Specimen Preparation
Without fossil preservation, paleontologists would not be able to accurately identify taxa and correlate coeval faunas; nor would they be able to solve the many anomalies of paleogeography and ancient environment(s). An unfortunate reality, however, is that most terranes of western North America have withstood relentless volcanism, tectonic forces, and mineral recrystallization that occurred over many millions of years. This effectively destroyed delicate microstructure and external morphologies of many shallow-water fossils that are presently preserved in limestones and shales, making them difficult to identify. To cope with this natural process, PERL uses a variety of preparation techniques that help to either free these fossils from their surrounding limestone matrix or highlight whatever morphologies remain imprinted in the rock after millions of years of geologic process.
Silicification
The mineralogical replacement of calcite or aragonite (comprising skeletons of most invertebrate marine organisms) by silica is one possible end member of the recrystallization process. This so called ‘silicification’ process often preserves important details in these shelly fossils, and this has occurred in certain key beds of various island arc terranes (such as the Wrangellia and Alexander terranes) of western North America. This phenomenon is interesting because under acidic conditions, limestone will dissolve and silica will not; and therefore emersion of limestone blocks (containing silicified fossils) in weak acid baths will effectively dissolve the enclosing carbonate matrix– leaving behind perfect renditions of these three-dimensional fossils. Many of which can be identified with accurate precision. Such beds are valuable as paleontological “gold mines” because they provide a wealth of information normally unavailable.
Moldic Impressions
Another unfortunate and somewhat common end member of fossil preservation is the imprint of external shell morphologies into a soft mud, shale, or other fine-grained sediment. Under these circumstances, the resulting negative impression of the original fossil shell is usually the only remaining evidence of ancient life in the respective rock body, and therefore identification depends solely on whatever details one can obtain from the moldic impression. To combat this type of fossil preservation, we use liquid latex to make a three-dimensional positive cast of the original negative impression which can then be identified.
Photography
Obtaining detailed photographs of fossils is critical, not only for making accurate identifications but also for effectively communicating results. Fossil photography at PERL is in accordance with Paleotechniques (Grant, 1989) and includes a multi-step process to elevate the contrast and bring-forth important morphological details. This involves: i) blackening with a water-soluble India Ink; ii) whitening and elevating the contrast of fossil details by heating granular ammonium chloride (to gas) and blowing onto the blackened specimen; and iii) photography using a high-resolution digital camera mounted to an adjustable light stand with basal illumination using an adjustable light board.
Reference
Grant, R. E. 1989. Extraction of fossils from carbonates by acid. In: Paleotechniques. Feldman, R. M., Chapman, R. E., and Hannibal, J. T., (ed.), The Paleontological Society Special Publication No. 4, p. 237-243.