Were bivalves ecologically dominant over brachiopods in the late Paleozoic? A test using exceptionally preserved fossil assemblages

Abstract

Interpreting changes in ecosystem structure from the fossil record can be challenging. In a prominent example, the traditional view that brachiopods were ecologically dominant over bivalves in the Paleozoic has been disputed on both taphonomic and metabolic grounds. Aragonitic bivalves may be underrepresented in many fossil assemblages due to preferential dissolution. Abundance counts may further understate the ecological importance of bivalves, which tend to have more biomass and higher metabolic rates than brachiopods. We evaluate the relative importance of the two clades in exceptionally preserved, bulk-sampled fossil assemblages from the Pennsylvanian Breathitt Formation of Kentucky, where aragonitic bivalves are preserved as shells, not molds. At the regional scale, brachiopods were twice as abundant as bivalves and were collectively equivalent in biomass and energy use. Analyses of samples from the Paleobiology Database that contain abundance counts are consistent with these results and show no clear trend in the relative ecological importance of bivalves during the middle and late Paleozoic. Bivalves were probably more important in Paleozoic ecosystems than is apparent in many fossil assemblages, but they were not clearly dominant over brachiopods until after the Permian–Triassic extinction, which caused the shelly benthos to shift from bivalve and brachiopod dominated to merely bivalve dominated.

Aragonite bias exhibits systematic spatial variation in the Late Cretaceous Western Interior Seaway, North America

“…Despite the potential relationships discussed earlier, we cannot report conclusive evidence for aragonite bias influencing the sampled diversity of molluscan fauna within the WIS. This aligns with other recent studies showing that despite evidence of widespread aragonite dissolution during early shallow diagenesis, perceived diversity is not largely affected by these processes (Behrensmeyer et al 2005; Kidwell 2005; Crampton et al 2006; Hsieh et al 2019). Hence, we must additionally look at external influences that might capture, enhance, or control the distribution of aragonitic faunas that would otherwise be lost to preferential dissolution.…”

“…Cherns and Wright (2000) argued that early-stage dissolution could be substantial and referred to the phenomenon as the “missing mollusk” bias. Subsequent work at a multitude of temporal and spatial scales (Wright et al 2003; Bush and Bambach 2004; Kidwell 2005; Crampton et al 2006; Valentine et al 2006; Foote et al 2015; Jordan et al 2015, Hsieh et al 2019) has debated the magnitude of this bias; however, there is a broad agreement on the potential for dominantly aragonitic shells to suffer greater postmortem diagenetic destruction in the taphonomically active zone (TAZ) (Davies et al 1989; Foote et al 2015). While the effect of dissolution on the global macroevolutionary record of mollusks has been found to be limited, possibly due to the potential of aragonite to recrystallize to calcite (Kidwell 2005; Paul et al 2008; Jordan et al 2015), it is conceivable that local or regional conditions could severely impact perceived patterns of biodiversity in restricted areas (Bush and Bambach 2004).…”

Fossil bivalves and the sclerochronological reawakening

“…Despite persistent methodological frustrations, the study of life-history evolution allowed by the combination of life-span and growth rate data archived in the shells of marine bivalves poses intriguing possibilities for macroevolution and ecology in deep time. Bivalves appear in the fossil record in the Early Cambrian (e.g., Jell 1980; Runnegar and Bentley 1983) and dominate shallow-marine settings from the Mesozoic to the present (e.g., Gould and Calloway 1980; Liow et al 2015; Hsieh et al 2019). Their preservation potential is high even in deep time (e.g., Jimenez et al 2019).…”

The effects of lithification on fossil assemblage biodiversity and composition: An experimental test

“…However, it is worth emphasizing that our cemented samples were equivalent to "poorly lithified" sediments, and the methodological effects of lithification may differ for more completely cemented rocks prepared with other methods. It is also worth emphasizing that studies of the effects of lithification on biodiversity have focused on late Mesozoic to Cenozoic molluscan shell beds (e.g., Sessa et al, 2009;Hendy, 2009;Nawrot, 2012;Sanders et al, 2015), although aragonite dissolution has been discussed more broadly (e.g., Wright, 2000, 2009;Cherns et al, 2008;Hsieh et al, 2019). What about Paleozoic and older Mesozoic rocks, which are often more completely cemented?…”