Scientists Unearth Fossils of Ancient Burrowing Mammal-Like Animals

Paleontologists have unearthed two new species of burrowing mammal-like animals that existed around 120 million years ago in what is now known as northeastern China.

The newly discovered species, which was unraveled Wednesday in study findings published in the journal Nature, are not closely related but have distinctly evolved traits to bolster their digging routine. They are the very first "scratch-diggers" found in this ecosystem.

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Many hypotheses have been raised on why animals burrow the soil and stay underground, researchers noted.

Fossiomanus senensis and Jueconodon cheni

(Photo: J. Meng/AMNH)
The holotype specimen of Fossiomanus sinensis used to describe the new species, shown in a photograph (left) and composite computed laminography (x-ray tomography) image.

"For protection against predators, to maintain a temperature that's relatively constant-not too hot in the summer and not too cold in the winter or to find food sources like insects and plant roots. These two fossils are a very unusual, deep-time example of animals that are not closely related and yet both evolved the highly specialized characteristics of a digger," lead author Jin Meng, a curator in the American Museum of Natural History Division of Paleontology, said in the study.

Two Mammaliamorth Fossils Discovered

The fossil of the mammaliamorph species, which are predecessors to mammals, were found in the Jehol Biota and identified to represent the Early Cretaceous era about 100 to 145 million years ago. One is the mammal-like reptile called a tritylodontid, which is the first of its kind to be retrieved in this biota. Around a foot in length, it was assigned the name Fossiomanus sinensis (Fossio means "digging," manus "hand," and sinensis "from China.") The other discovered fossil, measuring seven inches long, was given the name Juecondon cheni ("Jue" or digging from Chinese pinyin, the system of Romanized spelling for transliterating Chinese, and conodon or "cuspate tooth" and cheni for Y. Chen, who found the fossil). It is categorized as a eutriconodontan, a far-off cousin of modern placental mammals and marsupials, which was widespread in the habitat.

Mammals adapted to burrowing show exceptional traits for digging. Researchers saw some of the Fossiomanus sinesis and Juecondon cheni's identifiable features, including shorter limbs, sturdy forelimbs with strong hands, and a short tail. And these connote a type of digging behavior called "scratch digging," which is done mainly using the forelimbs' claws.

Fossil Had 38 Vertebrae

Ming noted that this is the first time researchers found and studied scratch diggers in the Jehol Biota, considered a home for diverse species, from plants to insects.

These animals also share another distinctive feature- an extended vertebral column. Normally, mammals have 26 vertebrae from neck to hip. But the Fossiomanus has 38 vertebrae-an additional 12 more than the ordinary mammal, while the Jueconodon had 28. Trying to know how these creatures got extended trunks, paleontologists sought recent studies in developmental biology, discovering that the variation could be due to gene mutations that identified the shape and number of the vertebrae at the start of the animal's embryonic development. This variation can be seen in present-day mammals as well, such as elephants, hyraxes, and manatees.