BIOMETRIC ANALYSIS OF SELECT NEOGENE MELLITID ECHINOIDS FROM THE SOUTHEASTERN U.S.: MORPHOLOGICAL VARIABILITY AND IMPLICATIONS FOR TAXONOMIC ASSIGNMENTS

Qualitative characteristics of skeletal morphology in fossil organisms serves as the foundation for establishment of taxonomic assignment for such fossils, yet interpretation of these descriptors may be difficult for subsequent workers to assess. Technological advances have allowed more quantitative approaches to taxonomy to become established, thereby reducing dependence on subjective interpretation of morphological traits to distinguish species. Biometric analyses also may help substantiate (or refute) taxonomic splitting or lumping of fossil taxa where disputes exist among described species.

Measurements were completed for up to 64 morphological traits, on each of 372 individuals, of several species of Encope and Mellita from the southeastern Coastal Plain (specimens collected from localities in SC and FL). These traits included the characteristics frequently used as part of species descriptors such as test length, width, height, peristome length and width, periproct length and width, petaloid ambulacra length and width, and ambulacral lunule length and width (among others). One of the goals for establishing this morphometric database was to help define the morphological variability within a fossil species population, and to use these data to help test for agreement with taxonomic assignments for the species.

Biometric data were tested between species using analysis of variance (ANOVA) as a simplified approach to verify true morphological differences for defined species. The results showed support for species currently established as valid (i.e., the “splitting” of taxa into more than one species group is appropriate). This is important because only modest morphological variation exists between some defined mellitid species, and our test appears to validate this distinction. A second point regarding variability within the mellitid species examined also warrants comment. Asymmetry (left vs. right) is present when detailed examination of ambulacral traits was completed. Petaloid ambulacrum I is not symmetrical with respect to ambulacrum V, nor is ambulacrum II symmetrical when compared to IV. This is not what is expected from the ideal model of symmetrical growth for echinoid individuals, but the significance of this observation is not yet clear.