Sunday, October 21, 2012


EURYPTERIDS (SEA SCORPIONS) were the largest ever arthropods. They belong to the chelicerates (“biting claws”), a group that includes scorpions and spiders. Sea scorpions appeared in Ordovician times and persisted into the Permian. Among the largest was Pterygotus, which lived more than 400 million years ago and could grow longer than a man. Before predatory fish evolved, sea scorpions were among the most dominant hunters of shallow seas. Some species even crawled ashore, where they breathed air by means of special “lungs,” like those of certain land crabs.

Like all sea scorpions, Pterygotus had a two-part body. Its prosoma (front) bore the mouth, one pair of large eyes, one pair of small eyes, and six pairs of appendages. The long opisthosoma (rear) had 12 plated tail segments called tergites. The first six tergites contained pairs of gills and included the creature’s sex organs. Pterygotus’s telson, or tail, formed a wide, short paddle. In some sea scorpions, the telson took the shape of pincers or a spike.

Pterygotus had big, sharp eyes that could detect the movement of small, armored fish on the muddy sea floor some way ahead. The hunter could have crawled or swum slowly toward its victim, then produced an attacking burst of speed by lashing its telson up and down. Before the fish could escape, it would be gripped between the pincers of a great claw with spiky inner edges. This fang would crush the struggling fish and feed it to Pterygotus’s mouth, which lay beneath its prosoma and between its walking legs.

Many species of sea scorpion were much smaller and less well-armed than Pterygotus. Eurypterus was only 4 in (10 cm) long and had two short fangs. It would not have been able to tackle the large prey that Pterygotus lived on. These creatures used their legs to pull tiny animals toward their fangs, which tore them up and fed them to the mouth.

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