Chuck Bednar for redOrbit.com – @BednarChuck
Researchers from the US and China have identified the oldest tree-dwelling mammal ancestor and the earliest-known subterranean relative of the phylogenic class, according to a pair of new papers published Friday in the peer-reviewed journal Science.
The tree-dwelling mammal ancestor (mammaliaform) is known as the Agilodocodon scansorius and it lived in China during the Middle Jurassic, according to the study authors. It had claws for climbing and unique dental characteristics indicating that its diet included tree sap.
The subterranean creature, Docofossor brachydactylus, had reduced digit segments and shovel-like paws similar to African golden moles, as well as distinct skeletal features similar to genetic patterns found in living creatures, suggesting those mechanisms pre-date modern mammals.
“We consistently find with every new fossil that the earliest mammals were just as diverse in both feeding and locomotor adaptations as modern mammals,” explained Dr. Zhe-Xi Luo, a professor of organism biology and anatomy at the University of Chicago and an author on both papers. “The groundwork for mammalian success today appears to have been laid long ago.”
Both newly identified creatures are members of the mammaliaform order Docodonta, and their discovery provides strong evidence that tree-dwelling (arboreal) and subterranean behaviors had developed early in the mammalian evolutionary process, convergent to those of true mammals. Each creature possessed unique adaptations designed specifically for their respective habitats.
Exhibit A
Agilodocodon, which the researchers said lived approximately 165 million years ago, had curved claws on its hands and feet, as well as limb proportions typical for mammals living in either trees or bushes. It also had special features that allowed it to adapt on the gum or sap of trees.
Its teeth resembled spades and allowed it to gnaw into bark, similar to some modern monkeys, and this adaptation marks the earliest-known evidence of this type of feeding in mammaliaforms. The creature also had flexible elbows and wrist and ankle joints that would have given it the mobility needed for it to climb trees, providing further evidence of its arboreal lifestyle.
“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 co-author David Grossnickle, a 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.”
Exhibit B
Docofossor, which lived around 160 million years ago, had a skeletal structure similar to the modern African golden mole, as well as shovel-like fingers that would have allowed it to dig. The creature also possessed upper molars that were short and wide, which is a common trait in mammals that forage underground, and a posture indicative of subterranean movement.
It also had reduced bone segments in its fingers, causing it to have shorter but wider digits – yet another trait it shares with African golden moles, and an adaptation that proves advantageous in mammals that dig. The characteristic is due to the fusion of bone joints during development, and the mechanism may have played a role in early mammal evolution, the authors explained.
‘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,” said Luo. “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.”
“We know that modern mammals are spectacularly diverse, but it was unknown whether early mammals managed to diversify in the same way,” he added. “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.”
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