China fish fossil reveals where our jaws come from
Oct 22 2016 by Michele Stevens
The skeleton has the form of a classic placoderm, displaying three bones which are also part of human jaws. It was a placoderm with thick, bony armor that covered most of its body and jaws and bony plates that acted as teeth to slice into prey. These are the maxilla and premaxilla, which make up the upper jaw, and the dentary, which makes up the lower jaw, Tech Times reports.
The picture began to change in 2013 when Zhu and his colleagues unveiled a fossil, called Entelognathus, that had a placoderm-like body but a three-part jaw in Yunnan.
It is universally accepted that the dentary, maxilla and premaxilla, bones found in the jaws of many animals, come from a shared heritage of bony fishes and tetrapods. "Most importantly, the character combination of Qilinyu and its phylogenetic position as the most basal maxillate vertebrate illuminate the relationship between the marginal jaw bones of osteichthyans and the gnathal plates of placoderms". Additionally, the jaw bones of placoderms were located slightly further inside their mouth, which led the researchers to believe that the jaws of placoderms have evolved independently and very distantly related to human jaws. A fossil of the fish was unearthed in China's Yunnan province and is described in the journal Science.
According to Long, evolutionary biologists have long discounted placoderms in their research, but now the new findings in Yunan will change that as they realize the importance of these prehistoric creatures in helping to understand "the early assembly of the vertebrate body plan".
Their new study says it also combines a placoderm skeleton with dentary, maxilla and premaxilla.
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Scientists had evidence about the presence of fishes called placoderms under water a hundred million years before the dinosaurs lived on the surface of Earth. The peculiarity of gnathal plates is incalculable. It contains dentary, maxilla and premaxilla bones. These bones, known as "gnathal plates", have always been regarded as unrelated to our jaws.
The simplest interpretation of the observed pattern is that our own jaw bones are the old gnathal plates of placoderms, lightly remodelled.
However, the discovery of a Qilinyu fossil and Entelognathus primordialis, another placoderm genus discovered in 2013, may have created the missing evolutionary links between the three bones first found in bony fish and the "sheet metal cutters" of the early placoderms.
But it took them another 80 million years to lose their fishy heads. The question is more complicated than it seems, because not all jaws are the same. No one is quite sure where to place them. Go back into prehistory and a species known as placoderms bear the signs of these jaw bones.