Small carbonaceous fossil

SCFs are typically preserved in fine-grained siliciclastic rocks, and are too small to be fruitfully examined on bedding planes. Instead, they are extracted by dissolving the rock in acid. Traditional palynological preparations involve high-energy steps such as centrifuging that destroy large and fragile fossils. In the more delicate technique pioneered by Butterfield,[2] individual microfossils are picked from sieved acid residues by hand. The sieving stage removes crystalline residue, making it easier to extract fossils, but introduces a filter: the smallest fossils (<~40 µm) pass through the sieve and are lost.[1] Once extracted, fossils can be mounted for light or scanning electron microscopy: transmitted light illuminates internal microstructures, whereas SEM picks out surface features.

SCFs are best preserved in environments that had anoxic conditions[3] and where the sediments have not been subject to high temperatures (limited thermal maturity);[4] the presence of oxygen is particularly deleterious at high temperatures.[5]

Scanning electron micrograph of a late Silurian chitinozoan, an enigmatic Palaeozoic SCF that can be recovered by the standard palynological technique. Scale bar: 50 µm

Traditional palynological methods are designed for extracting fossilized plant spores and other resistant organic microfossils such as acritarchs and chitinozoa. By using the modified SCFs extraction technique, more delicate fossil structures can also be recovered, including fragments of animals. In particular, this technique has been applied to sediments deposited during the Cambrian Period, since there is great interest in tracking how soft-bodied animals evolved during this time interval.[1][6][7][4] Animal SCFs extracted from Cambrian sediments include the minute scales of priapulid worms,[7] Wiwaxia sclerites,[8] and arthropod feeding parts,[6] for example. These organisms are not represented in the conventional (shelly) fossil record, and so the SCFs record provides data on their distribution and evolution that would not otherwise be available. Lagerstätten such as the Burgess Shale provide isolated snapshots of Palaeozoic life, whereas SCFs provide a more continuous record, albeit blighted by the fragmentary (and consequently enigmatic) nature of many of its constituents.[1] As such, SCFs can help to fill in some of the details of the fossil record outside the rare Lagerstätten sites: for instance, highlighting the rapid nature of the Cambrian explosion.[6]