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

Abstract

Lithification of unconsolidated sediment into solid rock can bias the tabulation of taxonomic diversity from fossil assemblages in several ways. Methodological biases result from the relative difficulty of extracting fossils from lithified sediments, whereas diagenetic biases result from poor preservation or destruction of fossils by dissolution or other processes. Here, we use an experimental approach to isolate the effects of methodological biases. Replicate samples of Pleistocene mollusks were collected from the same series of horizons at the same outcrop. One set of replicates was sieved and the other was cemented into artificial rocks before identifying and counting species. These procedures probably represent a best-case scenario for recovering biological signals: the artificial rocks were only poorly lithified, the sampling process was not highly destructive, and the smallest mollusks (< 4 mm) were excluded. However, minor biases were still introduced by the non-random nature of locating and identifying species in the lithified samples, despite efforts at random sampling. After standardizing sampling effort, the cemented samples actually appeared slightly more diverse than the uncemented replicates due to the oversampling of rare species. Assemblage composition was affected slightly by lithification, notably in the undersampling of oysters, whose irregular shells were probably more difficult to identify in matrix. The methodological effects of lithification doubtlessly vary-sample processing will be more destructive for some rocks and fossils-but methodological biases can be quite mild in some cases.

Virtual Paleobiology – Advances in X-Ray Computed Microtomography and 3D Visualization of Fossils

“…These shells are shaped by internal factors such as developmental interactions and modularity (Matsukuma, 1996;Vermeij, 2013;Sherratt et al, 2017;Edie et al, 2022b), and external factors such as selection on life history and ecological function (Stanley, 1970;Vermeij, 1987), providing key insights into questions on the evolution of form (Serb et al, 2011(Serb et al, , 2017Collins et al, 2016), the dynamics of mass extinctions and recoveries (Jablonski, 2005), the tempo and mode of evolution (Jablonski, 2017a), and the origins of spatial diversity gradients (Jablonski et al, 2013). However, such analyses require a wide-ranging inventory of shell form, and much of bivalve diversity is embedded as fossils in consolidated or lithified sediments (Foote et al, 2015; Daley and Bush, 2020). Manual excavation of delicate features, especially those important for taxonomic identification such as the hinge teeth, can require hours to days of preparation (Feldmann et al, 1989;Prôa et al, 2021), and is often impossible.…”

Relative oversampling of carbonate rocks in the North American marine fossil record

“…First, the type and degree of sampling effects may vary over time as the relative proportions of carbonate and siliciclastic marine sedimentary rocks change (Peters 2006). Paleontological sampling differs between these lithologies due to variation in the degree of lithification, which affects the ability to collect fossils (Hendy 2009; Hawkins et al 2018; but also see Daley and Bush 2020) and causes the loss of small fossils (Sessa et al 2009; but see Nawrot 2012). Carbonates may also be prone to early diagenetic dissolution, which may reduce the proportion of fossil-bearing carbonate rocks (Kidwell et al 2005; Best 2008), although significant disintegration also occurs in siliciclastic sediments (Aller 1982; Tomašových et al 2019).…”