Eldredge, Niles American Museum of Natural History, New York, New York.
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Living species very closely resembling fossil relatives in most anatomical details. The term is a relative one and, applied loosely, could embrace nearly all extant animals and plants. In its more restricted usage, the term applies to living species with four additional characteristics: (1) truly close anatomical similarity to (2) an ancient fossil species—generally at least 1 × 108 years old; (3) living members of the group are represented by only a single or at best a few species, which are (4) often found in a very limited geographic area. Examples are horseshoe crabs (found on the eastern shores of the two Northern Hemisphere continental landmasses, with closely similar relatives over 2 × 108 years old); ginkgo trees, dating from the Mesozoic era, which, until their recent introduction over the world, were restricted to Asia; and coelacanth fish, which have not changed much since the Devonian and which today are found only between eastern Africa and Madagascar and the Comoro Islands.
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