Researchers from China, the US, and Australia have conducted two new studies on Jurassic-era fossils that shed light on the evolution of mammals like humans, particularly how we came to have the teeth and ears we do today.
Between the two studies, a total of four fossils that are older than 164 million years have been found. All of them are fossils of the mammaliaform order, which includes extinct mammals and their closest relatives. Shuotheriids, tiny, mice-sized organisms that scurried alongside the dinosaurs, account for three of the four discoveries.
Experts are puzzled about how we made the transition from the era of the dinosaurs to the present when mammals rule the evolutionary tree, and how various evolutionary branches relate to one another based on the arrangement of the teeth and ears on these shuotheriids.
“Our study questions current theories and offers a new viewpoint on the evolutionary history of mammaliaforms,” remarked paleontologist Patricia Vickers-Rich, from Monash University in Australia.
Since the 1980s, paleontologists have been perplexed about shuotheriids since the individual teeth of these animals don’t resemble those of modern mammals.
Here, a more thorough examination revealed that the shape corresponded to the docodontans, a different group of mammaliaforms that split off early from the family of mammals that eventually became our progenitors. Thus, the researchers believe that shuotheriids belong more closely to docodontans in light of these new fossils.
Regarding the fossils’ ears, the scientists discovered important characteristics in the middle ear, which provides animals with some of the best hearing on the planet. It is believed that the middle ear originated from jaw joints. However, it has been challenging to piece together that shift from fragmented, badly preserved fossils.
The second study sheds light on the evolutionary transition from reptiles, which have one middle ear bone, to mammals, which have three, by contrasting an older, more reptile-like mammal fossil with the recently discovered shuotheriid species.
Redrawing a massive species map in both studies would be like erasing parts of the fixed boundaries between shuotheriids and other groups and adding new ones.
This type of remapping aids in illustrating how features evolved throughout time, either independently or in concert, as well as the various environmental factors that may have influenced them.
Put another way, the study advances our understanding of the how and why of mammal evolution, all of which may be attributed to tiny, scuttling animals that lived hundreds of millions of years ago. It serves as another reminder of how fresh insights into evolutionary puzzles might come from the discovery of new fossils and the subsequent study of those fossils.