Echinoderms are an ancient marine phylum with a unique water-based vascular system and frequent five-fold symmetry. They have two subphyla, the mobile Eleutherozoa and the sessile Pelmatozoa. Echinoderms are significant for their ability to survive in the wilderness of the world’s oceans and their skeletons provide important biogeographic information to paleontologists.
Echinoderms are a very ancient phylum of marine animals whose name means “thorny skin” in Greek. This name is a misnomer, however, because not all echinoderms have spines. Their most appropriate universal trait is common ancestry, including a unique water-based vascular system and frequent five-fold symmetry. Although they do not always show fivefold symmetry (sea cucumbers are echinoderms and have bilateral symmetry), echinoderms are known to often play with the typical tendency towards bilateral symmetry, such as in sea urchins (radial symmetry) and the numerous starfish and sand dollars (fivefold symmetry). Echinoderms are one of several exclusively marine phyla.
The earliest known echinoderm is thought to be Arkarua, an ancient disc-like fossil about 1 cm in diameter with a five-fold pattern of dents leading scientists to classify it as a probable echinoderm. This fossil dates to the Late Ediacaran, about 550 million years ago. Furthermore, the first certain echinoderms appear in the early Cambrian, about 530 million years ago. Containing 7,000 living species, echinoderms are the second largest phylum of deuterostomes after chordates (vertebrates), which are the dominant large phylum on earth.
Very flexible, echinoderms are found at every depth of the ocean, from the intertidal zone to the abyssal zone, miles and miles below the surface. There are two primary subphyla of echinoderms; the mobile Eleutherozoa, which includes starfish, brittle stars, sea urchins, sand dollars, sea daisies, and sea cucumbers; and the sessile Pelmatozoa, which includes the crinoids (feathered stars). The mobile subphyla crawl along the ocean floor using a muscular foot and are specialized to consume bottom-dwellers that few other oceanic animals can.
Echinoderms are significant because they are among the only large animals capable of surviving in the absolute wilderness that characterizes the vast majority of the world’s oceans. Their skeletons fossilize readily and provide important biogeographic information to paleontologists. Many limestone formations are made up of echinoderm skeletons, and some paleontologists believe that the evolutionary radiation of echinoderms was responsible for a sudden increase in the diversity of Mesozoic marine life.
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