Molecular palaeontological evidence for food-web relationships.

Abstract

The presence of consumer organisms in ancient ecosystems has been indicated mainly by the occurrence of fossilized mineral skeletons. In comparison with primary producers and bacteria, nonphotosynthetic enkaryotic organisms are minute contributors to organic carbon preserved in sediments. In principle, however, organic compounds produced by heterotrophs have the potential to enter the sedimentary record. The biomass of respiring heterotrophs is enriched in ¹³C relative to its carbon source, and it has been shown in the modern environment that the carbon isotopic fractionation between photosynthetic primary producers and heterotrophs increases as a function of the trophic level of the consumer. In ancient sediments, identification of organic compounds derived from specific heterotrophs and determination of their isotopic compositions are potential tools to reconstruct palaeo-food-web relationships. Here, we present unprecedented evidence of the presence in sediments of organic compounds derived from insect waxes. Analyses of the ¹³C contents of these materials indicate that insects utilized the organic material of cyanobacterial mats as their main source of carbon and, thus, demonstrate that molecular palaeontology can reveal food-web relationships in ancient communities. The observations confirm that the identification in sediments of specific refractory organic compounds from higher organisms and the determination of their stable carbon isotopic compositions provide valuable information regarding trophic relationships at the time of deposition. The molecular-isotopic approach appears likely to provide one of the only means for secure detection of products of consumer organisms and for placement of those organisms within ancient trophic systems.