Beautiful Losers: The Rise and Fall of the Bivalve Genus Cucullaea

Tightly integrated studies quantifying changes in species richness, geographic range and morphological disparity are difficult to synthesize but necessary to advance our understanding of macroevolutionary processes operating on higher taxa. This research investigates the evolutionary trajectory of the bivalve genus Cucullaea, following a pattern of a symmetrical rise and fall in species richness, beginning in the Jurassic, peaking in the Late Cretaceous and decreasing to the present day, with a single species surviving throughout the warm waters of the Indo-Pacific region. Early cucullaeids departed morphologically from their ancestral parallelodontid stock by elaborating on the theme of taxodont dentition, but it was not until the early Cretaceous that the archetypal characteristics of the genus, including a posterior flange for muscle attachment and the shift to a less wing-like, more symmetrical shell shape, were fully developed. Once morphologically established, cucullaeid species modified this general shell form throughout the Cretaceous and during periods of extinction in the Cenozoic by shifting towards more arcid or glycymerid shell shapes and through alterations of the posterior shell margin. Today, the remaining Indo-Pacific species is a morphological anomaly when compared to its extinct counterparts, suggesting a novel evolutionary strategy.

To study Cucullaea's macroevolutionary history, surveys of the literature and museum collections documented changes in richness and biogeographic distribution. Large-scale shifts in morphology were quantified using outline and landmark morphometrics on hundreds of individual shells from multiple species reposited within the collections of 17 museums in the US, Europe, New Zealand and Australia. These quantitative results support the overall picture described above, but allow a more nuanced examination of Cucullaea's evolutionary pathway, including, 1) the relationship between taxonomic diversity, morphological disparity and geographic range through time, and 2) biogeographic and morphological transitions in the past that may relate to Cucullaea's persistence in the future.