Time-Averaging and Morphology: Variability in Modern Populations and Fossil Assemblages of Mercenaria (Bivalvia)
The morphologic variability of a fossil assemblage is of interest in many paleontological studies. However, many fossil assemblages are time-averaged; that is, many generations of non-contemporaneous organisms are mixed into the same fossil bed. Assemblages of robust mollusk shells deposited in nearshore marine environments are often time-averaged over 100's to 1000's of years. Mixing many generations of a taxon can increase measured morphologic variability over that of a single generation if morphology is changing during the interval of time-averaging. If morphology is changing, time-averaging can also alter observed correlations between morphologic variables, as well as allometric growth patterns. If morphology is static, then time-averaging will not increase variability or otherwise obscure patterns of morphologic variability. Testing the effects of time-averaging on morphology will help determine the reliability of information derived from the fossil record.
In this study, morphologic variability was compared between 6 standing crop, living populations of Mercenaria campechiensis (Bivalvia) and two fossil assemblages of M. campechiensis and M. permagna. One fossil sample was collected as a series of superposed units that could be analyzed individually or in aggregate. The x,y coordinates of 13 landmarks and pseudolandmarks were recorded on over 600 valves, and variability was calculated using Least Squares Procrustes Analysis. Once corrections were made for allometry, the variabilities of the samples drawn from single time-averaged fossil beds were indistinguishable from the variabilities of the recent samples. For this data set, the variabilities of the fossil samples could be used without reservation to estimate the variability of the standing crop populations from which they formed. Morphology was quite stable over the 100's to 1000's of years that likely passed as the assemblages accumulated.
A small amount of analytical time-averaging of the samples increases variability slightly, but additional analytical time-averaging causes no further increase. Very slight morphologic fluctuations are evident at time spans exceeding 100's to 1000's of years. Lumping geographically separated samples and samples of different species also increases variability.
Morphologic stasis is evident in Mercenaria over 100's to 1000's of years, but previous studies have indicated that evolutionary rates over this time frame are typically high. These studies are based on colonization events, however, and are biased towards high rates. Data gathered here and in previous studies suggest that local populations may evolve rapidly at their founding, but that stasis follows this initial burst of change. This model describes a pattern similar to Punctuated Equilibrium at a lower level of the genealogical hierarchy, and is here termed "Punctuated Equilibrium, Jr." This model can be further tested in empirical studies and should aid in determining the causes of species-level evolutionary patterns.