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states that may have been lost in extant groups. Second, extinct groups may reveal the pattern of character acquisition of traits that are now autapomorphic to the entire living group. This second point is crucial in understanding the morphological transformations that led to more derived body plans. We use this approach in chapter 6 to examine the origin of the mammalian condition, and in chapter 8 to revise previous notes of a Cambrian Explosion of taxa.

Some authors (Eldredge and Cracraft 1980; Patterson 1981; Schaeffer, Hecht, and Eldredge 1973) have claimed that the temporal sequence of fossils should never be used as evidence for ancestor-descendant relationships. In using the fossil record, one assumes at least that the fossil record is sufficiently complete to make a determi-

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