ENVIRONMENTAL CORRELATES TO MORPHOLOGICAL DISTINCTION IN LATE EOCENE OLIGOPYGUS HALDEMANI FROM FLORIDA

Oligopygus haldemani exists in two overlapping forms: the typical one that predominates in the peninsula of Florida and "O. colsoni" Lambert. the former is a guide fossil to, and is evidently restricted to, the middle part of the Late Eocene Ocala Limestone, whereas the latter is found in the upper part of the unit in the panhandle.

Three possible explanations for the change from one form to the other suggest themselves: ecological (different substrate preferences), temporal (stratigraphic differences), or spatial (morphological cline). We have modified our data set of characters relevant to the morphological differences to try to distinguish which environmental component best correlates to the change in form. This is complicated by the fact that the three explanations are all closely interrelated in the region.

In a strict north-to-south arrangement of localities or groups of localities specimens from the southern part of the peninsula do not conform well to a simple geographic cline, though otherwise the remainder of specimens in the peninsula does.

If localities are grouped into those most likely to be from lower, middle, and upper parts of the biozone in the peninsula, the smaller (younger) specimens reveals a subtle peramorphic trend upsection, but by the midpoint of the size range this is lost in the complete blending of the populations. We suspect therefore that there is some temporal control on the distinction, acting early in development.

Ecological differences across the region during this time, particularly substrate type, are rather subtle, and also closely tied to geography: water depth increased (and presumably mean particle size in the substrate decreased) northward. There is some indication that the forms of populations from the coarsest sediments (the most southern localities and "O. colsoni") are most similar in some respects, but whether or not this indicates an ecological component of the evolutionary pattern requires more work teasing apart the stratigraphic and purported ecological signals.