Paleontologists have discovered two new species of burrowing animals that lived about 120 million years ago in northeastern China. The results are published in the journal Nature.
The new species have distantly related but independently developed traits: both dig the ground with sharp claws. They represent the first digging animals found in this ecosystem.
“There are many hypotheses about why animals dig and live beneath the surface,” explains lead author Jean Man, curator of the paleontology department at the American Museum of Natural History. “For protection from predators, to maintain a relatively constant body temperature, or to find food sources such as insects and plant roots.”
Fossil mammalian species – amorphous – the precursors of mammals – were discovered in the Zehol biota, representing the Early Cretaceous era, from about 145 to 100 million years ago. One of them belongs to the tritylodont family, similar to mammals. It is named Fossiomanus sinensis and is about 30 cm long.
The other, Jueconodon cheni, is a eutriconodont, a distant relative of modern placental mammals and marsupials. Its size is about 20 cm in length.
Burrowing mammals have distinctive features. Some of these, the researchers found in new species: shorter limbs, strong front legs, and a short tail – in both Fossiomanus and Jueconodon.
Animals have another peculiarity, namely an unusually elongated spine. Usually, mammals have 26 vertebrae from the neck to the thigh. However, Fossiomanus had 38 vertebrae — 12 more vertebrae, while Jueconodon had 28. It is possible that such a long spine was the result of gene mutations. They determine the number and shape of vertebrae in animal embryos. By the way, differences in the number of vertebrae can also be found in modern mammals, including elephants, manatees, and hyraxes.
Recall that the Zehol biota includes all living organisms – the ecosystem – of northeastern China between 133 and 120 million years ago. It is a Lower Cretaceous ecosystem that has left fossils in the Yixian and Jiufotang Formations. These deposits are composed of layers of tephra and chalk deposits. The Lower Cretaceous ecosystem was dominated by wetlands and numerous lakes (rather than rivers, deltas, or marine habitats). Precipitation was seasonal, with semi-arid and mesic conditions. The climate was moderate. The ecosystem was periodically disrupted due to ash eruptions from volcanoes in the west. The word “Jiehol” is a historical transcription of the former province of Rehe in China.