Cenozoic increase in samplelevel diversity in the marine benthic fossil record: shift in sampling techniques, loss of aragonite, or gain in gastropods?

Biological veracity of the sharp diversity increase observed in many analyses of the post-Paleozoic marine fossil record has been debated vigorously in recent years. This pertains both to global synoptic curves as well as sample-level species richness patterns. Here, we add more fuel to the debate by comparing Jurassic, Late Cretaceous, and Neogene shelly fauna sampled at comparable spatial scales and restricted to comparable open marine siliciclastic facies. Taken at face value, the results suggest that standardized sample-level species/genus diversity more than doubled between Jurassic and Neogene. The Cretaceous data show intermediate patterns. This increase persists when data are constrained latitudinally or confined to unlithified deposits only. However, when restricted to bivalves (the only group well represented in all analyzed time intervals), the Mesozoic diversity estimates drop only slightly suggesting that bivalves are primary component of the observed alpha diversity in that time interval. The other two higher taxa common in the Mesozoic (brachiopods and gastropods) account for a minor fraction of diversity in most samples. In contrast, the Cenozoic diversity drops dramatically when data are restricted to bivalves. Consequently, alpha diversity of the Mesozoic and Neogene bivalves are nearly identical and indistinguishable statistically. Cenozoic gastropods yield higher diversity than Cenozoic bivalves or any Mesozoic higher taxon. The observed increase in whole-fauna species richness appears to be driven entirely by the addition of diverse gastropods. This may represent (1) true diversification, perhaps reflecting increased ecospace utilization; (2) aragonite loss, resulting in underestimation of Mesozoic gastropod diversity due to shell-less mode of preservation; and (3) differences in sampling techniques, with field surveys of Mesozoic molds/casts in lithified deposits capturing much fewer small gastropods than sieved bulk samples of Neogene sediments. Although we add questions rather than provide answers, it is clear that dissecting diversity trends across higher taxa is a fruitful direction of research that provides new specific target questions and should help us to decouple artifacts from true evolutionary signals entombed in the fossil record of diversity.