Testing the Role of Biotic Factors in Taxonomic, Morphologic, and Functional Diversity: Analysis of Sister Clades Anomalodesmata and Imparidentia (Mollusca: Bivalvia)

Second chances are rare in macroevolution. Most clades show monotonic decline or plateau at low taxonomic richness once the initial decline begins. Classic examples of declining clades show evidence for various drivers of their decline, including biotic interactions and differential rebound from extinction events. Cases of rediversification are particularly valuable because changes at the tipping point can provide insight into factors driving the initial decline. The clam superorder Anomalodesmata (Mollusca: Bivalvia) presents such a rare example of diversity revival. Anomalodesmata showed high diversity in the Paleozoic, low diversity in the Mesozoic, and increasing diversity in the Cenozoic surpassing its Paleozoic peak. Its sister clade Imparidentia has been hypothesized as a potential competitor due to similar ecological functions and shared ancestry. Imparidentia showed reciprocal diversity with Anomalodesmata until the end of the Mesozoic. After the K-Pg extinction, both clades diversified in parallel, with Imparidentia maintaining around 10 times higher diversity. The tipping point in the Early Cenozoic when Anomalodesmata switched from a low plateau to continuous rise in diversity is crucial for understanding the drivers of diversity. I show that ecological partitioning alone did not drive this revival, but did increase the morphological disparity of Anomalodesmata. A combination of ecological and geographical partitioning drove the Cenozoic revival of Anomalodesmata.