DIFFERENTIATION OF FOSSIL AND RECENT VARIETIES OF MELONGENA (REXMELA) CORONA

A basic question in evolutionary paleobiology is the correspondence of species to morphospecies that are often found as fossils. The Melongena (Rexmela) corona complex is ideal for investigating this question, because some populations are living, some are both living and fossil, and some have been molecularly sequenced. This study addresses the extent to which eleven varieties, two of which have both fossil and recent individuals, of M. (Rexmela) corona can be differentiated by quantitative parameters, and how well morphometric discrimination agrees with perceived taxa. The earliest record of M. (Rexmela) corona is in the Ayer’s Landing Member of the Caloosahatchee Formation (1.6 mya), that is located in central Florida. The six varieties that are present in the southeastern United States and Mexico are found in differing habitats and geographic locale. M. c. var. corona and M. c. var. johnstonei are both found on the west coast of Florida in different environments while M. c. var. sprucecreekensis, M. c. var. winnerae and M. c. var. altispira are all found on the eastern coast of Florida. M. c. var. bicolor, found in the Florida Keys, lives in a marly sand environment. M. c. var. bispinosa is the only variety to live entirely outside of Florida and it is found on the Yucatan Peninsula in Mexico in turtlegrass beds.

This study measured seven distances (length, width, length and width of aperture, spire length, base length and base to suture length) and five angles (spire, shoulder, posterior aperture, body curvature and anterior aperture). In general, fossil specimens tend to be broader with a greater width to length ratio than Recent taxa. Even with the two varieties for which both Recent and fossil individuals were measured, analysis showed that the fossils were more similar to some other fossil taxa than to their Recent populations. When plotted on a canonical score plot, four fossil varieties and six varieties (including two that are known from the fossil record) of Recent individuals are distinguishable. The varieties have all been named in the literature as either species or subspecies; however, according to previous DNA sequencing, there is no genetic differentiation among the populations, even between the two Recent varieties that live in different habitats. This suggests that the different varieties recognized may be at an early stage of speciation not detectable by molecular methods.