THE EVOLUTION OF UNUSUAL SHELL MORPHOLOGIES IN FOSSIL AND LIVING TURRITELLIDAE (GASTROPODA)
Anderson, Brendan Matthew
Unusual morphologies are apparently novel forms or features, representing large increases in disparity within a clade. Turritellid gastropods are a Jurassic-Recent group of marine snails, with ~140 valid, named living species and ~800 valid fossil species. They are often the dominant macrofaunal in assemblages in which they occur, and are important for biostratigraphy and paleoclimatology. The unusual morphologies investigated in this study are uncoiling, occurring in the Vermicularia, and septation, which has been considered characteristic of the family Turritellidae, but unusual among gastropods as a whole. In chapter one, I construct a global molecular phylogeny for turritellids. Turritellids have notoriously simple shells, inhibiting understanding of supraspecific relationships among species and the widespread adoption of most proposed generic names. We constructed a molecular phylogeny of more than 30 species from a globally distributed dataset based on the 12S and 16S mitochondrial and nuclear H3 regions. Several distinct clades were recovered, with molecular distances indicating divergence times were likely older than the Miocene. A generic revision is proposed. In chapter two, I construct both molecular and morphological trees for fossil and Recent members of the Miocene-Recent group Vermicularia (Cerithioidea: Turritellidae). We provide a revised taxonomy with more detailed diagnoses for all known living and fossil species. A new species is described from the early Pliocene from the Dominican Republic. In chapter three, I examine the evolution of the Vermicularia in an evolutionary developmental biology framework. Isotopic sclerochronology was used to determine growth patterns and rates in both fossil and Recent species within the phylogenetic framework established in chapter two. This ontogenetic information allows study of the specific heterochronic mechanisms involved in the origin of these species’ morphologies. In chapter four, I demonstrate that septation in turritellids is not an anomaly, but is a widespread feature of high-spired gastropods. Adaptive hypotheses for turritellid septation do not survive strong scrutiny. I outline a methodology for testing the hypothesis that a feature originated as a spandrel, rather than the direct product of selection for that feature. I conclude that septa within turritellids are spandrels of shell thickening, rather than being adaptive themselves.
Evolution & development; Paleontology; Biological oceanography; Evo-Devo; Isotopic Sclerochronology; Molecular phylogeny; Spandrels; Vermicularia
Allmon, Warren D.
Lovette, John I.; Dietl, Gregory P.; Ivany, Linda C
Ph. D., Geological Sciences
Doctor of Philosophy
dissertation or thesis