Newly Discovered Squirrel-Like Creatures Suggest Mammals First Appeared In The Late Triassic

Chuck Bednar for – Your Universe Online
The discovery of three new small squirrel-like species lends evidence to the notion that mammals originated at least 208 million years ago in the late Triassic Period, according to new research appearing in a recent edition of the journal Nature.
The research, which was led by scientists at the American Museum of Natural History and the Chinese Academy of Sciences, places a poorly understood group of animals that lived in the Mesozoic era in the diverse family tree of mammals, which includes egg-laying monotremes like the platypus, marsupials like the opossum, and placentals like humans and whales.
“For decades, scientists have been debating whether the extinct group, called Haramiyida, belongs within or outside of Mammalia,” said co-author Jin Meng, a curator in the museum’s Division of Paleontology. “Previously, everything we knew about these animals was based on fragmented jaws and isolated teeth.”
“But the new specimens we discovered are extremely well preserved. And based on these fossils, we now have a good idea of what these animals really looked like, which confirms that they are, indeed, mammals,” he added. “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.”
Those three new species – Shenshou lui, Xianshou linglong, and Xianshou songae – were described using six nearly complete 160 million year old fossils originally discovered in China, the researchers said. They were placed in a new group known as Euharamiyida that resembled small squirrels. Meng’s team believe that they weighed between one and 10 ounces, and that they ate insects, nuts, and fruit with teeth that had raised points on the crowns.
According to Brian Switek of National Geographic News, the haramiyids lived in Jurassic China approximately 160 million years ago and were slender, graceful creatures that had a long-prehensile tail similar to modern monkeys. He added that they are unlike any living mammals, but closely resembled an extinct group known as the multituberculates.
“The picture that Mesozoic mammals were shrew-like insectivores that lived in the shadow of the dinosaurs needs to be repainted,” Meng told Switek. Oklahoma State University paleontologist Anne Weil added that the study helps demonstrate that different mammal body types developed early on, noting that mammals and harmiyids are similar to modern small rodents in that it’s easy to tell by looking that mice and squirrels are different creatures.
Mammals are believed to have evolved from a common ancestor that had three raised points, or cusps, on the crowns of their teeth, though human molars can have up to five, the researchers said. The newly discovered species, however, had two parallel rows of cusps on each molar, with up to seven of them on each side. Scientists have long been puzzled by how this comparatively complex tooth pattern evolved in these creatures.
Aside from the teeth, the overall morphology observed in the new haramiyidan fossils is clearly mammalian, the study authors added. For instance, the specimens show evidence suggesting they possessed the same type of middle ear (the region just inside the eardrum that converts airborne vibrations into ripples in ear fluid) as is typically found in mammals. Mammalian middle ears are unique, they noted, in that they have three bones.
“However, the placement of the new species within Mammalia poses another issue,” the museum said. “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).”
“What we’re showing here is very convincing that these animals are mammals, and that we need to turn back the clock for mammal divergence,” added Meng. “But even more importantly, these new fossils present a new suite of characters that might help us tell many more stories about ancient mammals.”
Image 2 (below): The holotype specimen of Senshou lui, which represents a new species of euharamiyidan mammal. It is a nearly complete skeleton that indicates a gracile body with a tail and long fingers that were adapted for an arboreal life in Jurassic forests. Credit: ©AMNH/J. Meng