Abstract

Exceptionally preserved fossils provide major insights into the evolutionary history of life. Microbial activity is thought to play a pivotal role in both the decay of organisms and the preservation of soft tissue in the fossil record, though this has been the subject of very little experimental investigation. To remedy this, we undertook an experimental study of the decay of the brine shrimp Artemia, examining the roles of autolysis, microbial activity, oxygen diffusion and reducing conditions. Our findings indicate that endogenous gut bacteria are the main factor controlling decay. Following gut wall rupture, but prior to cuticle failure, gut-derived microbes spread into the body cavity, consuming tissues and forming biofilms capable of mediating authigenic mineralization, that pseudomorph tissues and structures such as limbs and the haemocoel. These observations explain patterns observed in exceptionally preserved fossil arthropods. For example, guts are preserved relatively frequently, while preservation of other internal anatomy is rare. They also suggest that gut-derived microbes play a key role in the preservation of internal anatomy and that differential preservation between exceptional deposits might be because of factors that control autolysis and microbial activity. The findings also suggest that the evolution of a through gut and its bacterial microflora increased the potential for exceptional fossil preservation in bilaterians, providing one explanation for the extreme rarity of internal preservation in those animals that lack a through gut.

Highlights

  • The preservation of the soft-bodied anatomy of organisms in the fossil record occurs only rarely in Konservat-Lagerstatten

  • The findings suggest that the evolution of a through gut and its bacterial microflora increased the potential for exceptional fossil preservation in bilaterians, providing one explanation for the extreme rarity of internal preservation in those animals that lack a through gut

  • That fossils exhibiting soft tissue preservation can be among the most difficult to interpret [2]. This is because fossils preserving soft tissues are invariably a melange of pristine to decayed structures preserved through mineral replication, while other aspects of anatomy are not preserved at all

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Summary

Introduction

The preservation of the soft-bodied anatomy of organisms in the fossil record occurs only rarely in Konservat-Lagerstatten. Our results show that the carcass of the brine shrimp is consumed rapidly by gut-derived microbes after rupture of the gut wall and consequent release of endogenous bacteria to the rest of the body cavity, followed by microbially mediated tissue replacement through biofilm development, irrespective of external controls. These biofilms mediate authigenic mineralization of soft tissues and of the biofilms themselves, within both the gut and body cavity. The use of these indices depends on description and assignment of a numerical value to each stage of decay (table 1)

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