THE EVOLUTIONARY HISTORY AND PALEOBIOLOGY OF THE SINISTRAL CONES

Despite the great species diversity, cosmopolitan distribution, and 55 MY evolutionary history of the contemporary marine gastropod genus Conus, sinistral (left-coiling) taxa are only known from Pliocene and Pleistocene deposits in the southeastern United States. The fossil record of sinistral Conus shells displays a pattern of morphological stability through most of the Pliocene, encompassed within the single highly variable species C. adversarius. This period of stability was followed by the origin of several novel morphotypes near the Plio-Pleistocene transition, just prior to the extinction of all sinistrally-coiling Conus taxa in the Pleistocene; these novel morphotypes can be distinguished by discrete differences in the architecture and ornamentation of the shell shoulder and sutural ramp, the general shape of the shell body whorl, and subtle differences in coloration patterns on the body whorl as revealed by ultraviolet light.

Based on comparisons with living Conus taxa, the large protoconch size of C. adversarius (~1.0 mm in diameter) suggests that this species likely had a non-feeding (lecithotrophic), primarily benthic developmental mode. It is inferred that the novel Plio-Pleistocene sinistral Conus morphotypes likely had larval shells similar in size to those of C. adversarius, and, thus, a similar developmental mode, though specimens of the Plio-Pleistocene types with well-preserved protoconchs are lacking, and protoconch size had to be estimated based upon the breakage pattern of shell apices. The benthic developmental mode suggested by the fossils may have played an important part in the maintenance of sinistrality in the genus Conus following its origin by concentrating aberrant sinistral individuals within an ancestral dextral population.

While the monophyly of the sinistral cones remains to be tested, several unique character state combinations in addition to coiling direction suggest that these snails form a clade. These include relatively large larval shells, similar color patterning themes (e.g., prominent banding), and fine spiral ribbing on the anterior ends of their body whorls. If the sinistral cones form a monophyletic group, their radiation may offer insights into the interaction between developmental mode, intraspecific variability, and diversification in marine gastropods.