Morphological and spatial influences on molluscan macroevolution

Predicting the capacity of a lineage to survive, disperse, or diversify when faced with a changing environment is a fundamental aim in both ecology and evolution and one that is increasingly critical in a rapidly altered world. Here, by using morphological and phylogenetic measures of similarity as proxies for ecological similarity, I examine how the presence and absence of similar taxa across both space and time impacts extinction risk, morphological evolution, and colonization success. Through use of temporal and taxonomic replicates, the consistency of these patterns can be characterized, leading to a better understanding of the degree to which biotic responses to novel conditions are predictable./nChapter 1 examines the degree to which predictors of extinction risk remain consistent over time. I approach this using the fossil record of ammonites across the Cretaceous Period (145-66 Ma), incorporating measures of morphological similarity, in addition to classical descriptors of shell coiling and non-morphological traits, into stage-level extinction models. I find that predictors of background extinction are highly variable in importance and magnitude of selectivity from stage to stage with few consistent relationships that can be applied predictively across time. These results highlight the temporally variable nature of background extinction, the importance of context, and the challenge this poses in the search for generalizable rules of extinction./nChapter 2 assesses whether occupation of a newly-formed environment coincided with consistent morphological shifts independently across taxa. I use ammonite occurrences in and around the North American Western Interior Seaway (WIS) during its formative stage and outline analysis of shell aperture shapes to test whether species inhabiting a deepening seaway occupy similar positions in morphospace relative to their congeneric, non-seaway counterparts. I find that some genera spanning the boundary between the Western Interior Seaway and the Gulf and Atlantic Coast region to the seaway's south exhibit similar shape differences between the two regions, indicating some predictability in the direction of morphological evolution given access to the same environment. This spatial pattern, however, is not reflected within wide-ranging species. The consistency across multiple taxa suggests that the onset of novel environmental conditions may be capable of influencing the trajectory of morphological evolution in a clade as a whole./nChapter 3 characterizes the relationship between evolutionary relatedness to native taxa and successful colonization within a late Cenozoic embayment. Darwin's naturalization hypothesis predicts that potential colonizers more closely related to incumbent taxa are less likely to successfully establish due to competition and limiting similarity. I approach this using a large phylogeny of extant bivalve genera and the rich fossil record of bivalves along the Pacific Coast of North America. Specifically, I examine patterns of colonization from the open ocean, through restricted connections, into the embayment formerly present in the San Joaquin Basin of central California from 27 Ma to 2.5 Ma. By comparing the relatedness of successful colonizers to the native fauna in the basin with the relatedness expected through random assembly, I find that colonization success in the San Joaquin Basin is not strongly linked with unusually close or unusually distant relatedness in any of the time bins considered.

Advisor: Marshall, Charles