Into The Trees And Under The Soil

Mammals occupy a diverse range of environments. Some scuttle through the undergrowth. Some glide through the air. Others are active fliers or have taken to the water. A handful live beneath the ground. Most of this diversity is recent, having developed in the wake of the dinosaur extinction when ecological niches were newly unoccupied. Yet it was not the first time in evolutionary history that mammals took to entirely new environments. Recent fossil evidence shows that mammals went into the trees and underground as early as the Jurassic period.

The 165 million year old arboreal docodont
Agilodocodon scansorius

'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 from the University of Chicago and lead author on the study. 'The groundwork for mammalian success today appears to have been laid long ago.'

One of these species Agilodocodon scansorius, from the Daohugou Fossil Site of Inner Mongolia, had hands and feet with curved horny claws and limb proportions found in modern arboreal mammals, for example the primates, alongside spade-like front teeth designed to gnaw into bark to access nutritious sap. Agilodocodon also had well-developed, flexible elbows and wrist and ankle joints that allowed for much greater mobility, all characteristics of climbing mammals.

'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.' Another new species Docofossor brachydactylus, discovered at the Ganggou fossil site in Hebei Province of China, displayed very different adaptations for a subterranean lifestyle.

The 160 million year old subterranean
docodont Docofossor brachydactylus

It had reduced bone segments in its fingers, leading to shortened but wide digits. African golden moles, a modern subterranean species, possess almost the exact same adaptation. This characteristic is due to the fusion of bone joints during development, a process influenced by the genes BMP and GDF-5.

Due to the many anatomical similarities, the researchers hypothesise that this genetic mechanism may have played a comparable role in early mammal evolution, as in the case of Docofossor. The skeletal proportions and the morphology of the teeth were similarly diagnostic of a subterranean species. Intriguingly, despite their differences, Agilodocodon and Docofossor are actually closely related mammals, both belong to an order of mammals known as the docodonts.

This shows that mammals adapted quickly to the habitats they lived in. What is more, both species showed signs of modernisation in terms of their anatomy. Agilodocodon had a sharp boundary between the thoracic ribcage to the ribless lumbar vertebrae. Docofossor showed a gradual thoracic to lumber transition. These shifting patterns of thoracic-lumbar transition are regulated by genes Hox 9-10 and Myf 5-6 in modern mammals. The similar developmental patterns in the docodonts is evidence that these gene networks could have functioned in a similar way long before true mammals evolved.

'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.' Indeed it is likely that the degree of ecological diversity attained by Mesozoic mammals helped them survive through the K-T boundary extinction responsible for wiping out the dinosaurs. Modern mammals may well owe their own evolutionary success to the successes of their ancestors.