Smart mouth: Chinese fish fossil sheds
light on jaw evolution
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[October 21, 2016]
By Will Dunham
WASHINGTON (Reuters) - A bottom-dwelling,
mud-grubbing, armored fish that swam in tropical seas 423 million years
ago is fundamentally changing the understanding of the evolution of an
indisputably indispensable anatomical feature: the jaw.
Scientists said on Thursday they unearthed in China's Yunnan province
fossils of a primordial fish called Qilinyu rostrata that was about 12
inches (30 cm) long and possessed the telltale bones present in modern
vertebrate jaws including in people.
Qilinyu was part of an extinct fish group called placoderms, clad in
bony armor covering the head and much of the body and boasting jaws
armed with bony plates that acted as teeth to slice and dice prey.
Fish were Earth's first vertebrates when they appeared more than half a
billion years ago, but they were primitive and jawless, with sucker-like
mouths. Placoderms were the first vertebrates with jaws, a pivotal
evolutionary advance that enabled them to grasp prey, but they had no
teeth. Teeth appeared for the first time in later fish.
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Qilinyu had three bones, the dentary, maxilla and premaxilla, that
characterize the modern vertebrate jaw seen in bony fish, amphibians,
reptiles, birds and mammals, though they are absent in the cartilaginous
sharks and rays.
Scientists long viewed placoderms as a fascinating evolutionary
dead-end. But the fossils of Qilinyu and another placoderm called
Entelognathus that also possessed the three bones indicate that the
elements of the modern jaw first appeared in placoderms.
The maxilla and premaxilla are bones of the upper jaw. The dentary is a
bone of the lower jaw.
It appears they evolved from the bony plates that placoderms used to
sheer flesh in lieu of teeth, said paleontologist Per Ahlberg of
Sweden's University of Uppsala, who helped lead the study published in
the journal Science.
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![](../images/102116pics/news_L21.jpg)
This illustration shows the fish called Qilinyu that lived 423
million years ago during the Silurian Period. A fossil of the fish
was unearthed in China’s Yunnan province and is described in the
journal Science. Dinghua Yang/Chinese Academy of Sciences/Handout
via REUTERS
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"In us, the lower jaw is made entirely from the dentary. Most of the
upper jaw is composed from the maxilla, but the bit that carries the
incisor teeth is the premaxilla," Ahlberg said.
The findings contradict the long-held notion that the modern jaw
architecture evolved later, in the earliest bony fish.
"Now we know that one branch of placoderms evolved into modern jawed
vertebrates," said study co-leader Zhu Min, a paleontologist at
Chinese Academy of Sciences' Institute of Vertebrate Paleontology
and Paleoanthropology. "In this sense, placoderms are not extinct."
Qilinyu had a flat underside, ate soft-bodied mud-dwelling
invertebrates. It was a modest member of the placoderm group, which
included Earth's first true monster, a fish called Dunkleosteus with
huge, powerful jaws that was bigger than a great white shark.
(Reporting by Will Dunham; Editing by Sandra Maler)
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