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Updated: May 6, 2025
The "pavement-toothed" sharks, which at first represent three-fourths of the Elasmobranchs, dwindle in turn, and in the formidable spines which develop on them we may see evidence of the great struggle with the sharp-toothed sharks which are displacing them. The Ostracoderms die out in the presence of these competitors. More evident still is the effect of the new conditions upon the Crustacea.
The animal is probably to be placed with the jawless lampreys and hags, a group too low to be included among true fishes. OSTRACODERMS. This archaic group, long since extinct, is also too lowly to rank among the true fishes, for its members have neither jaws nor paired fins. These small, fishlike forms were cased in front with bony plates developed in the skin and covered in the rear with scales.
Some of the members of this order were, like the ostracoderms, cased in armor, but their higher rank is shown by their powerful jaws and by other structures. Some of these armored fishes reached twenty- five feet in length and six feet across the head. They were the tyrants of the Devonian seas. GANOIDS. These take their name from their enameled plates or scales of bone.
Their fore wings were still membranous and delicately veined, and used in flying; they had not yet become thick, and useful only as wing covers, as in many of their descendants. FISHES still held to the Devonian types, with the exception that the strange ostracoderms now had perished.
They were of use in swimming, and sections of them were stiffened with rods of cartilage, and became the pairs of fins. Some experts think, however, that the fish developed directly from a Crustacean, and hold that the Ostracoderms are the connecting link. Directly or indirectly, the fish is a descendant of some Archaean Annelid. It is most probable that the shark was the first true fish-type.
The fish had already been developed, and the Devonian rocks show it swarming, in great numbers and variety, in the enclosed seas and round the fringe of the continents. The first generation was a group of strange creatures, half fish and half Crustacean, which are known as the Ostracoderms. The hind-part of their bodies was remarkably fish-like in structure.
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