The new species have distantly related but independently evolved traits: both digthe ground with sharp claws.
“There are many hypotheses about whyanimals dig in and live under 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."
This portrait depicts tritylodontidFossiomanus sinensis (top right) and eutriconodont Jueconodon cheni in burrows; both lived in the Early Cretaceous Jehol biota (about 120 million years ago) in northeastern China and showed converging skeletal features adapted to the fossil lifestyle. Credit: © Chuang Zhao.
Fossil mammals - amorphous -mammalian precursors - 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.
A sample of the Fossiomanus sinensis holotype. Optical image (left) and composite image of computed laminography. Credit: J. Meng © AMNH
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 feature, namelyan 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.
Let us remind you that the Zehol biota includes everythingliving 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 consist 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 temperate. The ecosystem was periodically disrupted by ash eruptions from volcanoes in the west. The word "Jiehol" is a historical transcription of the former Rehe Province of China.
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