Deciphering Earth’s history, the trajectory of organic evolution, and the ecological attributes of ancient environments demand an approach different than that of any other science: Mother Nature has accomplished much in 4.5 billion years, and it is our job to tell her story as best we can from what we can observe in rocks.
Fossils, sedimentary structures and geochemical signals preserved in strata are the proxies geologists use. They show that for more than 3 billion years, Earth’s seas and oceans were a microbial world in which bacteria had seized control of most low-temperature geochemical pathways, including the creation of an oxygen-rich atmosphere. Then, beginning some 550 million years ago, the Cambrian Explosion—the rapid diversification of invertebrate clades—saw a dramatic and irreversible increase in ecological complexity in the marine realm. Evolving membership of these ancient communities led to distinctive and unique sedimentary rock types.
While it is usually easy to make generalized evolutionary and environmental interpretations of fossiliferous strata, a detailed understanding the geological context is critical. For instance, one of the classic examples of species evolution by punctuated equilibrium, the stratigraphic distribution of snails and clams in several million-year-old sediments of an East African rift lake, turns out to be wrong because it was interpreted on the basis of geological data now known to be inadequate and misunderstood. The opposing mechanism, that of gradual change, cannot be falsified.
Extracting ecological-scale data is typically frustrated too because: (1) the life habits of, and interactions between, extinct animals can only be inferred; and (2) the rate of sediment accumulation is theoretically much slower than the life span of the organisms that wind up preserved as fossils: soft-bodied organisms decayed, multiple populations of shell- and skeleton-secreting animals are mixed, and fossils are transported.
Remarkably, in some cases a considerable degree of ecological and biological fidelity is present, frozen in action, so to speak, by fortuitous combinations of geological phenomena. Burrows and trackways, reef deposits, encrustations, permineralized soft tissues, and certain miraculous fossil beds (Lagerstätten) present opportunities for a close approximation of original ecological attributes. This talk explores a number of examples to show how geology and biology overlap.