An ancient fish that swam in tropical seas during ancient times may have something to do with the development of the human jaws.
The armored fish from China, which lived 423 million years ago, suggests that the jaws of humans and other modern-day land vertebrates and bony fishes may have their origins in an extinct fish group known as placoderms.
In a new study published in the journal Science on Oct. 21, paleontologist Per Ahlberg, from the University of Uppsala in Sweden, and colleagues reported a primordial fish known as Qilinyu rostrata.
The creature, which belonged to the placoderm group, had a bony armor that covered the head and much of its body, as well as jaws with bony plates that functioned like teeth to slice its prey.
Fish became the first vertebrates on Earth when they appeared more than half a billion years ago. The first fishes though were jawless and featured sucker-like mouths. Placoderms, on the other hand, were the first vertebrates to have jaws, an important evolutionary advance that allowed them to grasp their prey albeit the ancient marine animals did not have teeth.
Placoderms have long been viewed as an evolutionary dead end, but the fossils of the Qilinyu and those of another placoderm, the Entelognathus, were found to have three bones: the dentary, maxilla and premaxilla, which are also seen in the jaws of modern-day vertebrates.
The maxilla and premaxilla, the upper jaw's bones, and the dentary, a bone of the lower jaw, appear to have evolved from the bony plates that placoderms used to slice flesh in lieu of teeth. This suggests that the elements of the modern jaw first appeared in this group of ancient marine animals.
"We propose that the maxilla, premaxilla, and dentary are homologous to the gnathal plates of placoderms and that all belong to the same dental arcade. The gnathal-maxillate transformation occurred concurrently in upper and lower jaws, predating the addition of infradentary bones to the lower jaw," researchers wrote in their study.
The findings are in contrast with the long-held idea that the architecture of the modern jaw evolved later in the earliest bony fish. The study provides evidence that the Qilinyu and Entelognathus were nearly perfect intermediates between placoderms and bony fishes.
Researchers likewise think that the key jaw elements of bony fishes and all terrestrial vertebrates including humans evolved from the bony blades of placoderms.
"This is part of our own early evolutionary history," Ahlberg said. "It shows where our own jaws came from."
