An Ontogenetic Approach to Understanding Changes in Shell Morphology Over Time: The Strombus Alatus Complex in the Plio-Pleistocene of Southern Florida

David Hargreave

Abstract


The study seeks to focus upon the process of shell growth, as well as on the end product of that process, as a way of understanding molluscan shell morphology and how it changes over time.  Techniques developed for analyzing individual ontogeny, when applied to the members of a sample, yield a multifaceted mean growth profile for that sample.  The growth profiles of multiple samples of the same putative taxon are then compared to obtain a more generalized set of growth patterns for that taxon.  Finally, the process is applied to the various members of a fossil lineage to provide insight into changes in the process of shell growth over time.  Presented herein are the results of a detailed ontogenetic study of Recent and fossil material from a number of biostratigraphic horizons in southern Florida involving the mesogastropod subgenus Strombus sensu stricto, a common component of the shallow-water marine fauna.  Evidence from this study indicates that Strombus floridanus Mansfield, 1930, the earliest member of the subgenus in the Florida Pliocene, differs from the extant species Strombus alatus Gmelin, 1791, in both adult morophological characteristics and patterns of growth, and that the former represents the ancestral form of the latter. Strombus floridanus is restricted to the Cancellaria zone of the Jackson Bluff Formation in the Florida Panhandle and to the lower Pinecrest beds in southern Florida, whereas S. alatus makes its first appearance in the post-Bermont Pleistocene.  Connecting these two taxa are a suite of fossil forms representing a single, morphologically changing lineage.  Within this lineage growth patterns in the larval stage undergo a series of incremental changes in one direction during this time interval, while growth patterns in the benthic stage and adult morphological features exhibit a complex set of changes, some being incremental and others undergoing a single large-scale transformation at one point in the geologic record.  The transformation from S. floridanus to S. alatus appears to have taken place over about a one million year interval in the fossil record (from about 2.4 Ma to 1.4 Ma), and has been followed by a period of apparent morphologic stasis. Two intervening morphological forms or chronospecies can be distinguished, separable in their growth patterns and some adult morphological features from the end-point taxa and, to a lesser extent, from one another. With some reservation, Strombus evergladesensis Petuch, 1991, and Strombus lindae Petuch, 1991, have been accepted as valid names for these two forms. The former, as interpreted herein, ranges from the upper Pinecrest to the basal Bermont, while the latter is restricted to a higher Bermont horizon. The existence of two clearly differentiable forms in different units of the Pinecrest beds, as exposed in the APAC Pit near Sarasota, supports the hypothesis of a substantial hiatus separating Units 2-4 from the underlying Units 5-10, and suggests that only the lower units are correlated with the Jackson Bluff Formation of northern Florida. The presence of S. alatus throughout the section exposed in the Leisey Shell Pits of southern Hillsborough County raises serious doubts about the placement of much of that section in the Bermont Formation


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