The earliest vertebrates were fish, and all of them were marine, with fish not migrating into fresh water until the Devonian. Most of these early fish lacked jaws. Any mineralization was concentrated on the teeth or on armor plating outside the body. Jaws evolved in the Silurian and this group, sometimes known as gnathostomes, quickly came to dominate fossil fish assemblages. Primitive gnathostomes evolved sequentially into the two most common modern fish groups, cartilaginous and bony fish, as well as into several extinct groups such as the placoderms and acanthodians. One group of bony fishes, the lobefins, evolved in turn into tetrapods - our direct ancestors (Fig. 11.2).
The most commonly preserved early vertebrates are con-odont animals, an eel-like primitive fish. These organisms are known best from their teeth, the microfossils called conodonts (for more details see "Microvertebrates" in Chapter 13). Conodont animals were all marine and their remains are found mainly in shallow water sediments from the Cambrian to the Triassic. Their teeth were phosphatic, made from dentine and enamel, but represent the only mineralized elements of the skeleton. Soft-bodied remains show that these organisms were active swimmers, with an elongated body between 5 and 50 cm in length, large eyes, and no jaws. They were almost certainly predators or scavengers, although reconstructions have been attempted showing the conodont elements arranged for filter feeding. Wear patterns on well-preserved conodont elements are consistent with the pattern expected for grasping and cutting teeth.
Other primitive fish are known from Cambrian and Ordovician rocks, all of them lacking preserved jaws. Together, these jaw-less fish are known as agnathans, and they formed a common element of Lower Palaeozoic fish faunas. Today the group is represented by hagfish, which are scavengers, and lampreys, which live parasitically. Earlier species were probably more diverse in their ecology and may well have included some of the first marine predators. Others appear to have been well adapted to feed on sediment or on plankton.
The evolution of fish since the Silurian has led to significant changes in jaw apparatus and in the animal's ability to swim accurately and at speed. Paired fins, a strong, flexible body, and a mouth that can be stuck out to suck up food, are all adaptations that have appeared within the group.
Fishes with jaws, the gnathostomes, evolved in the late Silurian. Jaws probably originated from the bony arches that support the gills, and may have been modified in order to improve the flow rate of water through the gills as much as for feeding purposes. However, this argument is based on modern fish that may be poor analogs for their Palaeozoic ancestors.
Gnathostomes diversified rapidly across the Silurian-Devonian boundary, making evolutionary relationships difficult to determine. Fish faunas of the time included heavily armored placo-derms and the lightly armored acanthodians, characterized by having spines supporting the front of each fin. Placoderms such as Dunkleosteus reached 10 m in length and had a predatory habit. Some acanthodians had spikes on their gill arches, which might have allowed them to sieve plankton from water as modern baleen whales do. Primitive jawed fish were common in the Devonian, and are known from freshwater and marine sediments worldwide. They declined thereafter, and the last acanthodians became extinct in the end-Permian mass extinction.
Modern sharks and rays have a skeleton built from cartilage that is not mineralized. However, their teeth and scales are characteristically vertebrate as is their overall body plan. These fish are known as chondrichthyans and their first clear appearance in the fossil record is Devonian. Two radiations of sharks and their relatives have occurred, one in the Carboniferous and one in the Triassic/Jurassic. Most modern forms can be traced back to the Mesozoic, including predators and giant filter feeders such as the basking shark.
Modern bony fishes, the osteichthyans, appeared in the late Silurian. During the Devonian they diverged rapidly into two main groups, the ray-finned fish that dominate modern aquatic environments and the lobe-finned fishes. This latter group includes modern lungfish and the coelocanth, a famous "living fossil". Although rare in the modern world, this is the group that gave rise to all terrestrial vertebrates - including us.
Ray-finned fish have flexible fins supported by a lightly built fan of radiating bones, the rays. Most are rapid swimmers, and the power for this swimming is generally provided by movements of the body or of the tail, with the fins serving a steering function. Over time, these fish have tended to become more lightly built, and the external skeletal elements have been abandoned. There have been three main radiations of ray-finned
fish, more properly known as actinopterygians. These took place in the late Palaeozoic, the late Triassic/Jurassic, and the late Jurassic/Cretaceous. The latest of these radiations saw the spread of teleost fish, whose mouthparts can be pushed outwards to form a delicate sucking or plucking shape (Fig. 11.3). This innovation may have contributed to the success of the group, which now includes at least 20,000 species.
Lobe-finned fish are known as sarcopterygians. Their fins are sturdy and supported by a few large central bones, usually supported by a strong linkage to the skeleton. In contrast to actinopterygians, the fins produce a power stroke to move the fish along. This strongly built, powerful fin is a useful preadap-tation to life on land. Another useful adaptation is the ability to breathe air. This is relatively common in fish, especially those living in warm, shallow water, which is prone to become stagnant. In this environment many fish will suck in bubbles of air from which they extract oxygen. Modern lung-fish can breathe air indefinitely. Sarcopterygians reached their maximum diversity during the Devonian, and also gave rise to amphibians in that period. They have formed a very minor component of fish faunas since that time.
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