early vertebrates and fish

early vertebrates and fish Vertebrates are distinguished from the other members of the phylum Chordata by the presence of a brain, specialized paired sensory organs (hearing, sight, smell), and an internal skeleton. The earliest vertebrate fossils have recently been reported from the Middle Cambrian of Chengjiang, southern China; however, they are not well known until the Middle Ordovician. These organisms had a cartilaginous internal skeleton and a bony exoskeleton. Bone may have developed initially as a protective armour, perhaps as a defence against invertebrate predators such as eurypterids, or it might have developed as a storage site for calcium and phosphate, both of which are necessary for muscle activity and metabolic processes.

Agnathans

The earliest fossil vertebrates belong to a group called the Agnatha or jawless vertebrates. They are represented now only by the lampreys, eel-like forms that are parasites on fish, and the hagfish, also eel-like but feeding on dead or dying animals. Ordovician vertebrates occur in the Harding Sandstone of North America and were first reported by Charles Walcott of the US Geological Survey in 1892. More recently they have been discovered in rocks of similar age in Australia and Bolivia. Work on these new forms by Pierre-Yves Gagnier of the Natural History Museum in Paris, and reassessment of the North American material by David Elliott of Northern Arizona University, has shown that these animals were about 15 cm long with numerous separate gill openings and no paired or dorsal fins. All of them were found in shallow-marine rocks, and it is generally accepted that the vertebrates originated in marine conditions, a view supported by the recent Chinese discoveries.

Related agnathans with external armour occur widely from the Late Silurian to the Devonian, but they all became extinct before the start of the Carboniferous. There are two major groups, one of these, the osteostracans (‘bone-shelled’) had a broad triangular bony shield covering the front of the body (Fig. 1). The eyes were close-set on the top of the shield and between them was a nasal opening and an opening for the pineal organ, a light-sensitive structure. Also in the mid-line was an elongated area covered by small polygonal plates, which together with two similar areas, one on each side of the head, constituted the main vibration receptors for the animal. The mouth and the gill opening were below the head-shield, and this together with the shape of the body indicates that these animals were bottom-dwellers. Paired pectoral fins and a dorsal fin were present, and the caudal fin was asymmetrical with a large upper lobe. These fish were neither fast nor agile swimmers, but probably wriggled along the bottom sucking up organic-rich material.

The second major group of agnathans, the heterostracans (‘different shelled’), were animals in which the head and front of the body were covered by bony plates. There were no paired or dorsal fins, although some forms developed bony plates extending out from the body to act as control surfaces (Fig. 1). They had no median nostril. A pineal eye was present, although not usually open to the surface. The water from the gills exited through a single branchial opening on each side of the body. A related group, the thelodonts, were covered by small bony elements. Although they are rarely well preserved as complete animals, their bony elements are sometimes very abundant in marine sediments and they have been used extensively in the biostratigraphic classification of the Late Silurian and Devonian.

Early jawed fishes

The development of jaws was a major step forward in the evolution of vertebrates, opening up many new possibilities in feeding and defence. Evidence from embryology and comparative anatomy shows that jaws developed from an anterior pair of gill arches (bony supports within the gills) that moved forward and became supports for the edges of the mouth. At the same time the upper part of the next gill arch (the hyoid arch) also became enlarged and formed a brace (the hyomandibular) between the back of the jaws and the braincase. There are, however, differences between the gill arches of modern jawless and jawed fishes, indicating that neither group could have been developed from the other and that they must have had a common ancestor far back in vertebrate history.

The first jawed fish in the fossil record are the acanthodians (‘spiny ones’), which first occur in the Late Silurian. These generally small fish were laterally compressed and were characterized by the presence of long spines in front of the two dorsal fins and the pectoral fins, and in a double row along the underside of the body. Some had small sharp teeth and fed on small invertebrates and fish; some of the later forms had no teeth and filtered suspended particles. The acanthodians became extinct in the Permian and are thought to be related to the modern bony fish.

The placoderms (‘plate-skinned’) are the second group of early jawed vertebrates and have a purely Devonian distribution. These fish were characterized by bony armour over the skull and front of the body, with a joint between the two areas. They developed a great diversity of body types, particularly within the major group, the arthrodires, some of which were predators up to 6 m long. Another placoderm group, the antiarchs, had a long box-like armour and a pair of arm-like pectoral appendages that may have helped them to move across the bottom. Although in the past they have been thought to show a relationship to sharks or to bony fish, they are now considered to be an entirely separate group.

Chondrichthyans

The advanced jawed fish the chondrichthyans (Fig. 1) first appeared in the Late Silurian. Their internal skeleton is composed of cartilage rather than bone and does not preserve well, so their fossil record consists mostly of teeth, fin spines, and scale-like dermal denticles, which are ossified. The teeth appear to have been formed from dermal denticles that migrated to the edge of the jaw and developed there. In the sharks, a method of tooth replacement was developed in which teeth migrated from the inner to the outer edge of the jaw, becoming functional and then being lost. Because of this replacement system, one shark may produce 20 000 teeth during its lifetime. Well-preserved specimens of the shark Cladoselache from the Late Devonian Cleveland Shale of Ohio show that this animal was an active predator with a streamlined body form. The pectoral fins were broad-based where they were attached to the body, which suggests less flexibility than in modern forms; the mouth and eyes were also further forward than is the case in modern sharks. Sharks of this type persisted until the Triassic, when they were replaced by the hybodonts. The hybodonts were dominant in the oceans until the Early Cretaceous, when the ancestors of the modern sharks appeared. The main advances seen are in the jaws, which became shorter and more curved, allowing for a more powerful bite. In addition, the upper jaws lost their contact with the brain-case, allowing the jaws to be protruded during biting. The snout was also developed in many sharks to accommodate the very sensitive electroreceptor system. Sharks include the largest modern fish predator, the Great White Shark, as well as the largest fish, the Whale Shark, which may reach 18 m in length. A separate development of animals adapted to life on the bottom produced the skates and rays, in which the pectoral fins enlarged to take over the main propulsive function. In addition, the chimaerids or rat-fish represent a separate radiation of deep-water fish in which large crushing toothplates are fused to the brain-case.

Osteichthyans

The osteichthyans or bony fish also appeared in the Late Silurian, together with the chondrichthyans and the early jawed vertebrates. They were streamlined, laterally compressed fish with a bony skeleton and thick bony scales. The gill openings were covered by a large plate, the operculum, and the head was covered by interlocking bony plates. Paired pectoral and pelvic fins and a large dorsal fin provided stability and control, while a heterocercal tail (in which the vertebral column projects into the upper lobe) provided the motive power. At the time of their first appearance the osteichthyans had already separated into two groups, the actinopterygians (‘ray-finned’) which includes most modern fish, and the sarcopterygians (‘lobe-finned’) which include the ancestors of the tetrapods, or land vertebrates.

The actinopterygians have only one dorsal fin and all the fins are supported by numerous fin rays controlled by muscles inside the body wall. Although they were initially a rather conservative group of visually oriented, active predators, they began in the Early Mesozoic a series of modifications that resulted in the enormous range of forms that we see today. Thinning of the scales, strengthening of the vertebral column, the forward movement of the pelvic fins, and the development of a homocercal, or symmetrical, caudal fin resulted in faster and more manoeuvrable animals. At the same time, shortening of the jaw and uncoupling of the upper jaw from the skull made possible a new feeding mechanism in which rapid opening of the mouth caused suction, pulling food into the mouth. These advances culminated in the modern bony fish, or teleosts, which show a tremendous diversity of mouth parts and feeding specializations. Although about 23 000 species of teleosts are alive today, there are also some examples of the earlier actinopterygians still living. The sturgeon is an example of the initial radiation of Palaeozoic chondrosteans, while the gar pikes are representatives of the initial radiation of neopterygians in the Mesozoic.

The sarcopterygians include the lungfish, together with the crossopterygians, which are composed of coelacanths and the extinct rhipidistians. These forms had two dorsal fins rather than one, and all the fins were supported by an internal skeleton and musculature. The stout fins may have been useful in propelling them across the bottom. The lungfish use lungs as accessory breathing organs, and during droughts modern forms can survive for several years in burrows. Massive crushing teeth on the upper and lower jaws are used to feed on shelled molluscs. The lungfish were extremely conservative and show little change through their evolutionary history. The crossopterygians, however, developed a diverse group of medium-sized freshwater predators. They were thought to have become extinct in the Cretaceous until, in 1948, a coelacanth was caught off the coast of East Africa. Rhipidistians did become extinct in the Early Permian, but at some time in the Devonian the osteolepiform rhipidistians had given rise to the first land animals, the amphibians.

David K. Elliott

Bibliography

Carroll, R. L. (1988) Vertebrate paleontology and evolution. W. H. Freeman, New York.
Colbert, E. H. and and Morales, M. (1991) Evolution of the vertebrates. Wiley-Liss, Inc., New York.