Paleontologists have discovered a tiny skull that belongs to the closest relative yet discovered of living mammals. Despite its small size--just 13 millimeters long--the skull preserves evidence of middle ear bones, demonstrating that this key mammalian innovation arose at least 45 million years earlier than previously known--and before true mammals appeared. The fossil, described in the 25 May issue of Science, could also help clarify the relationships of mammal ancestors.
Living mammals consist of three groups. The earliest fossils of all three--placentals, marsupials, and monotremes--are about 110 million years old. The extinct group from which all these descended is called the Mammaliaformes, members of which lacked several mammalian traits, including a large brain case relative to its body.
Into this picture steps a new mammaliaform, Hadrocodium wui. Discovered in the Lufeng Basin of China in 1985, the specimen was almost entirely encased in rock and was tentatively identified as a lizard-relative. In 1993, the tiny skull was carefully prepared at Harvard; it was studied by Harvard's Alfred Compton, Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh, and Ai-Lin Sun of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. "We could not believe it," Luo says. "It was entirely different from any species."
The find is remarkable for several reasons. First, it's the tiniest Mesozoic animal yet discovered, with an estimated weight of 2 grams. (The smallest living mammal, weighing in at about 1.5 grams, is the bumblebee bat from Thailand.) Second, it contains several characteristics that until now were thought to exist only in true mammals, such as middle ear bones separated from the lower jaw, an advance that helps mammals hear so well. Third, the animal must have had a brain that was big compared to its body size, based on a well-known correlation of skull size to body mass in mammals. Because the brain is an energy guzzler, the team thinks H. wui must have spent most of its time eating.
"It's a spectacular little fossil," says mammal paleontologist André Wyss of the University of California, Santa Barbara. "It's going to be important for determining what traits are ancestral to mammals and how the major groups of mammals relate to each other."