Phanerozoic trends in ecospace utilization: The bivalve perspective

Ecologic diversity within marine invertebrates has typically been reconstructed by examining three components: mobility, feeding habit, and tiering. Based on various combinations of these three features, a wide range of hypothetical ecospaces or “cubes” (the agglomeration of which forms a “Bambachian cube”) can be delineated. Given the range of constraints on a specific clade's morphologic diversity, however, only a relatively limited proportion of these possible combinations are actualized. Previous studies investigating Phanerozoic trends have documented the general pattern of ecologic diversity – measured as the total number of filled cubes – of all metazoan groups which displays a major increase in the range of ecospace inhabited. However, detailed, clade-specific studies are limited. In this Phanerozoic-level study on the Class Bivalvia, a group with a very robust and virtually unparalleled fossil record, the clade's ecologic diversity is reconstructed based on a set of newly compiled taxonomic and ecologic databases that were initially derived from a combination of the Sepkoski Compendium and the Paleobiology Database. These data have been binned into 94 intervals, from the Fortunian up to the Piacenzian. The ecologic position of each genus was determined relative to the 140 ecologic cubes that reflect the hypothetical range of ecospace bivalves may have inhabited (i.e. fundamental cubes or ecospaces) and which are based on a substantially modified version of the earlier Bambachain cube (Bambach et al., 2007). The overall results show several features: (1) given the various morphologic constraints, bivalves have only occupied 44 of the total, hypothetically available cubes which represents the clade's realized ecospace; (2) the most significant and dramatic phase of increase in the range of ecologic types occurred during the Ordovician; following this event, new ecospace was exploited in a much more limited fashion; (3) most changes in ecospace utilization involved increased packing within a relatively limited suite of cubes; and (4) mass extinctions had a very limited impact on ecospace utilization despite their impacts on bivalve taxonomic richness; only the K-Pg extinction had significant effect on the overall ecologic diversity of bivalves, largely reflecting the demise of a novel reef-building group, the rudistids.