Two new species of extinct mammals have been discovered in Jurassic sediments in China. One lived up in the trees, the other lived below ground, and both sported sophisticated, specialized features that researchers thought evolved millions of years later. The findings, published as two studies in Science this week, indicate that early, rodent-looking relatives of modern mammals were exploiting a huge diversity of habitats—and they had unique adaptations that made them well suited to their specific niches.
The Mesozoic—which includes the Triassic, Jurassic, and Cretaceous—has long been thought of as a very dinosaur-dominated era. Well not anymore! "We consistently find with every new fossil that the earliest mammals were just as diverse in both feeding and locomotor adaptations as modern mammals," Zhe-Xi Luo from the University of Chicago says in a news release. "The groundwork for mammalian success today appears to have been laid long ago."
Luo led an international team in describing the 160-million-year-old Docofossor brachydactylus, the oldest subterranean mammal known. It was about 7 centimeters long, weighed up to 16 grams, and lived in burrows on the lakeshore where it fed on worms and insects in the soil. The skeletal features, body proportions, and multiple other adaptions of Docofossor were similar to today’s African golden moles: shovel-like paws for digging, a sprawling posture good for subterranean movement, and short, wide upper molars for foraging underground.
Additionally, the reduced bone segments in its fingers—which led to short, wide digits for burrowing—is due to the fusion of bone joints during development. This process is influenced by the genes BMP and GDF-5, suggesting how these genetic mechanisms operated long before the rise of modern mammals. Pictured to the right is Docofossor preserved in rock slabs unearthed in Late Jurassic lake sediments of the Ganggou fossil site in Hebei Province of China.
Luo was also part of another other team, led by Qing-Jin Meng from the Beijing Museum of Natural History, which described the 165-million-year-old Agilodocodon scansorius—the earliest tree-dwelling mammal known. Found in Middle Jurassic lake sediments from the Daohugou fossil site of Inner Mongolia of China, this agile arboreal animal had a 13-centimeter-long body and weighed between 27 and 40 grams.
Agilodocodon had curved, horny claws on its hands and feet, as well as limb proportions good for living in trees and bushes. Its well-developed, flexible elbows and wrist and ankle joints also allowed greater mobility when climbing. Agilodocodon used its spade-like incisors for gnawing on bark and striping it away to feed on gum and tree sap. They’re the earliest-known evidence of gumnivorous feeding in mammals. Marmosets and other small primates today feed on plant sap, and you can find similar teeth adaptations in today’s New World monkeys.
Furthermore, the variations in the vertebrae and ribcage of both Agilodocodon and Docofossor appear to be influenced by the Hox 9-10 and Myf 5-6 genes identified in modern mammals. "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,” Luo adds.
Images: April I. Neander, the University of Chicago (top and bottom, full reconstruction available here), Zhe-Xi Luo, the University of Chicago (middle)