Exceptionally preserved fossil assemblages through geologic time and space

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 3 Ma). 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 nonmarine 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, nonmarine 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.

Soft‐Bodied Fossils Are Not Simply Rotten Carcasses – Toward a Holistic Understanding of Exceptional Fossil Preservation

“…Konservat-Lagerstaẗten occur throughout the geological record in a diversity of paleoenvironmental settings and sedimentary rock types. [3] Soft parts of organisms can be preserved in a variety of ways: as carbonaceous compressions ( Figure 3A and E); via early (authigenic) mineralization in iron sulfide (pyrite) ( Figure 2F and 3B) and apatite (calcium phosphate) ( Figure 2C); and by early cementation or entombment, such as in concretions ( Figure 3D) or within amber ( Figure 2D). Within a single specimen, a combination of these preservational pathways can account for the preservation of the whole organism and different tissues follow particular preservational pathways.…”

Variable preservation potential and richness in the fossil record of vertebrates

“…Lagerstätten distribution is variable in time and space; for example, conservation Lagerstätten are particularly common in the Cambrian and Jurassic (Allison & Briggs ; Muscente et al . ). This variation arises from real changes in environment, making exceptional preservation more likely in some regions and time periods.…”

“…Mineral replacement allows for less recalcitrant tissues to be preserved, a greater variety of taxa are fossilized, and preservation potential is higher in these deposits (Muscente et al . ). Additionally, palaeontological research effort is often concentrated on exceptional deposits, as ‘systematists follow the fossils’ (Raup ).…”

“…Lagerstätten are traditionally thought of as representing unusual, localized depositional environments, but there is strong evidence for clustering in space and time, perhaps reflecting global‐scale phenomena that enable several similar Lagerstätten to occur in an area or within a time bin (Muscente et al . ).…”

“…For the rest of this study we follow the narrower definition of Muscente et al . () who focused on ‘exceptionally preserved fossils’, so we exclude concentration deposits such as bone beds, comprising mainly hard tissues, often in abraded condition. Lagerstätten are traditionally thought of as representing unusual, localized depositional environments, but there is strong evidence for clustering in space and time, perhaps reflecting global‐scale phenomena that enable several similar Lagerstätten to occur in an area or within a time bin (Muscente et al .…”