Reconstruction of the newly described species Feredocodon chowi. Credit: Chuang Zhao
New sets of fossils from the Jurassic period bring to light a critical missing link in the evolutionary shift from lower jaw joint bones in reptiles to tiny bones in the middle ears of mammals. The fossils offer important evidence of the transitional phases—a progressive decline in the load-bearing capacity of the main jaw joint leading to the separation of postdentary bones from the dentary.
The back-to-back studies, published in Nature, could change how scientists reconstruct the earliest branches in the mammalian tree of life. Both papers hinge on the finding of two new genus and species that provide a missing link in the evolution of hearing.
In the first paper, the researchers describe a new genus and species of shuotheriid, which they named Feredocodon chowi. Shuotheriids are a family of mouse-sized mammals with molars that are different from those of any living mammal. The Feredocodon chowi lived between 168–164 million years ago in what is now Inner Mongolia.
The second study is based on the fossil skulls of Feredocodon chowi as well as a second new species, named Dianoconodon youngi, which lived between 201–184 million years ago.
The modern mammalian middle ear, the area just inside the eardrum that turns vibrations in the air into ripples in the inner ear’s fluids, has three bones—a feature that is unique to mammals. Reptiles and birds only have one middle-ear bone. Scientists know that during the early evolution of mammals from the group that includes lizards, crocodilians and dinosaurs, bones that formed the joints of the jaw were separated and became associated with hearing. And now, Feredocodon chowi and Dianoconodon youngi provide convincing fossil evidence of this transition in action.
The transition started from an ancestral animal that had a double jaw joint, a feature with the joint of a mammal on the outside and a reptilian joint on the inside.
The research team, comprised of paleontologists from the American Museum of Natural History and the Chinese Academy of Sciences, say analysis of the older Dianoconodon youngi fossil showed that one of its two joints—the reptilian one—was starting to lose its ability to handle the forces created by chewing. Meanwhile, the study results show that the younger Feredocodon chowi specimen had already developed a mammalian middle ear adapted exclusively for hearing.
“When you look at the fossil record, both for mammals and many other sorts of animals, teeth are the part of the body that you are most likely to recover,” said Jin Meng, curator in the American Museum of Natural History’s Division of Paleontology and a corresponding author on both papers. “Yet since the 1980s, the perplexing tooth shape seen in shuotheriids has been a barrier to our efforts to understand early mammal evolution. These new specimens have allowed us to solve this longstanding problem.”
Meng and co-author Fangyuan Mao say their new research strongly supports and enhances the view that the gradual evolution of the mammalian middle ear is a classic example of vertebrate evolution.
“Scientists have been trying to understand how the mammalian middle ear evolved since Darwin’s time,” said Meng. “While paleontological discoveries have helped reveal the process during the last a few decades, these new fossils bring to light a critical missing link and enrich our understanding of the gradual evolution of the mammalian middle ear.”