A New Outline Method to Identify Specialized Shell-Wedging Behavior In Gastropod Mollusks

Recent development of outline analysis techniques have opened the door to the quantification of shape variation within and between taxa where homologous points are difficult to define. New applications of these methods are therefore allowing detailed exploration of structures and questions previously thought to be unsuitable for geometric morphometric examination. Here we present preliminary results from a new application of eigenshape analysis designed to explore variation in the shape of the apertural lip of gastropods. Historically, the identification of specialized feeding behaviors with any degree of confidence has relied on direct observation of living animals and the trace fossil record of the interaction. This actualistic approach, although adequate for some species interactions, has limited our ability to unravel the evolutionary history of many other behaviors.

We targeted a number of marine gastropod taxa that utilize a variety of feeding behaviors, including grazing, filter feeding and predation to attempt identification of specialized feeding modes from shell morphology, particularly shell-wedging behavior in which predators force open their bivalve prey using the convex edge of their apertural lip - a behavior that does not always leave a trace fossil record. Results from the eigenshape analysis of apertural lip shape variation suggest extant predatory taxa using this behavior (e.g. Busycon, Fasciolaria, Buccinum) occupy a distinct region of morphospace compared with species known to exploit other feeding behaviors. This association between feeding mode and shell morphology in extant shell-wedging taxa permits a more accurate identification of the behavior in extinct taxa, supplementing analysis of the trace fossil record. This is particularly important because shell-wedging is hypothesized to have evolved independently multiple times to improve competitive performance. If corroborated, such repeated evolution would permit a comparative approach to ask where, when and under what circumstances innovations, such as the convex lip, are acquired and secondarily lost.