Before an asteroid completely changed the course of Earth’s evolutionary trajectory, the world was filled with scaled and feathered dinosaurs, web-winged pterosaurs, and, as a new study describes, precursors to modern mammals that eked out existence in burrows they dug into the Cretaceous soil.

The paper details two newly identified species: the reptilian Fossiomanus sinensis and the mammalian Jueconodon cheni, both dating to roughly 120 million years ago in what is today northeastern China. The former was a mammal-like reptile from a group called the tritylodontids, and the latter was a eutriconodontan, a many-syllabled way of saying it was evolutionarily close to the common ancestor of modern marsupials and placental mammals. Both animals were less than a foot long, and they were diggers.

Fossiomanus sinensis (upper right) and Jueconodon cheni bottom left. Illustration: © Chuang Zhao

The tritylodontids in general were mammal like: They were probably warm-blooded and had several skeletal characteristics, especially with their limbs, that aligned more with what mammals had going on than their reptilian contemporaries.

“Each of the two represents the only known digging species in its own group (tritylodontids and eutriconodontans, respectively); in other words, the other members of their own group do not dig,” study author Jin Meng, a paleontologist at the American Museum of Natural History in New York, said in an email. “The common ancestors of mammals are likely not burrowers—burrowing is a relatively specialized behavior for fossorial [digging] lifestyle.”

The mammal-like reptile F. sinensis. Image: J. Meng/ © AMNH

As to the reason for their digging, it could’ve been any number of things. Meng said that evading dinosaurs and other predators, staying warm or cool at different times of the year, and foraging food all could have been factors for their lifestyle. The fact that both animals—which were not closely related, though they coexisted—developed such burrowing niches in their ecosystem suggests it was a useful way of getting by.

The really intriguing thing about these animals, and thus the heft of the paper, dealt with the elongated vertebral columns of the two diggers. F. sinensis had 38 vertebrae, and J. cheni had 28. Mammals typically have 26 from the neck to the hip. The researchers posit that the increased number of vertebrae could be attributed to genetic mutations early in embryonic development, though, being so ancient, any answer to such a genetic question remains unanswered.

The Cretaceous mammal J. cheni. Image: J. Meng/ © AMNH

“Animals that live in burrows tend to have more vertebrae, probably to make their body more flexible so that they can make turns back and forth in the narrow burrows,” Meng said. “They move slowly so that their backbones remain stable; this is one of the reasons that their vertebral column can change.”

Other morphological characteristics that suggested the two fossils were diggers, included stubby limbs, strong forelimbs, and some claws reminiscent of modern moles, which indicated they’d do well getting through the dirt.

Long extinct now, the two animals highlight the great variety of life during the Cretaceous, which was crowded with the charismatic carnivorous dinosaurs that tend to steal our attention. One reptilian and one mammalian, both operated far below the furor other creatures were dealing with.