SEDIMENT EFFECTS ON THE PRESERVATION OF BURGESS SHALE-TYPE COMPRESSION FOSSILS

L A Wilson,N J Butterfield

doi:10.2110/palo.2013.075

Abstract

Experimental burial of polychaete (Nereis) and crustacean (Crangon) carcasses in kaolinite, calcite, quartz, and montmorillonite demonstrates a marked effect of sediment mineralogy on the stabilization of nonbiomineralized integuments, the first step in producing carbonaceous compression fossils and Burgess Shale–type (BST) preservation. The greatest positive effect was with Nereis buried in kaolinite, and the greatest negative effect was with Nereis buried in montmorillonite, a morphological trend paralleled by levels of preserved protein. Similar but more attenuated effects were observed with Crangon. The complex interplay of original histology and sediment mineralogy controls system pH, oxygen content, and major ion concentrations, all of which are likely to feed back on the preservation potential of particular substrates in particular environments. The particular susceptibility of Nereis to both diagenetically enhanced preservation and diagenetically enhanced decomposition most likely derives from the relative lability of its collagenous cuticle vs. the inherently more recalcitrant cuticle of Crangon. We propose a mechanism of secondary, sediment-induced taphonomic tanning to account for instances of enhanced preservation. In light of the marked effects of sediment mineralogy on fossilization, the Cambrian to Early Ordovician taphonomic window for BST preservation is potentially related to a coincident interval of glauconite-prone seas.

Highlights

Carbonaceous compression fossils are a major source of paleobiological data, in particular as it relates to capturing the early Paleozoic record of ‘‘exceptional’’ Burgess Shale–type (BST) macrofossils (Butterfield 1990, 1995; Gaines et al 2008; Page et al 2008; Orr et al 2009) and their microscopic counterparts (Butterfield and Harvey 2012)
Sedimentary matrix is fundamental to the fossilization process, as basic packing material and based on its potential to alter the chemistry of early diagenesis
The sensitivity of such systems to local circumstance suggests that exceptional BST preservation is likely to be the exception, even where conditions are generally favorable— the vast volumes of unoxidized marine mudstones lacking any hint of BST fossils

Summary

Introduction

Carbonaceous compression fossils are a major source of paleobiological data, in particular as it relates to capturing the early Paleozoic record of ‘‘exceptional’’ Burgess Shale–type (BST) macrofossils (Butterfield 1990, 1995; Gaines et al 2008; Page et al 2008; Orr et al 2009) and their microscopic counterparts (Butterfield and Harvey 2012). Despite reports of accelerated carcass decay associated with some sediments (e.g., Plotnick 1986; Allison 1988; Briggs and Kear 1993), most models for BST preservation invoke mineralspecific diagenesis as an essential factor, either by suppressing normal enzyme–microbial-based decay processes (e.g., Butterfield 1990, 1995; Gaines et al 2005, 2012) or secondarily by enhancing the recalcitrance of relatively labile substrates (e.g., Orr et al 1998; Petrovich 2001). In this study we adapt the seminal, sediment-free decay experiments of Briggs and Kear (1993, 1994a) to investigate the effect of sediment mineralogy on the decay and early diagenesis of two ‘‘model’’ nonbiomineralizing marine invertebrates: Nereis virens (polychaete annelids with collagenous cuticle, sclerotized collagenous jaws, and sclerotized chitinous chaetae) and Crangon crangon (crustacean arthropods with variably sclerotized chitinous cuticle)

Methods

Results

Discussion

Conclusion