Assessing Intraspecific Variation in the Fossil Record: A Key to Understanding the Pace of Evolution

A fundamental question in evolutionary biology is the pace at which evolutionary change occurs in a given taxon. To determine this, paleobiologists measure morphological changes among temporally separated groups of related specimens. Such analysis assumes 1) that individual populations are likely to be less variable than the species as a whole; and 2) that specimens found in a single deposit constituted a single population. One potential challenge in such studies is the identification of “real” populations in the fossil record. We compared the intraspecific variability of two species of scaphitid ammonites from the Western Interior Seaway (late Cretaceous of South Dakota). To lessen the degree to which time averaging influences the validity of the second assumption, we used specimens collected from carbonate concretions. Each concretion contained from twenty to several hundred specimens, as well as strong evidence of rapid, autochthonous formation. We measured six shell characters and analyzed morphological variation among the populations from each concretion. In addition, we compared the variation within a concretion to the variation among specimens collected at random from the same deposits. In general, morphological variation within concretions was less than that of the species as a whole, though the differences were not always significant. Groups of specimens collected at random tended to exclude smaller specimens, which were present in concretions from the same deposits. We suggest that morphological changes in scaphitid ammonites from this area depended strongly on local conditions, and that such changes may have led to evolution within the species. The implications are 1) that rates of morphological (and often evolutionary) change depend on how populations are defined; and 2) that concretions may represent variation within ammonite populations better than randomly collected specimens from the same deposit.