Specimen suggests people and ancient fish had more in common than previously thought.
A new fossil fish (Qilinyu rostrata) found in Yunnan, China, has filled in a gaping hole in how researchers thought the vertebrate jaw evolved.
The 423 million years old specimen — part of an ancient group of armoured fish called placoderms — is the oldest fossil ever found with a modern three-part jaw, which includes two bones in the upper jaw and one in the lower jaw. Researchers reported their find on 20 October in the journal Science1.
Scientists thought that placoderm jaws, such as those seen in Dunkleosteus, were completely unrelated to the three-part jaw that humans have. This was because the bones in placoderm jaws sat further inside their mouths than they do in humans and didn’t contribute to the outer structure of the face like human jaw bones do, says Per Ahlberg, palaeontologist at Uppsala University in Sweden and a study co-author.
The placement of bones in this new fossil jaw is halfway between an ancient placoderm jaw and a modern jaw, the kind that bony fish and land vertebrates, including humans, have. “They contribute to the face, but the bits inside the mouth look suspiciously like” placoderm jaw bones, says Ahlberg. This rewrites the previous understanding that placoderm jaws and modern jaws evolved completely independently and were only very distantly related.
“You know that old thing where you have a picture of a vase and you suddenly realize that it’s two human profiles facing each other? It was like that,” Ahlberg recalls. “You realize that what everybody else has ruled out is in fact not only not ruled out, but is in fact the crushingly obvious interpretation.”
Something to chew on
This new view of jaw evolution was first suggested, though not entirely confirmed, by the 2013 discovery of a fossil fish called Entelognathus primordialis. A group led by paleontologist Min Zhu at the Chinese Academy of Sciences in Beijing discovered this new fossil fish at the same site. The current fossil is older than the 419-million-year old Entelognathus, and provides an example of a jaw that is more clearly intermediate between ancient placoderms and modern bony fish.
Qilinyu rostratawas about 20 cm long and looked like a catfish with a flat pointy bit sticking out of its snout. “It reminded me of a platypus, having this big snout with a flat bill,” says John Long, a palaeontologist at Flinders University in Adelaide, Australia.
The most complete fossil, which was used to describe the new species, was uncovered from the Xiaoxiang site’s muddy limestone in 2012, but it was missing its lower jaw. Early in 2016, Zhu’s group found a lower jaw from another specimen, then four more lower jaws from other individuals, giving them a complete picture of the animal’s jaw bones.
Ancient tropical paradise
“It confirms what the scientific consensus has been leaning toward for the last two years,” since Zhu’s group found Entelognathus, says Sam Giles, a palaeobiologist at Oxford University in the UK. The find also shows how early placoderms filled very different ecological niches, she says. Qilinyu’s mouth is located on its underside, indicating it was most likely a bottom-feeder, whereas Entelognathus’s forward-facing mouth indicates a different style of feeding.
Xiaoxiang is “a spectacular site,” says Giles. “Before this locality, everyone thought there wasn’t much going on in this period.” But it’s turned out to be incredibly diverse, she notes.
Theancient tropical bay at this site was both geographically and biologically isolated from other oceans, says Zhu. That isolation cultivated a diversity that he has not even come close to fully cataloguing. Zhu says his team has collected over 20 other new fossil forms that are waiting to be prepared and described.
Zhu, M. et al. Science http://dx.doi.org/10.1126/science.aai8828 (2016).
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Nowogrodzki, A. New fish fossil upends scientists' view of jaw evolution. Nature (2016). https://doi.org/10.1038/nature.2016.20848