighed in at 9 kg. “It’s exciting that we kind of busted the old myths that early mammals came from a very humble generalized ancestor,” says Luo.

The finds are not solely from China. Important fossils are also coming from the United States, Spain, Brazil, Argentina, Madagascar and Mongolia.

Plenty of great illustrations at the link.