name: Platysomus time: Early Carboniferous to Late
Permian locality: Worldwide size: 7 in/18 cm long One family of palaeoniscids became disk-shaped — rather like such modern teleosts as the reef-dwelling blue tang or the Amazon discus fish. Platysomus lived in both freshwater and the sea.
Its deep body was fringed toward the rear by the elongated dorsal and anal fins. The pectoral and pelvic fins were tiny. Platysomus must have sculled along, bending its body from side to side in a slow swimming action. A fairly straight course could have been maintained, since the tail was deeply forked and symmetrical, although the main propulsive force still came from the tail's upper lobe which was stiffened by rows of stout, interlocking scales.
Like Canobius (above), the jaws of Platysomus were suspended vertically from its braincase, giving it the advantage of a wide gape and bulging gill chambers when the mouth was open. Platysomus probably also ate plankton.
name: Palaeoniscum time: Late Permian locality: Europe (England and
Germany), Greenland and North
America (USA) size: up to 1 ft/30 cm long Torpedo-shaped, with a high dorsal fin and powerful, deeply forked tail, Palaeoniscum was built for speed. It must have been a ferocious predator of other freshwater bony fishes. Its jaws were set with numerous sharp teeth, which were constantly being replaced.
Like all the early ray-finned fishes, Palaeoniscum had a pair of air sacs connected to the throat, which could be inflated to act as a primitive buoyancy device (sec p. 21).
name: Saurichthys time: Early to Middle Triassic locality: Worldwide size: up to 3 ft 3 in/1 m long
The long, narrow body of this freshwater palaeoniscid is reminiscent of that of the modern pike. Similarly, its dorsal and anal fins were placed well back on the body, near the equal-lobed tail.
Saurichthys probably also behaved like a modern pike. It may have lurked among the water weeds, or lain still on the river bed, seizing passing fishes in its toothed jaws. These were elongated into a long beak, almost a third of the body length, which greatly extended the fish's predatory range. The symmetrical placing of its fins, together with a great reduction in its bony coat of scales, also made Saurichthys a powerful swimmer.
name: Perleidus time: Early to Middle Triassic locality: Worldwide size: 6 in/15 cm long
Perleidus and its relatives evolved from the palaeoniscids, and survived during the 35 million years of the Triassic period. They were all freshwater predators with strong, toothed jaws, which could be opened wide due to their vertical suspension from the braincase.
A notable feature of this group was the flexibility of their dorsal and anal fins. This was made possible by a reduction in the number of bony rays within each fin, a thickening of the bases of those that remained, and an alignment with their bony supports within the body. Such mobile fins, together with an almost symmetrical tail, allowed greater swimming maneuverability.
name: Lepidotes time: Late Triassic to Early
Cretaceous locality: Worldwide size: 1 ft/30 cm long Toward the end of the Paleozoic Era, many new types of ray-finned fishes evolved from the marine palaeoniscids. Grouped together as the neopterygians, these fishes show many of the features developed in the modern bony fishes, which descended from this group.
Lepidotes (a member of the semi-onotids, see pp. 18—19) had evolved a new jaw mechanism, which allowed it to feed differently from earlier fishes. Its upper jaw bones were shortened and freed of their connection to the cheek bones, to which they had formerly been fused. This new mobility allowed the mouth to be formed into a tube, and prey could be sucked from a distance toward the fish, rather than engulfed at close quarters as early fishes had done.
name: Dapedium time: Late Triassic to Early Jurassic locality: Asia (India) and Europe
(England) size: 14 in/35 cm long
The deep, round body of Dapedium (another member of the semionotids) was stabilized by long dorsal and anal fins at the rear. Its body was covered in heavy, protective scales with a thick outer layer of enamel.
Dapedium had long, peglike teeth in its short jaws and crushing teeth on its palate. These, combined with its body shape, suggest that it was a mollusk-eater, weaving slowly through the coral reefs of the Early Mesozoic seas.
name: Pycnodus time: Middle Cretaceous to Middle Eocene locality: Asia (India) and Europe
(Belgium, England and Italy) size: 5 in/12 cm long
Although belonging to a later group (the pycodontids), Pycnodus had evolved the same deep-bodied shape and grinding teeth as Dapedium (above), probably in response to living in the same type of environment — calm reef waters — and eating similar food — hard-shelled mol-lusks, corals and sea urchins.
name: Aspidorhynchus time: Middle Jurassic to Late
Cretaceous locality: Antarctica and Europe
(England, France and Germany) size: 2 ft/60 cm long Superficially, Aspidorhynchus looked like the modern gar, or garpike (Lepisos-teus) of North America, although there is no evolutionary relationship between them. Like it, Aspidorhynchus must have been a ferocious predator. Its elongated body, protected by thick scales, was perfectly adapted for fast swimming. The symmetrical tail propelled it, the dorsal and anal fins stabilized it, and the paired pectoral and pelvic fins kept it on course. The jaws were studded with sharp, pointed teeth, and the upper jaw was elongated into a toothless guard.
Aspidorhynchus and its relatives (the aspidorhynchids, see pp. 18-19) are closely related to the modern teleosts (see pp. 38-41), and most probably shared a common ancestry with them. Only a single species of aspidorhynchid survives today — the bowfin, Amia calva, of North American freshwaters.
Modern ray-finned fishes
Modern ray-finned fishes
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