The dioramic landscape illustrates the Early Cretaceous Jehol Biota with emphasis on mammaliamorphs. © Chuang Zhao
The Jehol Biota in northeastern China was home to a great diversity of life between 145 to 100 million years ago, from dinosaurs to insects to plants. Now, two new species of mammal-like, burrowing animals join their ranks as the first “scratch-diggers” discovered in this ancient ecosystem.
“There are many hypotheses about why animals dig into the soil and live underground,” said Jin Meng, a curator in the Museum’s Division of Paleontology and lead author of a study out today in the journal Nature that describes the new species. “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.”
The two species are members of a larger group called mammaliaforms from which mammals themselves emerged. One, measuring about a foot in length, is a mammal-like reptile called a tritylodontid and is the first of its kind to be identified in this biota. It was given the name Fossiomanus sinensis. The other, named Jueconodon cheni, is a eutriconodontan, a distant cousin of modern placental mammals and marsupials. It is about 7 inches long.
Upon analyzing the well-preserved fossil specimens, the researchers found some of the hallmark traits of burrowing behavior, including shorter limbs, strong forelimbs with robust hands, and a short tail. In particular, these adaptations indicate that the animals used “scratch digging,” which involves moving soil with the claws of the forelimbs.
“This is the first convincing evidence for fossorial life in those two groups,” Meng said.
Both animals also have an elongated vertebral column. Mammals usually have 26 vertebrae from the neck to the hip. However, Fossiomanus had 38 vertebrae, while Jueconodon had 28.
To try to determine how these animals got their elongated trunks, the paleontologists turned to recent studies in developmental biology, finding that the variation could be attributed to gene mutations that determine the number and shape of the vertebrae in the beginning of the animals’ embryotic development. A number of modern mammals also have variation in the number of their vertebrae, including elephants, manatees, and hyraxes.