A tiny fossil skull found within 195-million-year-old Chinese sediments provides evidence that crucial features of mammal anatomy evolved more than 45 million years earlier than previously recognized.
The well-preserved fossil shows several characteristics of mammals, says Zhe-Xi Luo, a vertebrate paleontologist at the Carnegie Museum of Natural History in Pittsburgh. Most notably, there's no groove at the rear of the jawbone. This indicates that the three bones of the middle ear had separated from the ancient animal's mandible. This separation occurs in modern mammals but not in reptiles.