4 Fossils From the Jurassic Era Offer New Insight About Human Ears, Teeth Evolution

Two studies offer fresh insights about evolution, mainly how humans grow their teeth and ears. The fossils obtained from the Jurassic era belonged to mammaliaforms, including mammals and their extinct relatives.

Human Evolution From Jurassic-Era's Shuotheriids

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, mainly how we came to have the teeth and ears we do today.

Four fossils older than 164 million years have been found between the two studies. All of them belong to the class mammaliaforms.

Shuotheriids, tiny, mice-sized animals that scurried alongside the dinosaurs, account for three discoveries.

Experts are puzzled about how we transitioned from the era of the dinosaurs to the present when mammals rule the environment 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," said paleontologist Patricia Vickers-Rich from Monash University in Australia. "We provide vital insights into the phylogenetic relationships and evolutionary trajectories of shuotheriids, little known until recent discoveries in China."

Since the 1980s, paleontologists have been perplexed about shuotheriids since the individual teeth of these animals don't resemble those of modern mammals. In the study "Jurassic shuotheriids show earliest dental diversification of mammaliaforms," researchers found that the shape was similar to the docodontans, an early branching off of the group of mammals that eventually became our progenitors. Thus, the researchers believe that shuotheriids belong more closely to docodontans in light of these new fossils.

In the second study, "Fossils document evolutionary changes of jaw joint to mammalian middle ear," the researchers discovered essential characteristics in the middle ear of the fossils, which provides animals with some of the best hearing on Earth. It is believed that the middle ear originated from jaw joints. However, it has been challenging to piece together that shift from fragmented, poorly 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.

Per Jin Meng from the American Museum of Natural History in New York, scientists have been trying to figure out how the mammalian middle ear evolved since Darwin's time.

"These new fossils reveal a critical missing link and enrich our understanding of the gradual evolution of the mammalian middle ear," Meng added.

The two experiments are similar to redrawing a massive species map, erasing some of the shuotheriids' fixed boundaries, and drawing new ones. This form of remapping shows how various environmental factors may have contributed to the independent or synchronous development of features across time.

The studies contribute to our understanding of the how and why of mammalian evolution, all because of tiny, scuttling animals that lived hundreds of millions of years ago. It is another reminder of how fresh research on fossils and discoveries might illuminate some of evolution's puzzles.

ALSO READ: Living Fossils Offer Fundamental Demonstration of Evolutionary Process [Study]

Evolution of Mammals

Mammals descended from Therapsida reptiles during the Triassic Period, which occurred between 252 and 201 million years ago. Compared to other reptiles of their epoch, the therapsids, which belong to the subfamily Synnapsida, were typically considered uninteresting.

Synapsids are among the oldest known groups of reptiles, having existed during the Carboniferous Period. They dominated the Permian Period's reptile population, and despite their predaceous lifestyle, they also produced herbivorous species through adaptive radiation.

The archosaurs, or "ruling reptiles," were the most significant synapsids during the Mesozoic Era, while therapsids were primarily small, active carnivores.

Therapsids tended to develop a specialized heterodont dentition and enhance locomotion mechanics by moving the limbs' plane of action near the trunk. The temporal muscles involved in closing the jaw were enlarged, and a secondary palate emerged.

At any moment during the transition from reptiles to mammals, some forms incorporated diverse traits of both groups since the characteristics that set reptiles and mammals apart evolved at different rates and in response to various connected situations. This kind of evolutionary pattern-known as a mosaic-occurs frequently during transitions that signify the emergence of significant new adaptive kinds.