A group of paleontologists led by Dr Jin Meng of American Museum of Natural History has discovered three squirrel-like mammals that lived in what is now China during Jurassic, about 160 million years ago.
Mammals in a Jurassic forest: the three animals on the left side represent the three newly discovered species – Shenshou lui, Xianshou linglong, and Xianshou songae; the other two represent a gliding species and another euharamiyidan, respectively, which were described earlier. Image credit: © Zhao Chuang.
The three new species, named Shenshou lui, Xianshou linglong, and Xianshou songae, are described from six nearly complete 160-million-year-old fossils.
These animals were successful tree-dwellers, weighing between 30 and 300 grams.
“They were good climbers and probably spent more time than squirrels in trees. Their hands and feet were adapted for holding branches, but not good for running on the ground,” said Dr Meng, who is the senior author of a paper published in the journal Nature.
They likely ate insects, nuts, and fruit with their ‘strange’ teeth, which have many cusps on the crowns.
“Mammals are thought to evolve from a common ancestor that had three cusps; human molars can have up to five. But the new squirrel-like mammals had two parallel rows of cusps on each molar, with up to seven cusps on each side.”
The paleontologists have placed them in a new group of mammals, named Euharamiyida.
Left: holotype specimen of Senshou lui. Top right: specimen of Xianshou linglong. Bottom right: specimen of Xianshou songae. Image credit: © AMNH / J. Meng.
Despite unusual tooth patterning, the overall morphology seen in the new fossils is mammalian.
“For example, the specimens show evidence of a typical mammalian middle ear, the area just inside the eardrum that turns vibrations in the air into ripples in the ear’s fluids. The middle ears of mammals are unique in that they have three bones, as evidenced in the new fossils.”
“However, the placement of the new species within Mammalia poses another issue: based on the age of the Euharamiyida species and their kin, the divergence of mammals from reptiles had to have happened much earlier than some research has estimated.”
Instead of originating in the middle Jurassic (between 176 and 161 million years ago), mammals likely first appeared in the late Triassic (between 235 and 201 million years ago).
_____
Shundong Bi et al. Three new Jurassic euharamiyidan species reinforce early divergence of mammals. Nature, published online September 10, 2014; doi: 10.1038/nature13718
