Fish are the most numerous vertebrates on Earth today, outnumbering the others 10 to one. There was, however, a time when they were much more diverse. We call it the Devonian, the so-called Age of Fishes. From about 419 to 358 million years ago, the evolution of fish had gone into overdrive. Even by this time though, the fish were already pretty much an established animal group. They had already evolved relatively recognizable forms more than 200 million years earlier, from some of the first vertebrate ancestors.
These backboned pioneers had come a long way since their beginnings as minor parts of the fauna like Pikaia, one of the forefathers of all vertebrates. Soon, the earliest fish had begun to look somewhat like their future descendants, at least superficially. The standard body plan was already present. Most of these early fish lacked jaws. They breathed through gills supported by loop-like arches of bone or cartilage and had a lateral line system along their bodies to sense vibrations in the water.
There are still many types of fish that have no jaws, like lampreys and hagfish which have both been called “living fossils” by some. The extinct jawless fish are placed under an umbrella group known as ostracoderms, and were all slow swimmers with armored heads. They had fleshy mouths to siphon up tiny living things from the sand on the seabed. It is these armored forequarters that get preserved in the fossil record, and we currently have a variety of strange jawless forms starting at more than 480 million years ago, far before the Devonian even began.
However, jawed forms began appearing around 419 million years ago, with the appearance of the first cartilaginous and bony fish. Their appearance was during the later Silurian period, and the rocks of China shows us the best remains. Some of the earliest bony fish were lobe-finned fish, the same group as coelacanths and lungfishes. Then there were the so-called spiny sharks, a short-lived group of small shark-like fish with immovable spines running along their bodies. All jawed fish had to modify their first gill arches – a series of bony loops supporting the gills – thus turning them into their lower jaws.
Both sharks and bony fish evolved from a single common ancestor. We have fossils of one of these fish, – or at least its closest relative – and the find is quite a revolutionary one. It is known as Janusiscus. It dates back to the very start of the Devonian, hailing from Siberia. It may not be the true common ancestor, but this is as close as it gets. Its secrets were revealed by high-powered CT scans of its tiny skull. Janusiscus’ body has been restored as being somewhat shark-like. This seems like a stretch, considering that it is known from a partial skull roof but it is reasonable. This little sliver of skull is made of bone. The blood vessels that carried blood to the brain though, had left scars in the bone and they were most similar to those of sharks.
This leads to the new theory that the common ancestor of the two did not have cartilage. Instead, Janusiscus rewrites the story of fish as a whole, putting bone into the skeleton of the very first ancestor of two of the most prolific groups of fish today. Sharks are certainly not primitive animals, instead evolving over the last 400 million years. We know that to get to the way they are now, they had to lose their bony skeletons altogether and replace it with a framework of light but strong cartilage.
A separate family of bony fish developed their jaws independently of the others. These strangers are known as placoderms. Placoderm fish developed their jaws at around the same time as these other fish, and the earliest examples hail from Silurian China. But the feature that made the placoderms famous was their armored head and neck.
Entelognathus
