Bivalve shells contain growth lines which are formed as a result of periodic environmental or physiological stress, analogous to tree rings. The study of these regular growth increments in the hard parts of bivalves and other calcifying organisms is called sclerochronology. In this thesis, I have examined the ways that sclerochronology can be useful in fields where it is underutilized. The first chapter involves aggregating hundreds of past bivalve seasons of growth worldwide to help answer the question: why do bivalves stop growing in certain seasons and where does it happen? We discuss the primacy of temperature over seasonal food supply as the major determinant of seasonal growth in bivalves, and the latitudinal gradients of likelihood of shutdown that result from this. The second chapter is an investigation of bivalve growth in deep time, looking at the rate of growth of an enigmatic Early Jurassic group called the Lithiotids. For the first time, we have isotopically calibrated their growth, determining that it is quite fast in comparison to other giant bivalves through time but does not corroborate prior hypotheses that they harbored photosymbiotic algae in their tissue. Finally, in chapters three and four we report on investigations of carbon, oxygen and nitrogen stable isotopes of giant clam shells in the Red Sea and relate their growth rate to those environmental proxies. In chapter three, we report interspecific and intrashell differences in carbon and oxygen isotopes for different Tridacna species, corroborating the proposed shallow life habit of the rare endemic T. squamosina, and discuss how the outer shell layer of the giant clams records higher formation temperatures than the interior. Chapter four reports on the unexpected acceleration of growth of modern giant clams in the Northern Red Sea, and we propose that fertilization by anthropogenic nitrate aerosols is recorded in the nitrogen isotopes of their shell organic material. Together, these chapters represent applications of sclerochronology to understand bivalve physiology in the deep and near past, the present and potentially the future.

Brachiopods dominated the seafloor from the Ordovician to the Permian as one of the primary members of the Paleozoic fauna. Despite the devastating effects of the Permian-Triassic extinction, the group mounted a successful recovery during the Triassic and Jurassic, which was followed by their final decline. One proposed cause of this decline is the large increase in bioturbation associated with the Mesozoic Marine Revolution, leading to brachiopods shifting to harder substrates. This hypothesis was explored using occurrence and abundance data downloaded from the Paleobiology Database, with carbonate lithologies serving as a proxy for hard substrates and siliciclastics as a proxy for soft substrates. Brachiopods were more common on carbonate substrates in the Mesozoic which suggests a shift to harder substrates due to rapidly increasing bioturbation during the era. The resulting restriction to harder substrates is a contributor to the Mesozoic decline of brachiopods. Though increasing bioturbation has been previously proposed as a cause of brachiopod decline, this study provides additional quantitative support for this hypothesis.