Fossils of two strange creatures found in northeastern China show the earliest dental diversification among ancestors of mammals.
One of the species, Feredocodon chowi, was found in the Daohugou Formation in Inner Mongolia. The rocks in which they were found date to the Middle Jurassic (174–163 million years ago). Two specimens assigned F. chowi were examined in a paper published in Nature.
Another paper published in Nature analysed both F. chowi and another new species, Dianoconodon youngi. The bones of D. youngi were found in the Lufeng Formation in the Yunnan province. These rocks are older, dating to the Early Jurassic (201–174 million years).
Both animals belong to a group known as shuotheriids. These animals are Jurassic mammaliaforms – a clade which contains both mammals and also their closest extinct relatives. It was in the Jurassic that the first shrew-like mammals emerged in the shadows of the dominant dinosaurs.
Shuotheriid fossils have been found in rocks in China, England and possibly Russia. They aren’t true mammals but are ancestors of mammals, including humans.
Studying the evolution of Jurassic mammaliaforms could shed light on what makes mammals the way they are. The new research on the Chinese fossils has thrown some previous concepts into question.
“Our study questions current theories and offers a new viewpoint on the evolutionary history of mammaliaforms,” says co-author of one the papers Patricia Vickers-Rich, a professor at Melbourne’s Monash University.
Studying tooth shape and occlusion, the way teeth in the upper and lower jaws come together, Vickers-Rich says, is one way palaeontologists can understand the diversification of Jurassic mammaliaforms.
Shuotheriids have what are called “pseudotribosphenic teeth” with a basin-like structure in front of the trigonid – the first three cusps of a lower molar. This is different to the pattern seen in today’s mammals where the basin-like structure is behind the trigonid.
Traditionally, shuotheriids have been grouped with australosphenidans – a clade of mammals from the Jurassic and mid-Cretaceous, but some studies controversially include monotremes (such as the living echidna and platypus) as well.
But the relationship between shuotheriids and australosphenidans has been debated due to inconsistencies in geography and morphology.
The teeth of the Chinese shuotheriid fossils suggests these creatures are more closely related to another extinct mammaliaform which lived during the “Age of Dinosaurs”, docodontans.
It suggests shuotheriids belong to a different group called Docodontiformes.
“The study emphasises the presence of a significant variety of tooth morphology in early mammaliaforms, indicating unique ecomorphological adaptations throughout the evolutionary development within mammals,” Vickers-Rich explains.
“In 1982, a single Jurassic small lower jaw with four teeth was placed at one point on the mammalian family tree by Minchen Chow and myself,” says co-author Thomas Rich from the Museums Victoria Research Institute. “Now two virtually complete specimens, analysed in a different way, places all of them in a quite different place on the mammalian family tree. Additional specimens and different methods suggest different interpretations. This is how science often works.”
