The fossils were found in sedimentary rock commonly used to make house bricks in England, quarried from what is called the Oxford Clay Formation.
The findings are reported in the May issue of the journal Geology (now online to subscribers.) Fabien Kenig, associate professor of earth and environmental sciences at the University of Illinois at Chicago, is the article's lead author. Contributors include John Hudson of the University of Leicester, Jaap Sinninghe Damsté of the Royal Netherlands Institute for Sea Research and Brian Popp of the University of Hawaii.
Kenig and his colleagues have spent almost 15 years trying to learn why the bacteria -- which thrive only in oxygen-free but hydrogen sulfide-rich aquatic environments -- apparently co-existed with ancient, oxygen-breathing sea reptiles and other bottom-dwellers in a shallow part of a sea that's now the region of east-central England around Peterborough.
"It should have been one or the other," said Kenig. "You can't have both at the same time. They cannot co-exist."
Using solvents to extract oily lipids from sediments, the geologists identified the signature organic compounds of green sulfur bacteria using gas chromatographs and mass spectrometers.
"Molecules of green sulfur bacteria have a very specific, recognizable pattern or trace that allowed us to identify them in every sample of the Oxford Clay shale that we studied," said Kenig. "We found that their presence was not just an accidental find in a few samples, but corresponds to a recurring, ubiquitous process during deposition of those sediments."
A casual look at a piece of sedimentary rock embedded with fossils suggests a broad-strokes picture of what organisms may have lived in a given epoch of time. Kenig's samples included seemingly incompatible creatures living together. The standard interpretation of the fossil record needed some refinement.
"The animal fossils and bacteria organic matter, though contained in the same packet of rock, were not deposited at the same time," he said. "There was the deposition of organic matter and clay during one set of environmental conditions, then when conditions changed and became favorable, different organisms colonized the environment and left their remains, some as a result of a mass kill by the next anoxic -- or complete lack of oxygen -- event."
"There are indications that conditions in the Oxford Clay-sea were extremely dynamic at the time, probably in response to climatic changes," said Kenig.
The finding suggests that use of fossils to reconstruct environmental records of the past needs to be interpreted with greater care because details from sedimentary deposits, as Kenig and his colleagues found, suggest that these fossils may only represent end-member environments.
The research was supported by grants from the National Science Foundation and the National Environmental Research Council.