Temporal trends in allometry of shell calcification in northeastern Pacific venerid bivalves: Implications for predicting responses to climate change

Despite a literature spanning almost a century, how allometric relationships of phenotypic traits behave over evolutionary time still remains poorly known for most marine species. In particular, the fossil record is seriously underutilized in this context despite harboring a rich archive of traits. Here we use the late Pleistocene fossil record in San Diego, California, in conjunction with archival and field-collected specimens, to quantify temporal changes in allometric relationships between shell size and calcification, two important functional traits, in five different species of marine bivalves. Our results reject the traditional hypothesis that allometric relationships are statistically invariant over time. They also show that temporal changes in allometric relationships are species-specific, with closely related species showing divergent trends. Finally, we argue that information about the nature of long-term changes in allometric relationships of functionally important traits can provide an important yet underappreciated tool for understanding species and population responses to climate change.