Feature This rat-sized Liaoconodon hui is one of many fossils from northern China that are sharpening the picture of how mammal traits evolved. THE MAKING OF MAMMALS An explosion of fossil finds is allowing our early mammal ancestors to leap out of the shadow of the dinosaurs. By John Pickrell ight is falling in the early Jurassic dug up the block and shipped it back to the cusp of mammalhood — and researchers say 185 million years ago, and the Kay- laboratory for safekeeping. It wasn’t until that it provides crucial insights into which traits entatherium is tending to her newly nine years later that a specialist preparing the define mammals and which were present in hatched brood. Heavy rains pum- fossil for study noticed something startling: their earlier relatives. mel the bank above her den as she embedded in the block were tiny teeth, and Kayentatherium’s skeleton is mammal-like looks over her dozens of tiny young. jawbones just 1 centimetre in length. “Immedi- in many ways, but the fossil suggested that it She is about the size of a large cat ately they stopped the preparation and thought still reproduced very much like a reptile, giving and could easily pass for a mammal, about ways of non-destructively examining birth to large litters of small-brained offspring. Nbut her large jawbone, characteristic teeth and the babies,” says Eva Hoffman, at Texas with By contrast, “mammal moms invest a lot in a lack of external ears give her away: she is a cyno- Rowe at the time and now a palaeontologist smaller number of babies, each of which has dont, a member of the group from which mam- at the American Museum of Natural History a better chance of surviving”, says Hoffman. mals evolved. At some point without warning, in New York City. Instead of breaking into the Mammal babies spend longer under their the sodden bank collapses, entombing the rock, Hoffman and Rowe digitally extracted parents’ care, developing relatively large hatchlings and their mother in mud. the bones with a microcomputed tomography brains, whereas these fossil hatchlings had There they remained until the summer of (microCT) scanner, which uses X-rays to create well-developed bones and teeth, hinting that 2000, when a fossil-hunting crew led by Tim- fine-grained 3D images. they could fend for themselves and were not othy Rowe at the University of Texas at Austin What they found inside the rock were the first nourished by milk, as all mammals are today. 2011 , 181–185; 472 chanced upon their scattered bones among known babies of mammals or their relatives The find is among a mass of discoveries rocks of the Kayenta Formation in northern from the Jurassic — and not just one, but 38 of in the past 10–20 years that are illuminating Arizona. them, placing this among the most significant milestones in mammalian evolution. Although ET AL. NATURE That initial encounter with the fossils did discoveries related to mammal origins made major finds are emerging all over the world, 1 little to impress the palaeontologists. They in the past decade . Kayentatherium is at the the largest number are coming out of China; J. MENG 468 | Nature | Vol 574 | 24 October 2019 ©2019 Spri nger Nature Li mited. All ri ghts reserved. ©2019 Spri nger Nature Li mited. All ri ghts reserved. together, they have overturned the now dated interested in understanding the evolution of belong to long-extinct subgroups, says Meng. belief that dinosaur-era mammals were small, the mammalian ear, for instance. In contrast to the panoply that existed in the unremarkable insectivores, eking out a life in The finds overturned previous dogma. “We Mesozoic, mammals today come in just three the shadows of the giant reptiles. used to say that during the time of dinosaurs, varieties: placentals, which make up the major- The fossils have revealed that early mammals mammals were totally unspectacular. That they ity of species and include humans; marsupials, were ecologically diverse and experimenting were just these little mousey things scamper- such as kangaroos and koalas, in which gesta- in gliding, swimming, burrowing and climbing. ing around in the shadows,” says Brusatte. But tion in the womb is brief and development The discoveries are also starting to reveal the these animals “were undergoing their own continues in a pouch; and the egg-laying mono- evolutionary origins of many of the key traits evolutionary explosion”, he says. tremes, represented only by the platypus and of mammals — such as lactation, large brains Mammals first appeared at least 178 million several echidnas. “But in geological history, and superbly keen senses. years ago, and scampered amid the dinosaurs there were many other groups such as multitu- “The explosion of early-mammal discover- until the majority of those beasts, with the berculates, triconodonts and haramiyids,” says ies, particularly from China, over the last two exception of the birds, were wiped out 66 mil- Meng. “Mammals were actually very diverse in decades has been eye-opening, mind-numbing lion years ago. But mammals didn’t have to wait the Jurassic.” and absolutely dazzling,” says David Krause, for that extinction to diversify into many forms Some, such as the shrew-like Juramaia — a vertebrate palaeontologist at the Denver and species. “These new discoveries document described by Luo’s team in 2011 and dated to Museum of Nature and Science in Colorado. a huge, hitherto-undreamed-of ecological 160 million years ago — are among the earliest This avalanche of discovery is also stirring up diversity,” says Richard Cifelli, a palaeontolo- placental mammals and therefore have the debate: some researchers disagree over which gist at the University of Oklahoma in Norman. potential to be our ancestors11. fossil groups are true mammals and the shape Among the first innovations that researchers And a few dinosaur-era mammals were of the mammal family tree. “We want to under- began to find in fossil form were those to much bigger than suspected, too. Repe- stand our early history in the language of evolu- do with locomotion. In 2006, Meng’s team nomamus was 12–14 kilograms, and the tionary biology, and that’s what fires me up,” says reported the first gliding mammal3, 164-mil- racoon-sized Vintana weighed in at 9 kg. “It’s Zhe-Xi Luo, a palaeontologist at the University lion-year-old Volaticotherium, which had wing exciting that we kind of busted the old myths of Chicago in Illinois. “That’s why this entire membranes made of furry skin, like today’s that early mammals came from a very humble field is so interesting, because the fossil record flying squirrels. In 2017, Luo’s team added generalized ancestor,” says Luo. is getting better and better, and we are starting Vilevolodon and Maiopatagaium4,5, which The finds are not solely from China. Important to really tackle some of these questions.” fossils are also coming from the United States, Spain, Brazil, Argentina, Madagascar and Mon- Out of the shadows golia. Some of the most intriguing and oldest In 1824, at the Geological Society of London, “THESE NEW fossils — as well as the biggest gaps in our knowl- naturalist William Buckland presented bones DISCOVERIES edge — relate to the southern continents, where from one of the first known dinosaurs, Megalo- only five genera of Mesozoic mammals and their saurus. At the same talk, he revealed tiny mam- DOCUMENT A relatives are known, compared with more than malian jaws that had been found in the same 70 genera from northern latitudes. In the past fossil deposit. Their presence suggested that HUGE, HITHERTO- two decades, Brazil has yielded several Trias- mammals had a very deep history, but as would sic fossils that are more than 200 million years happen repeatedly, the dinosaur discoveries UNDREAMED-OF old. Guillermo Rougier, a palaeontologist at the completely overshadowed the mammal ones. University of Louisville in Kentucky, describes The slow trickle of mammal finds from ECOLOGICAL DIVERSITY. them as “incredible discoveries” that are right around the world continued for 150 years. Then on the cusp between mammals and their cyno- in 1997, researchers described the first ancient dont ancestors. “These forms really show a very mammal from the fossil-rich rocks of Liaoning transitional progression from things that are in northeastern China2, and the floodgates lived at around the same time and belonged typically non-mammalian, to things that pretty opened. Since then, 50 or more near-complete to a group called the haramiyids. These animals much have all the features of early mammals.” and “beautiful specimens” have been found swooped between the trees alongside some there, according to Jin Meng, a palaeontologist of the first flying dinosaurs, taking advantage Mammal must-haves at the American Museum of Natural History. of previously unexploitable food resources. The latest finds are also offering clues to Like the dinosaur fossils, they are dug up by Researchers found other specializations the evolution of key mammal features. For local farmers and sold on to museums. that they assumed had evolved only later: instance, the keen hearing of mammals is But the dinosaurs continued to get the vast Agilodocodon could climb trees and gnawed partly down to tiny bones in the middle ear — majority of the attention, says palaeontologist into bark to feast on sap6; the platypus-sized the malleus, incus and ectotympanic. But in the Steve Brusatte at the University of Edinburgh, river-dweller Castorocauda had webbed feet reptilian ancestors of mammals, these bones UK. “It’s only that very recently, through and a beaver-like tail for swimming7; and Doco- were part of the jaw, and were used for chewing the work of Luo, Meng and others, that the fossor had paws and claws for digging, and instead of hearing.