Researchers have discovered what may have been the world’s first burrowing and tree-climbing mammals—two shrew-sized critters that lived in what is today China during the age of the dinosaurs. The fossils add to the growing evidence that, far from cowering in the dinosaurs’ shadow, early mammals were highly successful, specialized animals in their own right.
The new finds “give us a very different view of mammal life during the age of dinosaurs,” says John Wible, an expert in mammalian evolution at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, who was not involved in the work.
Dinosaurs first appear in the fossil record about 230 million years ago, and paleontologists have recently discovered fossil mammals dated to just 20 million years later. Yet scientists had long assumed that the earliest mammals were mostly nocturnal, staying out of sight and living largely on insects and other food that dinosaurs were not interested in. That picture has begun to change, however, with the discovery of a number of early mammals that were well adapted to a variety of ecological niches, including eating plants that huge vegetarian dinosaurs also munched on.
Among these small but versatile creatures were the docodonts, shrew-sized mammals that looked somewhat like a cross between a rat and a squirrel, bearing sharp-edged molars good for shearing, crushing, and grinding plants and other food. In 2006, for example, a team led by paleontologist Zhe-Xi Luo, now of the University of Chicago, reported the discovery in China of a 164-million-year-old docodont named Castorocauda lutrasimilis, which apparently not only swam but might also have eaten fish—adaptations not previously known among such early mammals.
In two papers in this week’s issue of Science, a team led by Luo and paleontologist Qing-Jin Meng of the Beijing Museum of Natural History reports the discovery of two new, shrew-sized docodonts that boost this picture of early mammalian diversity. The team claims that one of them, a 165-million-year-old creature named Agilodocodon scansorius, is the earliest known tree-climbing mammal, possibly eating tree sap with its specialized teeth; the other, 160-million-year-old Docofossor brachydactylus, had shortened, shovel-like paws and may be the earliest known burrowing mammal.
The two fossils were found over the past several years by farmers in the fossil-rich shale outcrops called the Tiaojishan Formation in northeast China and acquired by the Beijing Museum of Natural History in the form of shale slabs in which the delicate bones were embedded. Mindful of numerous controversies concerning the authenticity of fossils found in China, the team verified that the fossils really did come from the Tiaojishan Formation, matching both the geology of the slabs as well as Jurassic invertebrates also found in them. The researchers then prepared the fragile fossils for study, using CT scanning—the same CT scanning doctors use to diagnose human bone and soft tissue ailments—to reveal the details of their tiny skulls and teeth (as shown in the video below).
Agilodocodon scansorius (meaning “agile docodon” with a scansorial, or climbing, adaptation) was about 13 centimeters from head to tail and weighed about 27 grams, roughly the size of a house mouse. It looked like a squirrel with a long snout instead of a short one, the team reports, with curved, horny claws and flexible ankle and wrist joints typical of modern climbing mammals like squirrels and monkeys. Moreover, its front teeth were shaped like spades, leading the researchers to conclude that it could gnaw tree bark and feed on gum or sap.
Docofossor brachydactylus (“Doco” for docodont; “fossor” for fossorial, or digging, adaptations; and “brachydactylus” for short, broad fingers) had a striking resemblance to the modern African golden mole, a burrowing mammal that lives underground. It was about 7 centimeters long and weighed about 16 grams, the size of a small shrew, and had a wide stance typical of burrowing animals. Like the golden mole, Docofossor had shovel-like fingers ideal for digging. Moreover, unlike most mammals, Docofossor had only two phalanges (bone segments) in most of its fingers instead of the usual three, which led to shortened but wider digits.
The researchers note that in modern mammals the number of segments is largely controlled during development by two specific genes (called BMP and GDF-5), and they suggest that natural selection was operating on the same genes as early as 160 million years ago. If so, Luo says, it would imply that the evolutionary groundwork for this surprising diversity and adaptiveness among early mammals was laid down much earlier than researchers suspected. “With each new discovery it becomes increasingly clear that the [earliest] mammals had the same evolutionary mechanisms as modern mammals,” he says.
These adaptations may have played an important role in the later success of modern mammals once the dinosaurs went extinct about 66 million years ago, says Richard Cifelli, a vertebrate paleontologist at the University of Oklahoma, Norman, who was not involved with the work.
Still, Frietson Galis, an evolutionary developmental biologist at Naturalis Biodiversity Center in Leiden, the Netherlands, questions whether Agilodocodon really gnawed bark and ate tree sap. She says its teeth “are quite different” from the modern sap-eating monkeys that the team compared it with, and the long, thin lower jaw seems to her too weak for chomping on tree bark. She and Wible also question whether the reduction in finger digits in Docofossor really employed the same genes as in modern mammals.
Luo himself cautions against “overstretching” the conclusions about the versatility of early mammals. Although the new discoveries are an antidote to “stereotypical thinking” about cowering, timid mammals, he says, it is still true that our distant ancestors stayed pretty small until the monstrous dinosaurs were safely out of the way.
(Video credit: April I. Neander, University of Chicago)