PHYLOGENY, DEVELOPMENTAL MODE, AND EVOLUTION IN GEMINATE SPECIES OF TURRITELLINE GASTROPODS FROM THE CENTRAL AMERICAN ISTHMUS REGION

Turritelline gastropods are conspicuous, diverse, and abundant in Miocene-Pleistocene deposits of northernmost South America, southern Central America, and adjoining areas of the Western Caribbean. Yet the patterns and processes of their evolution are almost unknown. In this study, we are generating the first phylogeny of fossil and Recent turritelline species on both sides of the Central American Isthmus based on both “traditional” shell characters and a relatively new approach using continuous variables. On this phylogeny we also are plotting evidence of developmental mode, inferred from protoconch size and shape.

Preliminary results suggest that our new data are similar to the few previously-published observations of protoconch diameter and Diameter/Number of Volutions (D/Vol), which has been used by previous authors as an indicator of larval developmental pattern. Three Recent Western Atlantic species yield an average maximum protoconch diameter of 428.4 ± 79.2 µm and a D/Vol ratio of 2.97 ± 0.72. Three Recent Eastern Pacific species have an average maximum diameter of 295.5 ± 93.7 µm and a D/Vol ratio of 2.20 ±1.41. The difference of diameter are significant (Student's t-test, α = 0.044) as is the D/Vol difference (α = 0.206). Whether egg diameters on the Pacific side of the Isthmus decreased or whether sizes in the Atlantic increased remains unclear; however, measurement of the fossil species Turritella gatunensis reveals a maximum diameter of 486.7 µm and a D/Vol ratio of 3.66, suggesting that egg sizes in the Atlantic remained constant. These results suggest that Atlantic species had and continue to have larger egg sizes compared with Pacific species, a pattern seen in other marine invertebrate taxa. The evolutionary significance of this pattern, however, remains unclear, as Atlantic and Pacific species may not form monophyletic clades.