THE EVOLUTION AND DIVERSIFICATION OF THE “VERMICULARIINAE” (GASTROPODA: TURRITELLIDAE)

The Vermiculariinae is a long-recognized Miocene-Recent clade of turritellid gastropods with unusual uncoiled shell morphology. Modern representatives include Callostracum gracile and at least 5 species of Vermicularia: V. bathyalis, V. fargoi, V. knorrii, V. pellucida, and V. spirata. Fossil species include V. milleti, V. recta, V. weberi, V. woodringi, and an unnamed Miocene Vermicularia sp. Here we present a phylogeny of fossil and Recent Vermicularia species – based on genetic and morphological data -- to better understand the evolution of this group.

Oxygen isotopic sclerochronologies were used to determine how Vermicularia achieve their unusual morphology and rapid upward growth compared to other turritellids. These sclerochonologies were generated for multiple individuals of V. spirata (Recent) (including a potentially dwarfed form), V. fargoi (Recent), V. recta (Pliocene), V. weberi (Pliocene) and V. woodringi (Pliocene) and compared to data on other fossil and Recent turritellids. A variety of peramorphic heterochronic changes to the ontogenetic growth pattern of typical turritellids are present in Vermicularia species to differing degrees. A trend observed in some turritelline species towards loosening of coiling in late ontogeny has been pre-displaced in these taxa. An unusually long lifespan contributes to peramorphosis (hypermorphosis) in some, but not all species. Peramorphosis is also achieved by acceleration (more rapid growth), through the elimination of the decrease in growth rate typical of other turritellines after reaching 1 year in age.

For the majority of Vermicularia species, uncoiled morphology, facilitated by the protection afforded from close association with other reef formers, appears adaptive for rapid upgrowth, which in turn facilitates suspension feeding in a reef environment. In other species, uncoiled morphology may be functional for living on soft-substrates. While many turritellines became extinct after the closure of the Central American Seaway, Vermicularia continued to diversify in the Plio-Pleistocene and Recent. The relative success of the Vermicularia in the Plio-Pleistocene Western Atlantic, may be attributed to their association with (or construction of) reef environments, allowing them better access to dwindling food resources.