Your Squirrely Cousins

A reconstruction of Jurassic forest mammals, depicting three new species of haramiyidans (from left Shenshou lui, Xianshou linglong, and Xianshou songae) as well as a gliding species and another previously-discovered haramiyidan (right). Illustration: Zhao Chuang

Fossils of Senshou lui (left), Xianshou linglong (Top right) and Xianshou songae (Bottom right). Photo: Jin Meng.

One of the oldest mammals yet discovered was an ancient squirrel-like creature named Shenshou lui, found in China's northeastern Liaoning province. Its name is derived from the Mandarin words for "heavenly animal" and Lu Jianhua, the man who discovered the specimen in 2013.

Shenshou's remains were found alongside fossils of two similar ancient tree dwellers, dubbed Xianshou songae ("celestial beast" found by Rufeng Song) and Xianshou linglong ("exquisite celestial beast"), in a layer of rock 160 million years old. Shenshou was the largest of the three species, weighing about ten ounces, and the size of a small squirrel, while the other species were the size of house mice.

All three are members of the haramiyid family, now shown to be among the first mammals. These three species thrived in what is now China during the Jurassic period, alongside jaw-droppingly huge titanosaurs such as the seven-story-tall titanosaurus.

The haramiyid fossils were unearthed by amateur paleontologists in a cornfield, in a region famous for its cornucopia of dinosaur fossils, all discovered within the last half-century. During the Triassic period in which the haramiyids first appeared, this region was blanketed by tropical forests which teemed with dinosaurs, mammals and pterosaurs, on the ancient supercontinent of Laurasia, which would eventually separate into the modern northern continents.

It was long believed that, rather than true mammals, the haramiyids were only protomammals from the Synapsid lineage. However, these newly-discovered specimens, including a complete skeleton, provided American Museum of Natural History paleontologist Dr. Jin Meng and colleagues with evidence that haramiyids were in fact true mammals. The newly-discovered fossils include a skull, teeth, and skeletons definitively showing that Shenshou and its haramiyid kin were basal mammals.
This discovery demonstrates that mammals evolved earlier than paleontologists had realized, pushing back the divergence of mammals from reptiles at least 25 to 50 million years; the earliest known member of the haramiyid group being between 220 million and 201 million years old.

Haramiyids lived an arboreal existence, scampering among the treetops, out of reach of larger predatory dinosaurs on the ground, as evidenced by their slim, light, and graceful bodies, their long, monkey-like prehensile tails, and their long fingers, which resemble those of more modern branch-grasping treetop-dwellers. They had teeth adapted to an omnivorous diet of fruit, nuts and insects. All three species also appear to have had ossicles - three uniquely mammalian middle ear bones, which amplify sound, providing excellent hearing in a terrestrial environment.

Because of its body features and large incisor teeth, Shenshou superficially resembles modern squirrels, though the species are not directly related. In fact, the haramiyids were quite different from any species alive today. Any similarities to be found between these ancient creatures and modern ones are the result of convergent evolution, the independent development of structures with similar form or function among creatures of very different ancestry, such as the independent evolution of wings among birds and bats.

Scientists have known about the haramiyids since Darwin, but only from fossilized teeth and fragmentary jaws. Because of this scarcity of well-preserved specimens, their status as mammals or mammal-like reptiles has long remained something of a mystery, according to Dr. Meng.

Living during the same period as the haramiyids were the small rodent-like, egg-laying members of the multituberculate family, a sister group whose Latin name refers to the complex cusps of their teeth. Multituberculates were long known to have been mammals, and were believed to have been the only major mammalian branch to have gone entirely extinct, leaving no surviving descendants. In their time, however, they were enormously successful, eventually spreading throughout the ancient world for over 100 million years, longer than any other mammal group.

Both the haramiyids and the multituberculates eventually died out however, leaving no living descendants, having long ago diverged from the lineage which would lead to Earth's modern mammals. They appear to have been outcompeted by early rodents, which first emerged some 34 million years ago, during the Oligocene period.

According to Dr. Meng, his team's findings mean we need to revise our picture of our prehistoric mammalian ancestors as "...shrew-like insectivores that lived in the shadow of the dinosaurs".

The earliest mammals evolved in many divergent directions from the outset - over the last half century, a number of fossil discoveries are showing that early mammals occupied a variety of ecological niches, swimming, walking on the ground, digging burrows, and gliding in forest canopies, he notes. Ancient mammals like the haramiyids and multituberculates were just as varied as modern-day rodents like beavers, gophers, mice, porcupines and squirrels.

Sources: Chisel-Toothed Beasts Push Back Origin of Mammals, Brian Switek, National Geographic, September 10, 2014;

Three Jurassic Squirrel Species Discovered in China, Sergio Prostak, Science News, Sep 11, 2014;

Three new Jurassic euharamiyidan species reinforce early divergence of mammals, Shundong Bi et al., Nature magazine, September 10, 2014;

Ancient Squirrel-Like Creatures Push Back Mammal Evolution, Charles Q. Choi, Live Science, September 10, 2014