Two species of Mammaliaforms which are predecessors of mammals are described in an article published in the journal Nature. They originate from northeastern China and belong to the Jehol biota, a vast concentration of fossils dating from the Lower Cretaceous, that is, between 145 to 100 million years ago.
Fossils with common characteristics
The two animals although belonging to two distant lineages show similarities. Here, they are two species suitable for digging and they are the first two burrowers discovered in this ecosystem. The first fossil corresponds to a tritylondont, a mammalian reptile, baptized Fossiomanus sinensis which was discovered in the formation of Jiufotang. It measured about thirty centimeters and represents a genus identified for the first time in this biota. The second is a triconodont, a distant cousin of modern placental mammals and marsupials. Appointed Jueconodon cheni, it comes from Yixian’s training and many other representatives of his family have already been discovered there. Smaller, it measures just under 20 centimeters.
Fossiomanus sinensis. Credit: Nature.
Both Fossiomanus and Jueconodon exhibit characteristics that indicate they could undoubtedly dig and burrow the ground. Thus, both species have hind legs that are shorter than their forelegs, a short tail, and wide forelimbs with sturdy claws. They also have an increased number of thoracic vertebrae. The authors, led by Jin Meng of the American Museum of Natural History, conclude that the characteristics shared by these two species evolved independently under similar selective pressures, a typical case of convergent evolution.
Fossil of Jueconodon cheni. Credit: Nature.
Jehol’s exceptional fauna
Over the past two decades, a large number of fossils have been collected from excavation sites in northeast China’s Liaoning and Heibei Province, including feathered dinosaurs as well as bird remains and of mammals. Most of these specimens belong to the Lower Cretaceous and are grouped under the name of “Jehol fauna”. However, in these same areas, paleontologists have also unearthed fossils dated to around 160 million years ago, a period corresponding to the Jurassic. Jehol fossils are found in three main formations: those of Jiufotang, Yixian and Huajiying. Since their discovery, during the Japanese occupation, the richness of the sites has not dried up. And it is more than 20 years now that the most iconic specimens have been unearthed. These are feathered dinosaurs: the discovery of these dinosaurs sheds light on the origin of these dander which will become the prerogative of birds. Birds which are also present in the Jehol register and which may be among the very first to take to the skies. Their study provides a better understanding of the evolutionary modifications that allowed their flight and confirms that birds do indeed belong to the dinosaur superorder of which they are the last representatives.