Order Actinistia

The actinistians, or coelacanths, have a long evolutionary history, longer in fact than anyone thought. They arose in the Middle Devonian and the last fossils found come from Late Cretaceous rocks, some 70 million years old.

Then, in 1938, a living coelacanth was caught in the deep waters of the trench that separates Madagascar from southern Africa. The people of the nearby Comoro Islands had known of this fish for generations, but it was new to science. The term "living fossil" was awarded to Latimeria chalumnae, the only surviving species of a group that evolved over 380 million years ago.

name: Eusthenopteron time: Late Devonian locality: Europe (Scotland and USSR) and North America (Canada) size: up to 4 ft/1.2 m long This large osteolepiform rhipidistian fish is considered by most paleontologists to be the direct ancestor of the amphibians. The pyramidal arrangement of the bones in its paired fins is strikingly similar to the arrangement of the limb bones in land animals (see p. 49). In addition, the structure of its backbone, the pattern of the skull bones and the complex, labyrinthine folding of the enamel inside each tooth all bear a remarkable resemblance to these features in the first amphibians (see p. 52).

Eusthenopteron was a long-bodied, predatory fish with a powerful 3-pronged tail, consisting of 2 equal-sized lobes on either side of the bony axis of the vertebral column. Its pectoral fins were well forward on the body, and articulated with the shoulder girdle, which in turn articulated with the back of the skull. The pelvic fins were well to the rear, as were the 2 dorsal and anal fins.

name: Macropoma time: Late Cretaceous locality: Europe (Czechoslovakia and England) size: 22 in/55 cm long Macropoma was only about one-sixth the length of its living relative, Lati-meria, but in all other respects the two fishes, separated in time by almost 70 million years, are remarkably similar.

Macropoma had a short, deep body and a bulbous, 3-lobed tail — a design characteristic of coelacanths. The only teeth in its mouth were concentrated at the front, but the hinge joint in the skull (the same arrangement as in the rhipidis-tians) ensured that the jaws could be opened wide and closed forcefully on prey. Its pectoral fins were set high on the flanks, to aid maneuverability, and the pelvic fins were placed midway along the belly. The first of the dorsal fins was sail-like and supported internally by long bony rays; the other fins were fleshy, muscular lobes.

The living coelacanth is one of the few bony fishes that give birth to live young. Whether this was the case among its ancient relatives is not known, but discoveries of fossil coelacanths in Niger and Brazil may shed light on their breeding habits.

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