Testing ecological function in the carving of anomalodesmatan morphospace (Mollusca: Bivalvia: Anomalodesmata)

Abstract

Today, the most taxonomically rich bivalve clade is Imparidentia. However, 300 million years ago, Imparidentia's sister clade, Anomalodesmata, was dominant among marine bivalves, while Imparidentia was much less diverse. Both clades are ecologically diverse but infaunal suspension-feeding is the dominant mode of life in each. When taxonomic diversity and morphological disparity are plotted through time for each clade, Imparidentia diversified faster in the Mesozoic than Anomalodesmata until the K/Pg boundary, when the extinction event interrupts the pattern, and both clades then diversified in parallel through the Cenozoic. In this paper, I test the hypothesis that life mode has an effect on morphological disparification and taxonomic diversification, that by ecologically separating themselves from the dominant infaunal suspension-feeding mode, some ‘divergent’ anomalodesmatan lineages may have been able to free themselves from direct competition with imparidents and once more start to rise in morphological and taxonomic diversity. During the Cenozoic, morphological disparity of divergent Anomalodesmata does increase compared to their infaunal suspension-feeding relatives. However, taxonomic diversity does not differ between divergent and infaunal suspension-feeding anomalodesmatan lineages. This suggests that functional partitioning drives morphological disparification, but not taxonomic diversification.