Researchers discovered the earliest-known tree-dwelling and underground-tunneling ancestral mammals, suggesting widespread ecological diversity as far back as 160-million years ago.
The two shrew-sized interrelated ancestral species were discovered in China, and provide insight into the roots of the diversity seen on Earth today, the University of Chicago reported.
"We consistently find with every new fossil that the earliest mammals were just as diverse in both feeding and locomotor adaptations as modern mammals," said Zhe-Xi Luo, PhD, professor of organismal biology and anatomy at the University of Chicago."The groundwork for mammalian success today appears to have been laid long ago."
Both fossils provide strong evidence that both arboreal and subterranean lifestyles evolved very early on in mammalian history. The first of the two species, Agilodocodon scansorius, had claws perfectly designed for climbing and teeth adapted for a tree sap diet. It also had flexible elbows and wrist and ankle joints, giving it a wider range of motion as it moved through the trees.
"The finger and limb bone dimensions of Agilodocodon match up with those of modern tree-dwellers, and its incisors are evidence it fed on plant sap," said study co-author David Grossnickle, graduate student at the University of Chicago. "It's amazing that these arboreal adaptions occurred so early in the history of mammals and shows that at least some extinct mammalian relatives exploited evolutionarily significant herbivorous niches, long before true mammals."
The second fossil, Docofossor brachydactylus, boasted digging adaptations comparable to the golden mole, such as "shovel-like" paws. The ancient animal had "shortened but wide" digits, also similar to the modern golden mole; the process of these bone joints fusing together is believed to be an influence of the genes BMP and GDF-5 in the modern mammals, and the same may have been true for their ancient ancestors.
"We believe the shortened digits of Docofossor, which is a dead ringer for modern golden moles, could very well have been caused by BMP and GDF," Luo said. "We can now provide fossil evidence that gene patterning that causes variation in modern mammalian skeletal development also operated in basal mammals all the way back in the Jurassic."
Early mammals were once believed to have had relatively low diversity, but these new findings provide strong evidence against this theory.
"We know that modern mammals are spectacularly diverse, but it was unknown whether early mammals managed to diversify in the same way," Luo said. "These new fossils help demonstrate that early mammals did indeed have a wide range of ecological diversity. It appears dinosaurs did not dominate the Mesozoic landscape as much as previously thought."
The findings were reported in a recent edition of the journal Science.
