Classification is based on recognition of similarities. The species, the category of classification immediately below the genus, comprises organ isms that are similar to one another and are capable of interbreeding and producing fertile offspring. In order that other paleontologists will have a concrete example for recognition of the characteristics of the species, the person who first describes and names it designates a specimen as the holotype, or sometimes he designates a group of like specimens as a type series, including one individual specimen as the holotype. This is the standard to which collectors and classifiers can refer, the keystone of the species.
Genus is an abstract concept by which classifiers can associate species in which they have reason to recognize biological relationships and similarities. The author of a genus designates one of the species included in his genus as the genotype, which then becomes the standard for reference to that genus. The cat, for example, is placed in the same genus as the lion, tiger, and cougar, while each belongs to its own species. Like the genus, the larger units — family, class, and phylum —are synthetic groupings based upon more and more general resemblances, just as the human family is included in the nation and the race.
A recent example affords glimpses of paleontologists at work studying and classifying a fossil. Near Bishop, California, 31 specimens of a fossil resembling the present-day sea cucumber were collected in Lower Cambrian shale. The organism was associated with fellow members of the echinoderms, to which crinoids and starfish belong, and with trilobites. Unlike the typical crinoid, however, it apparently was not held to the
bottom by a stalk but was free-living. Its closest relative in the fossil world appeared to be a group of starfishlike animals known as edrioasteroids.
What was unique about this new organism was the spiral arrangement of the tiny hard plates of its body. Later it was deduced after close study that the organism could expand and contract its armored skin.
Two species were distinguished — Helicoplacus gilberti and Helicoplacus curtisi, from the Greek helix and plakos, meaning "spiral plates" —and it was proposed to place them in a new class, the Helicoplacoidea, in recognition of their major differences from already classified fossils. In their announcement, Professor J. W. Durham of the University of California at Berkeley and Professor K. E. Caster of the University of Cincinnati asserted that from the discovery "it is apparent that considerable doubt is cast upon the generally accepted view that the ancestral echinoderm was attached. Need for a reconsideration of all subphyla of the Echinodermata hitherto proposed is also indicated." Holotypes of the two species were deposited in the University of California museum.
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