PATTERNS OF PHYLETIC EVOLUTION IN TWO LINEAGES OF LYMNOCARDIID BIVALVES (LAKE PANNON, CENTRAL EUROPE)

Patterns preserved in the fossil record are of the highest importance in addressing questions about long-term evolutionary processes, yet both the description of pattern and its translation into process can be difficult. With respect to identifying gradual phyletic change, randomly generated sequences may exhibit characteristics of a “trend”; apparent patterns, therefore, must be interpreted with caution. Furthermore, even when the claim of a gradual trend can be statistically justified, interpretation of the underlying mechanisms may be challenging. Given that we can observe populations changing rapidly over 10's or 100's of years, it is now more difficult to explain instances of geologically gradual (as opposed to punctuated) change.

Here we describe morphologic change in two bivalve lineages from the Late Miocene Lake Pannon. We evaluate change according to the model-based methods of Hunt (2006). Both lineages exhibit size increases and shape changes over an interval of approximately 3.5 m.y. Size increase in both lineages and shape change in one lineage (using eigenshape and PCA) are best fit by a model of directional evolution (general random walk); most remaining shape variables are best described as unbiased random walks.

Parallel trajectories of increasing log shell height over time suggest comparable directionality in the two lineages. We tested this possibility by fitting a set of models to the two sequences. The models differ in two ways: whether dynamics are the same or different in the two lineages, and whether evolution is directional (general random walk) or not (unbiased random walk). Of the four possibilities, the data overwhelming support the model in which evolutionary dynamics are directional and shared across the two lineages, suggesting that the size increases may be a shared response to the same cause.

Although the European Neogene has been well studied in terms of paleoenvironment, we can identify no environmental parameter that exhibits consistent change across the interval of gradual morphologic change. It may be that in Lake Pannon the long-term persistence of generally ameliorating conditions (plentiful resources and habitat space, few predators or competitors) resulted in anagenetic change (including size increase) in some lineages and numerous branching events in other lineages.