Fish

Jawless fish, which resemble living lampreys, first appeared in the early Cambrian and later evolved into many types, including Loganellia and Jamoytius. Some (e.g. Cephalaspis) were protected by thick scales or by a bony head shield. None had bones within their bodies (although they had a notochord, a primitive cartilaginous backbone) and so fossils are rare.

One group, the conodonts, were for decades known only from teeth found in Palaeozoic rocks, but very rare fossils of their soft, eel-like body have now been found.

Jawless fish ate tiny organisms and food particles from the sea floor and some may have sucked blood.

The development of jaws and teeth resulted in changes in diet — fish no longer had to eat small food particles, but could catch larger prey. Many different types of jawed fish evolved, but they can all be put in two large groups:

• those with a cartilaginous skeleton (chondrichthyes)
• those with a bony skeleton (osteichthyes):
  • placoderms (the earliest of all jawed fish)
  • acanthodians
  • ray-finned and lobe-finned fish

Chondrichthyians (cartilaginous fish)  such as sharks, rat fish, rays and skates (e.g. Hybodus, Ischyodus, Spathobatis and Sclerorhynchus) had skeletons made of cartilage. The use of cartilage rather than bone was once thought to be a primitive feature, but it is now believed to be an evolutionary advance to aid buoyancy.

The most primitive bony fish were the placoderms. They had a bony skeleton, an ‘armoured’ head shield and plates that covered the front part of their body (e.g. Groenlandaspis and Coccosteus).

‘Spiny sharks’ (acanthodians), such as Climatius, had a cartilaginous skeleton covered by a thin film of bone. Some were marine and others lived in fresh water. These two groups formed short-lived lineages prior to the evolution of the more successful ray-finned and lobe-finned fish.

Lobe-finned fish, which include coelacanths (e.g. Macropoma) and lungfish (such as Dipterus), first evolved during the late Silurian. They had a bony skeleton and scales like the ray-finned fish, but the paired pectoral and pelvic fins were long muscular lobes, with a bony skeleton and rays, that could be moved independently by muscles. Lungfish are still found living in parts of Brazil, western Africa and eastern Australia and coelacanths live off southern Africa and Indonesia.

The evolution of the lobe-finned fish was one of the most important steps in the evolution of vertebrates. Comparison with modern fish suggests that the pair of air sacs of primitive bony fish developed into the swim bladder of many ray-finned fish, but lungs in the lobe-finned fish. The lobed fins of Eusthenopteron evolved into legs and the early amphibians (e.g. Ichthyostega) developed.

Ray-finned fish get their name from the bony rays that support and stiffen the fins, but which lack muscles — all movement was controlled by muscles in the body. The fins of the most primitive, like Cheirolepis, were rigid, but gradually they evolved, through a stage of slight flexibility, into the mobile fins of today’s modern fish.

Evolution brought other refinements. The air sacs of primitive types developed into swim bladders that control buoyancy. The heavy ‘armour’ was replaced, first by thick, bony scales (e.g. Palaeoniscum) and then by light flexible scales (e.g. Naso). The skeleton of the tail became symmetrical with lobes of equal length, like in Pholidophorus, giving strong propulsion and improved mobility.

Ray-finned fish inhabited both fresh water environments and the sea. Many different types evolved, including the teleosts, the group that forms most of the fish of today. These have thin scales, symmetrical tails and very mobile jaws and fins. One of the earliest teleosts was the herring-like Pholidophorus, which had heavy, enamelled scales and only a partially ossified ‘backbone’. Salmon-like Enchodus had thin scales and an entirely bony skeleton. Later on, trout, cod, haddock and perch-like fish appeared.