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The discovery of a 423-million-year-old fossil in China has shed light on the evolution of the tripartite (three-part) jaw, revealing a previously unknown stage of jaw evolution in placoderms, an extinct group of early fishes.
Life reconstruction of Qilinyu along with Guiyu and Entelognathus in Silurian waters. Image credit: Dinghua Yang.
Jaws are an iconic and defining feature, not only of our own anatomy but of all jawed vertebrates. Jaws first appear in the developing embryo as a cartilage bar similar to a gill arch.
In a shark, this develops directly into the adult jaws, but in an embryo of a bony fish, or a human being, new bones appear on the outside of the cartilage.
In human skull, these bones – the dentary, maxilla and premaxilla – make up the entire jaws and carry our teeth.
It is widely accepted that the dentary, maxilla and premaxilla are a shared heritage of bony fishes and tetrapods.
But what about further back? Only one other group of fishes, placoderms, has a similar set of jaw bones. But these bones, known as ‘gnathal plates,’ have always been regarded as unrelated to the dentary, maxilla and premaxilla.
For one thing they are located slightly further inside the mouth, and in any case the general opinion has been that placoderms and bony fishes are only very distantly related.
In 2013, Dr. Min Zhu of the Institute of Vertebrate Paleontology and Paleoanthropology in China and co-authors unveiled a fossil, Entelognathus primordialis, that had a placoderm-like body but a three-part jaw.
However, there was some uncertainty as to whether it could, indeed, be classified as a placoderm.
Now, the same team has unearthed a new species, named Qilinyu rostrata, from the Kuanti Formation of Qujing in Yunnan, China.
The preserved part of the fossil is 5 inches (12.6 cm) long, with an estimated total body length of more than 8 inches (20 cm).
Sporting a dolphin-shaped head, Qilinyu appears to have dwelled and fed along the bottom of bodies of water.
A comparison of this ancient fish to 372 other species consistently placed it as a sister group of Entelognathus.
What’s more, Qilinyu’s jaw reveals the basis of a three-part complex, confirming that such a setup evolved from within the placoderm system, and did not arise independently of placoderms.
“Looking at the jaw bones of Entelognathus and Qilinyu we can see that they, in both fishes, combine characters of the bony fish jaw bones and placoderm gnathal plates,” Dr. Zhu and co-authors said.
“Another thing becomes apparent as well: it has been argued that placoderm gnathal plates represent an inner jaw arcade, similar in position to the coronoid bones of bony fishes, and if that were true we would expect to find gnathal plates just inside of the dentary, maxilla and premaxilla of Entelognathus and Qilinyu. But there is nothing there.”
“The simplest interpretation of the observed pattern is that our own jaw bones are the old gnathal plates of placoderms, lightly remodeled.”
“It seems like substantial parts of human anatomy can be traced back, not only to the earliest bony fishes, but beyond them to the strange ungainly armored placoderms of the Silurian period.”
The paper describing Qilinyu was published in the October 21, 2016 issue of the journal Science.
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Min Zhu et al. 2016. A Silurian maxillate placoderm illuminates jaw evolution. Science 354 (6310): 334-336; doi: 10.1126/science.aah3764
