Exceptionally preserved fossil assemblages through geologic time and space

A.D Muscente,James D Schiffbauer,Jesse Broce,Marc Laflamme,Kenneth O'Donnell,Thomas H Boag,Michael Meyer,Andrew D Hawkins,John Warren Huntley,Maria Mcnamara,Lindsay A Mackenzie,George D Stanley,Nancy W Hinman,Michael H Hofmann,Shuhai Xiao

doi:10.1016/j.gr.2017.04.020

Abstract

Geologic deposits containing fossils with remains of non-biomineralized tissues (i.e. Konservat-Lagerstätten) provide key insights into ancient organisms and ecosystems. Such deposits are not evenly distributed through geologic time or space, suggesting that global phenomena play a key role in exceptional fossil preservation. Nonetheless, establishing the influence of global phenomena requires documenting temporal and spatial trends in occurrences of exceptionally preserved fossil assemblages. To this end, we compiled and analyzed a dataset of 694 globally distributed exceptional fossil assemblages spanning the history of complex eukaryotic life (~610 to 3Ma). Our analyses demonstrate that assemblages with similar ages and depositional settings commonly occur in clusters, each signifying an ancient geographic region (up to hundreds of kilometers in scale), which repeatedly developed conditions conducive to soft tissue preservation. Using a novel hierarchical clustering approach, we show that these clusters decrease in number and shift from open marine to transitional and non-marine settings across the Cambrian-Ordovician interval. Conditions conducive to exceptional preservation declined worldwide during the early Paleozoic in response to transformations of near-surface environments that promoted degradation of tissues and curbed authigenic mineralization potential. We propose a holistic explanation relating these environmental transitions to ocean oxygenation and bioturbation, which affected virtually all taphonomic pathways, in addition to changes in seawater chemistry that disproportionately affected processes of soft tissue conservation. After these transitions, exceptional preservation rarely occurred in open marine settings, excepting times of widespread oceanic anoxia, when low oxygen levels set the stage. With these patterns, non-marine cluster count is correlated with non-marine rock quantity, and generally decreases with age. This result suggests that geologic processes, which progressively destroy terrestrial rocks over time, limit sampling of non-marine deposits on a global scale. Future efforts should aim to assess the impacts of such phenomena on evolutionary and ecological patterns in the fossil record.

Highlights

The bulk of the fossil record consists of skeletal materials, biologically produced by organisms that control the formation of minerals within their tissues
We argue that a single unifying conceptual model—rooted in sedimentary geology and geomicrobiology—can account for virtually all preservational styles of soft tissue fossils, if their origins are considered in terms of taphonomic processes with unique and shared environmental controls
Using this model as basis for interpreting distributions of exceptionally preserved fossils in geologic time and space, we present a quantitative metaanalysis of a new compilation of 694 exceptionally preserved fossil assemblages, which span the history of complex eukaryotic life (~ 635– 0 Ma)

Summary

Introduction

The bulk of the fossil record consists of skeletal materials (i.e. shells, bones, and teeth), biologically produced by organisms that control the formation of minerals within their tissues. Geologic deposits containing exceptionally-preserved fossils—such as certain Konservat-Lagerstätten (Seilacher, 1970)—offer relatively complete snapshots for exploring the ecology and dynamics of ancient ecosystems, as they generally contain remains of both biomineralizing and non-mineralizing taxa (Conway Morris, 1986) For these reasons, exceptionally preserved fossil assemblages—such as the Weng'an (Xiao et al, 2014), Burgess Shale (Conway Morris, 1986), Hunsrück (Briggs et al, 1996), and Mazon Creek (Cotroneo et al, 2016) biotas—have received special attention among scientists studying Earth history. Work on such Konservat-Lagerstätten has led to innovative hypotheses regarding patterns in diversity and extinction of softbodied organisms through time (Darroch et al, 2015; Labandeira and Sepkoski, 1993; Van Roy et al, 2010)

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