Earliest fossiliferous sites

Places of exceptional fossilization

Fossil sites with exceptional preservation—sometimes including preserved soft tissues—are known as Lagerstätten. These formations may have resulted from carcass burial in an anoxic environment with minimal bacteria, thus delaying decomposition. Lagerstätten span geological time from the Cambrian period to the present. Worldwide, some of the best examples of near-perfect fossilization are the Cambrian Maotianshan shales and Burgess Shale, the Devonian Hunsrück Slates, the Jurassic Solnhofen limestone, and the Carboniferous Mazon Creek localities.

Earliest fossiliferous sites

Lower Proterozoic Stromatolites fromBolivia, South America

Earth’s oldest fossils are the stromatolites consisting of rock built from layer upon layer of sediment and other precipitants. Based on studies of now-rare (but living) stromatolites (specifically, certain blue-green bacteria), the growth of fossil stromatolitic structures was biogenetically mediated by mats of microorganismsthrough their entrapment of sediments. However, abiotic mechanisms for stromatolitic growth are also known, leading to a decades-long and sometimes-contentious scientific debate regarding biogenesis of certain formations, especially those from the lower to middle Archean eon.

It is most widely accepted that stromatolites from the late Archean and through the middle Proterozoic eon were mostly formed by massive colonies ofcyanobacteria (formerly known as blue-green "algae"), and that the oxygen byproduct of their photosynthetic metabolism first resulted in earth’s massive banded iron formations and subsequently oxygenated earth’s atmosphere.

Even though it is extremely rare, microstructures resembling cells are sometimes found within stromatolites; but these are also the source of scientific contention. The Gunflint Chert contains abundant microfossils widely accepted as a diverse consortium of 2.0 Ga Microorganisms.

In contrast, putative fossil cyanobacteria cells from the 3.4 Ga Warrawoona Group in Western Australia are in dispute since abiotic processes cannot be ruled out. Confirmation of the Warrawoona microstructures as cyanobacteria would profoundly impact our understanding of when and how early life diversified, pushing important evolutionary milestones further back in time. The continued study of these oldest fossils is paramount to calibrate complementary molecularphylogenetics models.