PHYLOGENETIC ANALYSIS AND FUNCTIONAL MORPHOLOGY SUGGEST AN ALTERNATIVE KEY INNOVATION FOR THE RADIATION OF NATICID GASTROPODS

Naticid gastropods are perhaps best known for their activities as drilling predators, although this guild also includes living muricids, cassids, capulids, marginellids, and buccinids. Most of these drilling clades are thought to have radiated in the Cretaceous. In the case of naticids, drilling has been inferred from the fossil record to be the cause of their diversification. However, any other synapomorphy for the group also represents a potential key innovation that could explain their radiation, and the incorporation and sealing of large amounts of local sediment in naticid egg masses, or egg collars, meets these criteria. A feature proposed as a key innovation must serve a known function; thus a clear understanding of the sedimented egg collar is critical to elucidating its potential role in naticid evolution.

Two functional hypotheses for the sediment-impregnated egg collar are viable: 1) incorporated sediment serves to deter fish predation on embryos within the collar, and 2) incorporated sediment acts as ballast, freeing maternal snails from competition for a limited number of egg attachment sites. The former hypothesis is supported by comparison of egg capsule and sediment grain diameters, as well as facies data from the earliest fossil occurrences of naticids. However, potential anti-predatory benefits from included sediment are complicated by links to habitat and mode of development. The latter hypothesis is supported by quantification of egg collar densities and phylogenetic reconstruction of primitive egg collar morphology. Egg collar densities and morphologies covary with estimated densities at the sediment-water interface, suggesting that included sediment functions as ballast, whereas the collar morphology acts as a snowshoe that prevents collars from exceeding the shear strengths of marine sediments.

This study emphasizes that paleobiologists cannot rely solely on the fossil record to preserve potential key innovations. Phylogenetic analysis and reconstruction of basal character states allow detailed insight into the evolution of naticid egg collars as well as their potential importance in the diversification of the group, even though there is no known fossil record for these structures.