Hundreds of millions of years ago, armoured fish dominated the oceans, lakes and rivers of the world. These now-extinct creatures belonged to a class called Placoderms, meaning ‘plate-skinned’ in Greek. They had armoured plates covering their head and thorax, and they were the first creatures to develop teeth, pelvic fins, and most importantly, jaws. Jaws enabled them to become predators, which is thought to be a significant step towards the development of complex creatures like humans.
But for years, there’s been debate over whether placoderms are the ancestors of modern jawed vertebrates, because placoderms have a simple jaw made of one single bone while modern vertebrates have jaws made out of a more complex set of bones. There seemed to be something missing in the transition—evolutionary scientists have long puzzled over what happened in between the placoderms dying out and modern jawed vertebrates springing up. Now, though, a new discovery has shown that placoderms are, in fact, the ancestors we’ve been looking for.
In the suburbs of Yunnan in south-west China, a team of scientists recently found an exquisitely preserved fossil of a 419-million-year-old armoured fish. They dubbed it Entelognathus, meaning ‘complete jaw’. Entelognathus primordialis was part of the placoderm family. Around 20 centimetres long, it had bony plates around its head and front, just like a placoderm, but its jaw and facial structure were nothing like the placoderms. Entelognathus boasted a jaw composed of a complex arrangement of smaller bones. It had the same distinctive three-bone system that is still found in chewing vertebrates today: the lower jaw bone (the dentary bone), the two upper jaw bones (for holding the front teeth), and the maxilla (for holding the canines and the teeth further back in the cheek).
Entelognathus provides evidence that placoderms were actually the ancestors of all 30,000 species of living fish fauna today. The discovery adds to the list of “missing links