The bivalve family Cucullaeidae has persisted for the past 200 million years, with shifts in species richness, geographic range and morphological disparity occurring in response to extrinsic and intrinsic evolutionary pressures and opportunities. The purpose of this dissertation is to investigate the evolutionary trajectory of the cucullaeids in detail, by integrating multiple analytical approaches towards assessing changes in biodiversity, biogeography and morphology.
As one element of the Jurassic radiation of bivalves generally, and of arcoid bivalves specifically, the Cucullaeidae initially diverged morphologically from their ancestors through modification of their taxodont dentition into distinct sets of central and lateral hinge teeth, which enabled early cucullaeid species to live in semi-infaunal, endobyssate life habits. Facing unique functional obstacles to the infaunal adaptive zone, the family continued to evolve morphologically throughout the Jurassic and Early Cretaceous, most notably employing an innovative, internal flange to facilitate lengthening of the posterior margin, to eventually become the first, post-Paleozoic arcoid group to diversify into fully-infaunal, free-burrowing life habits. Based on a distinctive, powerfully-interlocking hinge structure, this morphological shift and expansion was paralleled by increases in species richness and geographic range. By the Late Cretaceous the family was represented by many species (~65) distributed throughout warm-water, sandy environments across the globe. During this diversity peak, innovative patterns of external sculpture within the “core” genus Cucullaea , along with extreme modifications to shell features and overall form among other cucullaeid genera, pushed the morphological limits of the family and expanded its adaptive range.
Following the extinction and lack of replacement of many species, the number of cucullaeid species plummeted into the early Cenozoic, especially for once highly-diverse assemblages in North America and Europe. At that time the family is composed primarily of large-bodied taxa, which utilized their size for enhancing stability in the substrate and avoiding the risk of excavation. Now represented entirely by the genus Cucullaea , the family maintained its worldwide distribution throughout much of the Paleogene, but a continued decline in global temperature eventually resulted in the extripation of individual species and, indeed, the entire family from several continents. This winnowing has continued to the present day, where the range and diversity of the group is a relict of a once richer past. Yet the family persists with four surviving extant species, and, beginning in the Miocene, has radically altered multiple aspects of the long-established cucullaeid body plan. Reduction to the hinge plate, ligament grooves, shell thickness and external sculpture, in conjunction with a maintenance of large body size and elaboration of the posterior flange, has completely shifted the family into new morphological territory, rivaling the Jurassic-to-Cretaceous transition whereby elongate-shaped, endobyssate species were replaced by trapezoidal, free-burrowing forms. In light of the prolonged morphological evolution of the Cucullaeidae, the magnitude of this recent shift is impressive, with the persistence of this relict family relying on the survival of and expansion around this modern morphotype.