Eod

10 mm

maxilla premaxilla maxilla

Figure 16.10 Evolution of the ray-finned bony fishes: (a) the Carboniferous palaeonisciform Cheirodus, a deep-bodied form; (b) the Triassic "holostean" Semionotus; (c) the Cretaceous teleost Mcconichthys; (d) evolution of actinopterygian jaws from the simple hinge of a palaeonisciform (left) to the more complex jaws of a holostean (middle) and the fully pouting jaws of a teleost (right). (a, b, based on Moy-Thomas & Miles 1971; c, based on Grande 1988; d, based on Alexander 1975.)

Figure 16.10 Evolution of the ray-finned bony fishes: (a) the Carboniferous palaeonisciform Cheirodus, a deep-bodied form; (b) the Triassic "holostean" Semionotus; (c) the Cretaceous teleost Mcconichthys; (d) evolution of actinopterygian jaws from the simple hinge of a palaeonisciform (left) to the more complex jaws of a holostean (middle) and the fully pouting jaws of a teleost (right). (a, b, based on Moy-Thomas & Miles 1971; c, based on Grande 1988; d, based on Alexander 1975.)

heterocercal tail

50 mm heterocercal tail

50 mm

highly mobile jaws

50 mm

50 mm

pectoral fin

Figure 16.11 Sharks and rays, ancient and modern: (a) the Jurassic shark Hybodus; (b) the modern shark Squalus; and (c) the modern ray Raja. (Based on various sources.)

highly mobile jaws

50 mm

50 mm

pectoral fin

Figure 16.11 Sharks and rays, ancient and modern: (a) the Jurassic shark Hybodus; (b) the modern shark Squalus; and (c) the modern ray Raja. (Based on various sources.)

Neoselachians radiated dramatically during the Jurassic and Cretaceous to reach their modern diversity of 42 families. These sharks can open their mouths wider than their precursors, and they have adaptations for gouging masses of flesh from their prey. The body shape (Fig. 16.11b) is more bullet-like than in their ancestors, and the pectoral fins are wider and more flexible. Neoselachians range in size from common dogfishes (0.2-1 m long) to basking and whale sharks (16 m long), but the monster ones are not predators: they feed on krill, which they filter from the water. The skates and rays, unusual neoselachians, are specialized for life on the seafloor, having flattened bodies and broad pectoral fins for swimming by sending waves of up-and-down motion from front to back (Fig. 16.11c).

Sharks and their kin radiated three times during the Paleozoic, Mesozoic and Cenozoic, and this seems to match the three-phase radiation of bony fishes, palaeonisciforms, holos-teans and teleosts. It is impossible to say which set of evolutionary radiations came first: the bony fishes had to swim faster to escape their sharky predators, and the sharks had to swim faster to catch their bony fish prey. This is a classic example of an arms race, where predator and prey keep upping the ante, but neither side wins.

Box 16.6 Fish teeth and scales

Isolated teeth and scale, commonly grouped as ichthyoliths ("fish stones"), can usually be assigned to their fishy originator (Fig. 16.12). However, there is great debate over the precise identity of many shark teeth, and especially over the amount of variation that may occur among the teeth from a single species - are they all the same, or do they vary in shape around the jaws? Also, many Cambrian and Ordovician ichthyoliths are a mystery - the original host animal is generally unknown. The thelodonts, an ostracoderm group, were known from a rich diversity of scales from the Ordovician to Devonian, but there are only a handful of partial or complete specimens of the whole fishes.

Ichthyoliths have been used to establish stratigraphic schemes in the Silurian, Carboniferous, Tri-assic, Cretaceous and Tertiary. In some Paleozoic strata, teeth and scales sometimes occur in association with conodonts, and sometimes in situations where there are no other biostratigraphically useful fossils. It is hard work to generate workable dating schemes using such ichthyoliths, not least because their morphology can be so intricate; but it is also frustrating that it is often impossible to relate the ichthyoliths to the complete fishes that might have produced them.

Figure 16.12 Some microvertebrate specimens: (a) thelodont scale (Devonian); (b) thelodont body scale (Devonian); (c) protacrodont shark tooth (Late Devonian to Early Carboniferous); (d) acanthodian scale (Devonian); (e) shark tooth-like scale (Triassic); and (f) shark scale (Triassic). (Courtesy of Sue Turner.)

Figure 16.12 Some microvertebrate specimens: (a) thelodont scale (Devonian); (b) thelodont body scale (Devonian); (c) protacrodont shark tooth (Late Devonian to Early Carboniferous); (d) acanthodian scale (Devonian); (e) shark tooth-like scale (Triassic); and (f) shark scale (Triassic). (Courtesy of Sue Turner.)

Figure 16.13 Skull of the Late Devonian amphibian Acanthostega, showing the streamlined shape, deeply-sculpted bones and small teeth, all inherited from its fish ancestor. (Courtesy of Jenny Clack.)
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