Innovation and evolution at the edge: origins and fates of gastropods with a labral tooth

Abstract

I combined data from the taxonomy, phylogeny, functional morphology, biogeography, and fossil record of gastropods to probe the origins, distribution, and fates of predatory gastropod clades characterized by the presence of a labral tooth, a downwardly projecting tooth or spine formed at the edge of the outer lip of the shell. A labral tooth occurs in at least 608 species, of which 251 are Recent. Studies of the type and position of the labral tooth, along with other characters, indicate that the labral tooth has evolved independently at least 58 times, beginning in the Campanian epoch of the late Cretaceous. The labral tooth plays a more or less active part in predation on relatively large prey animals that are protected by a hard skeleton. In the Recent fauna, tooth-bearing species are overwhelmingly warm-temperate to tropical in distribution (240 of 251 species; 96%). Within Muricidae (excluding Coralliophilinae), however, there is no discernible latitudinal gradient in the number of tooth-bearing species relative to total regional diversity. First appearances of clades with a labral tooth are overwhelmingly concentrated in the late Oligocene to Pleistocene interval, with the largest number appearing during the early Miocene (12 clades). The temporal pattern differs significantly from that expected on the basis of the number of faunas available per time interval, and is therefore not an artifact of sampling or fossil preservation. The most consistent factor associated with, and permitting the repeated evolution of, the labral tooth is high planktonic primary productivity. Two factors may account for the link between primary productivity and the evolution of labral teeth: (1) the general economic opportunity afforded by ready availability of an access to nutrients, and (2) the greater abundance and sizes range of available suspension-feeding prey animals. Incumbency—the presence of already well-adapted species—often controls evolutionary opportunity. The complementary distributions of major tooth-bearing clades in many parts of the world point to the role of well-adapted incumbents in limiting the adaptive exploration by other clades that could in principle evolve a labral tooth. The elimination of incumbents by extinction, however, does not provide opportunities for other clades to fill the adaptive void.