Jaws are crucial to the evolutionary success of many animals, yet their origins have long been shrouded in mystery. Now a new discovery is shedding light on how the jaws of ancient fishes are related to our own.
Prehistoric armoured fishes called placoderms were the first fishes to have jaws. They arose some time in the Silurian Period, about 440 million years ago, to become the most abundant and diverse fishes of their day.
Placoderms dominated the oceans, rivers and lakes for some 80 million years, before their sudden extinction around 359 million years ago. This is possibly due to the depletion of trace elements in our oceans.
But placoderm jaws bear no resemblance to those of any living animal. So the question was, how did they evolve, and what is their relationship to modern jawed animals?
A paper published today in Science by Zhu Min and colleagues at the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing, shows how placoderm jaws evolved and then rapidly changed. The key is a newly discovered fish named Qilinyu (pronounced “chee-lin-you”), which lived some 425 million years ago and was discovered at a site in Qujing, China.
It has an unusual set of jaws that is similar to both those of traditional placoderms and those of modern bony fishes, or osteichthyans.
This is significant because we are all ultimately descended from osteichthyans, as this group includes the lobe-finned fishes (Sarcopterygii), from which all four-limbed land animals arose.
As Zhu Min explained to me:
With this finding, we can now trace our dermal jaw bones (dentary, maxilla and premaxilla) to the most primitive jawed vertebrates. The gnathal plates of conventional placoderms, such as the gigantic Dunkleosteus, are the homologues of the marginal jaw bones of bony fishes and tetrapods.
From simple jaws to complex ones, or vice versa?
Before this new discovery, we had a very poor record of Silurian placoderms.
But the latest fossil is just the latest in a series of intriguing discoveries made over the past decade at Qujing. These include the oldest known complete bony fish, Guiyu, as well as the oldest known complete placoderms.
One of these, Entelognathus is arguably one of the most significant transitional fossils found in the past century, bridging a huge morphological gap between the extinct placoderms and the living bony fishes.