Measuring just 12 inches in length, the Qilinyu Rostrata, which lived more than 400 million years ago, was no apex predator. Belonging to the now-extinct armored fish group called placoderms, it was an unassuming bottom dweller, albeit one destined for posthumous fame thanks to a special feature – its jaws, which scientists claim are the evolutionary basis of the chompers of modern vertebrates, including ours.

Where did our jaws come from?

The evolution of the modern jaw is one of the major advances in vertebrate evolution (the other two being the evolution of bone and limbs). With the exception of a rare few, like the hagfish, practically every living vertebrate has a jaw comprising the same basic three-part structure – a mandible (lower jaw bone), maxilla (upper jaw bone) and premaxilla (that pair of dermal bones in the upper jaw where your incisors are).

We know gnathostomes (jawed vertebrates) evolved from their jawless ancestors over 400 million years ago, but how the biting lower jaw, which is a critical component in this evolutionary process, came into existence is a mystery that has baffled and divided scientists for ages.

Evolutionary theory states we are all ultimately descended from bony fishes, so it follows that the key to this mystery lies with one of the prehistoric specimens. Until recently, a popular view was that the common ancestor of jawed vertebrates was probably some kind of cartilaginous fish, like the shark, and that the modern jaw came into existence much later with the appearance of bony fishes.

There was another group of ancient fish – the placoderms – that also had jaws, but theirs were primitive beak-like structures which looked like "sheet metal cutters … that slice together" according to Per Ahlberg, a paleontologist at Uppsala University and co-author of the study that documents the discovery of the Qilinyu. They sat further inside their mouths and had no bearing on its facial structure, he says. Furthermore, they bore little resemblance to any modern living creature, hence most scientists dismissed them as an evolutionary dead-end.

The 2013 discovery of the Entelognathus at the Xiaoxiang fossil site in China's Yunnan province was paramount in shining new light on the subject; conventional placoderms had only one lower jaw bone. The Entelognathus had a complex set that resembled those found in early bony fishes and modern vertebrates. Yet, there were still doubts about the link between the Entelognathus and modern bony fish, namely the possibility that the former's lower jaw had somehow evolved independently as a fluke.

An artist's rendition of the Entelognathus primordialis(Credit: Brian Choo)

The recent unearthing of the Qilinyu, which was also found at the same site, by palaeontologists from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing and Uppsala University in Sweden puts an end to any lingering doubts about the link between placoderms and bony fishes.

See how far we've come: Charting the evolution of the placoderm jaw(Credit: Brian Choo and Zhu Min)

Old questions answered, new questions raised

Like the Entelognathus, the fossil of this newly discovered placoderm also has the three bones that characterize the modern vertebrate jaw, a trait that "more securely bridges the jawless toothlike plates of placoderms to the development of the jawed condition that ultimately led to the three-boned jaw in ancestors of modern vertebrates," write the authors of this study.

Paleontologist Gavin Young of the Australian National University, who was not involved with the work, tells New Atlas that the new fossil is "truly a missing link":

"The idea that the three 'marginal tooth bearing bones' in our own ancestry [premaxilla, maxilla, dentary] correspond to the three gnathal bones of placoderms is a surprise. We would have thought they actually corresponded to other bones well inside the mouth that carry teeth - for example the paired vomers on the front of the palate [still present in humans, now a part of the internal septum]. The new fossil is truly a missing link in that respect. It displays three external bones around the jaw margin [two on top, one on lower jaw] and they all fold inwards to form a biting surface. As Per Ahlberg says – once you see the evidence from this fossil it seems obvious, but we never imagined that before."

That said, while the Qilinyu appears to have answered the question of how the modern vertebrate jaw came to be, it does raise another one, namely, how did teeth evolve in our fishy ancestors? "Did they originate by subdivision of the three bones in Qilinyu, or were they new structures that evolved in some other unknown transitional form?" asks Young. Maybe another fish from the amazing Yunnan fossil site may answer this."

Prior to these two discoveries, not much was known about the Silurian era, let alone placoderms from this time period. However the Xiaoxiang fossil site, which in its heyday was like the prehistoric equivalent of today's Coral Triangle, promises to keep researchers busy. The Chinese team claims it has unearthed more than 20 new fossil forms that are still waiting identification.

In the meantime, where human evolution is concerned, "it seems like substantial parts of our anatomy can be traced back, not only to the earliest bony fishes, but beyond them to the strange ungainly armoured placoderms of the Silurian period," writes Ahlberg.

The team's results were published in Science.

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