The earliest examples of gliding mammals yet discovered, dated to the Jurassic period about 160 million years ago, suggest dinosaurs did not dominate the prehistoric Earth as much as has been believed.
As the first winged mammals, the identified fossils of two gliders demonstrate the wide ecological diversity attained by early mammals, says Zhe-Xi Luo, professor of organismal biology and anatomy at the University of Chicago, who is co-author of two new papers analysing the animals, published in Nature. This degree of early evolutionary diversity, he suggests, “means dinosaurs likely did not dominate the Mesozoic landscape as much as previously thought”.
Bearing some similarities in appearance to modern gliding mammals such as flying squirrels and possums, the “exquisitely fossilised” remains of the two animals unearthed from China’s renowned Tiaojishan Formation show wing-like skin membranes between long fore and hind limbs, and skeletal features in their shoulder joints and forelimbs that would make them capable gliders. Their long fingers (or toes) are suited to gripping branches, indicating trees were their natural habitat, while their teeth indicate they ate a mainly herbivorous diet.
Their capacity for aerial travel – what is known as ‘volant locomotion’ – evolved roughly 100 million before the earliest known gliding members of the family known as therians, to which squirrel and possum flyers and gliders belong. “These Jurassic mammals are truly ‘the first in glide’,” Luo says. “In a way, they got the first wings among all mammals.”
Luo and his colleagues from the University of Chicago, the Beijing Museum of Natural History and Hebei GEO University categorise the two gliders as belonging to the haramiyidan clade, an extinct branch of the mammalian evolutionary tree considered a forerunner of modern mammals. The clade extends back to the Late Triassic (201-252 million years ago).
For this reason the scientists consider the two newly discovered gliders as more antiquated than Volaticotherium antiquus, a squirrel-sized gliding mammal dug up in Inner Mongolia and presented to the world in 2006. It has been dated as being about the same age of the new fossils.
V. antiquus, however, belonged to the eutriconodont clade, which extends back only to about 170 million years ago and is technically part of the modern mammal family. “Eutriconodonts are rooted between modern monotremes and modern marsupials-placentals on the mammal evolutionary tree,” Luo explains. “Volaticotherium’s gliding evolved after the split of monotremes on one hand and marsupial-placental mammals on the other.”
So while in absolute geological terms all three fossils are about the same age, Luo says, the two new gliders evolved at an earlier point in mammalian evolution, prior to the diversification of modern mammals into monotremes, marsupials and placentals. “The evolutionary antiquity is much older for the newly found Maiopatagium and Vilevolodon than for Volaticotherium. That’s why we say they are the first winged mammals.”
One of the gliders, Maiopatagium furculiferum, was dug up from the Daxishan fossil site in Jianchang County, Liaoning Province. The other, Vilevolodon diplomylos, was unearthed at the Nanshimen fossil site in Qinglong County, Hebei Province.
Their evolution to glide between trees to forage for food demonstrates the the adaptability of early mammaliaformes to exploit new ecological niches otherwise inaccessible to competitors, Luo and his co-authors say. “Evolution of gliding behaviour is an important evolutionary transition between divergent land-based and aerial habitats,” they write in the paper analysing Maiopatagium furculiferum.
Together with many other fossils described by Luo and colleagues over the past decade or so, the new fossils provide strong evidence that mammals adapted well and were more ubiquitous in an age once presumed to have been the domain of dinosaurs.
“The traditional and historical view was that when dinosaurs dominated the world, mammals were small, generalised and without much functional or ecological diversity,” Luo says. “In simple terms, mammals were not able to diversify when dinosaurs dominated the terrestrial ecosystem. The popular version of this view was that mammals always lived in the shadow of dinosaurs. But that was then.”
A stream of new discoveries in the past 15 years has shown that mammals which co-existed with dinosaurs during the Mesozoic evolved into semi-aquatic forms, such as Castorocauda, subterranean forms, such as Docofossor, and many arboreal forms, such as Agilodocodon and Arboroharamiya.