A reconstructed illustration of Polistodon chuannanensis, an early mammal relative that had a uniquely shaped secondary jaw joint that is helping researchers better understand the evolution of the mammalian jaw joint.ZHAO Chuang ©️ PNSO
A new study led by researchers at the Museum and the Chinese Academy of Sciences finds that the evolution of the modern mammal jaw—which makes exceptional hearing and chewing possible for a wide range of mammals, including humans—is more complex than previously thought.
The researchers re-examined fossils of two early-mammal relatives from museum collections, describing a new species in the process: a squirrel-sized animal that lived in the early Jurassic, between about 174 and 201 million years ago.
In mammals, the joint that connects the skull to the lower jaw is made up of two bones: the squamosal and the dentary, the tooth-bearing bone in the lower jaw. This structure replaced an older jaw joint found in reptiles, which involved two different bones, the quadrate and the articular.
As animals evolved from early mammal-like reptiles into true mammals, several “experimental” versions of this new jaw joint appeared and helped support the pressure of chewing. Eventually, this led to the emergence of a double jaw joint in which the new dentary-squamosal joint carried most of the chewing workload while the reptilian joint continued to function, providing the initial system for airborne sound hearing.
Over time, the dentary-squamosal became the only joint, and the articular-quadrate joint transformed into the tiny bones of the mammalian middle ear, a key feature that helps mammals hear. But scientists still don’t fully understand how this new jaw joint evolved, mostly because the fossil record from this period is very limited.
“The evolution of the mammalian jaw joint is one of the most intriguing and incomplete chapters in vertebrate history, with key transitions obscured by gaps in the fossil record,” said Museum Paleontology Division Curator Jin Meng, one of the corresponding authors on the new Nature study, which was published this week.
The researchers used high-resolution computed tomography (CT) scanning to look at two fossil specimens from the Chinese Academy of Sciences’ Institute of Vertebrate Paleontology and Paleoanthropology: Polistodon chuannanensis, an opossum-sized animal that had a “horn” it likely used for digging, was named in 1984 but hasn’t been studied since, and a new species, which the researchers called Camurocondylus lufengensis.
The team found new jaw formations in both animals. In P. chuannanensis, the researchers identified a uniquely shaped secondary jaw joint between the jugal and the dentary bones, the first discovery of its kind in any four-limbed animal. And in C. lufengensis, they described a simple articular head of the dentary bone that appears to show an evolutionary transition to a form that fits into a socket of an initial new jaw joint.
These discoveries increase the diversity of jaw joints in mammalian evolution and expand researchers’ understanding of the evolutionary sequence of key mammalian characteristics that are critical for understanding how mammals process their food and hear airborne sounds.