- The Paleontological Society
Members of the neogastropod muricid subfamily Rapaninae are abundant, shallow-water predators whose phylogeny was previously investigated by Kool (1993b), who used mainly anatomical characters. In order to deepen understanding of the evolution of this important clade and to incorporate functional, ecological, and fossil evidence, we performed a phylogenetic analysis based on 34 shell characters in 45 genus-level taxa, including five muricid outgroups. Cladograms based on shell characters alone differed from those founded on anatomical features, and these analyses differed from the phylogenetic reconstruction combining all available morphological evidence. The preferred cladogram incorporates all evidence and reveals a “Thais group” and an “Ergalatax clade” that both emerge from the derived portion of a more primitive, paraphyletic group of other rapanines. The Ocenebrinae, the other four outgroup taxa, and three ergalataxine taxa all lie outside the rapanine clade that includes the remaining ergalataxines as a derived subclade.
We used the phylogenetic results to probe aspects of the ecological history of the Rapaninae. Our data imply that antipredatory shell defenses (elongated aperture, denticles on the inner side of the outer lip, and robust external spines and tubercles) evolved multiple times, mainly in post–early Miocene clades in the Indo–West Pacific region. These results support earlier nonphylogenetic inferences.
We compared known prey types and methods of predation of living rapanines with their distribution on our phylogenetic tree. The plesiomorphic mode of feeding in the Rapaninae is drilling of hard-shelled prey. Feeding by other means and on such soft-bodied prey as sipunculan and polychaete worms evolved several times independently among post–early Miocene rapanines in the Indo–West Pacific. Methods of predation on hard-shelled prey that involve edge-drilling or attack by way of the aperture also evolved independently several times, but did so throughout the geographical range of the subfamily.
Specialization for life on the upper shore occurred in at least eight lineages, all but two of which are confined to the Indo–West Pacific. Ecological diversification of the Rapaninae was therefore most common in the tropical Indo–West Pacific during and after early Miocene time. This diversification occurred in a setting of already high biological diversity and intense competition and predation.