Order Teleostei

By the close of the Mesozoic Era, some 65 million years ago, the teleosts were well established as the dominant bony fishes in the seas, lakes and rivers of the world.

The teleosts had evolved more than 150 million years before the end of the Mesozoic. They first appeared in the seas of the Late Triassic, some 220 million years ago. Initially, they were small herring-type fishes, with symmetrical tails and flexible jaws, rather like advanced neopterygians such as Aspid-orhynchus (see p. 37). But in the middle of the Cretaceous period, the teleosts underwent an explosive phase in their evolution, which resulted in more advanced fishes like the modern salmon and trout. These diversified rapidly, and by Late Cretaceous/Early Tertiary times, a second evolutionary burst gave rise to the highly advanced teleosts of today — the spiny-rayed, perchlike fishes (see p. 41).

name: Hypsocormus time: Middle to Late Jurassic locality: Europe (England and

Germany) size: up to 3 ft 3 in/1 m long The dividing line between the advanced neopterygians (see p. 37) and the primitive teleosts is unclear. Hypsocormus was a fast-swimming, fish-eating predator, which could have belonged to either group since it had both primitive and advanced features.

For example, it had the heavy, "old-fashioned" body armor of its palaeonis-cid ancestors — thick, enamel-covered, rectangular (rhomboid) scales. But the scales were comparatively smaller and allowed greater flexibility during swimming.

Its tail was symmetrical and half-moon-shaped, superficially like that of a modern mackerel, although there were many more bony fin rays supporting the lobes of the tail than in modern teleosts.

Its fins, too, were arranged differently on the body. Besides the long anal fin, there was only one dorsal fin. The extra-large pectoral fins were placed low on either side (rather than high up on the flanks, behind the gills, as in more advanced bony fishes). And the pelvic fins were unusually small and placed halfway down the belly.

Hyfuocormusdid, however, have fairly advanced jaws. They were flexible and mobile, and well equipped with muscle-attachment points to ensure a powerful bite. The upper and lower jaw bones were long, and equipped with teeth along their length.

name: Pholidophorus time: Middle Triassic to Late Jurassic locality: Africa (Kenya and Tanzania), Europe (England, Germany, Italy and USSR) and South America (Argentina) size: 16 in/40 cm long Pholidophorus is one of the earliest-known undisputed teleosts to appear in the sea. Superficially, it looked like a modern herring, with a symmetrical tail, a single dorsal fin halfway along its back, paired pectoral and pelvic fins on the underside, and an anal fin toward the tail. With large eyes and flexible jaws, equipped with small, rounded teeth, it was obviously a fast-moving, predatory fish, probably feeding on crustaceans in the plankton. Some specimens have also been found with the remains of other bony fishes in their stomachs.

Despite their "modern" appearance, Pholidophorus and its relatives were primitive bony fishes, betrayed by 2 main features. Their bodies were encased in the heavy, enameled scales of the earlier "ganoid" fishes, the palaeoniscids (see p. 36). And their "backbones" were only made of bone in places. Their successors, the leptolepids (below), were the first teleosts to have a backbone made entirely of bone.

name: Leptolepis time: Middle Triassic to Early

Cretaceous locality: Africa (Tanzania), Australia (New South Wales), Europe (Austria, England, France and Germany) and North America (Nevada) size: 1 ft/30 cm long Leptolepis and its relatives were herring-type fishes, like the pholidophorids (above). But unlike them, the leptolepids lived and moved in shoals, gaining safety in numbers as they fed on plankton in the surface waters. This gregarious lifestyle is deduced from the many fossil finds in which hundreds of these fishes have been preserved in the same slab of rock.

The leptolepids were also more advanced than the pholidophorids in 2 important respects. First, their skeletons were made entirely of bone, and second, their bodies were covered in thin, rounded scales with no enamel coat.

Both these developments aided swimming. The backbone formed a strong, yet flexible, rod to resist the pressures created by the S-shaped bending of the body during swimming. The thin scales reduced the fish's weight, and their rounded shape made the body more hydrodynamic.

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