CHAPTER 1: ABOUT PALEONTOLOGY AND FOSSILS – Flashcards

"The present is the key to the past," is the fundamental basis for all historical science.

States that the physical laws of the universe do not change with time. Therefore, insights gained about geologic processes in the recent may be applied to the interpretation of ancient rocks and events.

Was originally advocated by Sir Charles Lyell, postulated that geologic processes go on always at the same rates and with the same relative importance as today. This gradualistic view of earth history is not supported today.

States that, in a stratified sequence of rock layers, the older rocks are at the bottom and the younger ones on top. It is a simple but essential principle for interpretation of the relative sequence of depositional events.

Sedimentary layers are approximately horizontal at the time of original accumulation.

Sedimentary layers continue laterally as far as the conditions controlling them prevail, or until the edge of the depositional basin. This allows us to extend (correlate) relationships between outcrops, beneath soil and vegetation cover.

States that if a rock unit (igneous intrusive) or a structure (fold, fault) cuts across (interrupts, offsets) another rock unit or structure, it must be younger. This law makes it possible to add igneous intrusive events, structural deformation events, and other non-layered phenomena to the relative sequence of events based on sedimentary layering, and in this way to work out a complete relative geologic history.

(Smith's Law) States that fossils are not distributed at random in rocks. Rather, each kind of fossil occurs only within a relatively limited interval of the stratigraphic succession. Once the stratigrapher has worked out the sequence of strata exposed in a region and has catalogued the occurrences of all kinds of fossils in these strata, from then on the fossils provide as good a clue to position within this sequence as any other lithologic criterion. The fossils are indexes (indices) or indicators or relative position in a known stratigraphic sequence.

States that all life is cellular. The cell is the basis unit of organization of life. All organisms are cells or societies of cells.

States that species originate by modification of pre-existing species, by "descent with modification."

Paleobiology is a science, which means that we employ scientific reasoning (inductive reasoning). The scientific method distinguishes different qualities of statements: Facts (observations). Hypotheses (explanations for those observations, not yet fully tested). Theories (well-tested, widely-adopted explanations, which are supported by numerous observations). Unifying theories (complex theoretical models encompassing smaller theories). Examples are the Theory of Biological Evolution and the Theory of Global Plate Tectonics.

The term fossil (from Latin fossilis, loosely from the verb fodere, to dig) was coined by Agricola. In his books De Re Metallica (1556) and De Re Fossilium (1546) he explained fossils as the remains of once-living creatures, and thus he became one of the first advocates of the new "Organic Theory."

Defined as any evidence of past life. A fossil may be some part of an organism, or it may be a nonskeletal secretion, such as a burrow-lining, egg-case or reproductive cyst, or it may the the work of an organism, such as a burrow or trail. There is no restriction on age.

Most fossils represent hard parts, chiefly skeletal structures. Many skeletons are a composite of organic and mineral phases, of which only the mineral phase is preserved.

The pore spaces and interior cavities of a bone or shell have been filled by minerals deposited from interstitial waters. The mineral may be the same as that composing the skeletal remains, which are still present, or it may be different. A permineralized fossil is heavier, denser, more "stony," and generally resistant to further alteration. Such fossils are often said to have been petrified, and the process may be termed petrefaction.

During diagenesis the mineral grains of the skeleton may be metastable or unstable, and they may reorganize into a more stable configuration. Typically a fine-grained skeletal fabric will become coarser, composed of fewer but larger crystals. Aragonite may recrystallize to the more stable morph calcite, and high-Mg calcite may be converted to low-Mg calcite. Siliceous fossils (of opaline silica) become chalcedony, chert or quartz. Recrystallization commonly blurs or destroys the original skeletal microstructure and fine morphological details.

Skeletal components may react chemically with circulating interstitial or ground waters, so that the original material is dissolved and carried away, while its place is filled by simultaneous deposition of another mineral. This process may be so delicate that even very fine skeletal microstructure is preserved in the new material. Calcareous shells are often replaced by silica (silification) and less commonly by pyrite (pyritization). Siliceous skeletons may be replaced by calcite. More rarely, soft tissues may be replaced by calcium phosphate salts or pyrite.

Organic remains buried in dysaerobic or anaerobic environments, where aerobic decay bacteria cannot live, will be degraded by distillation. The increased temperature and pressure associated with compaction cause more volatile hydrocarbons to distill away, leaving more reduced compounds and elemental carbon as a flat smear on a bedding plane. The extreme compaction in black shales renders such fossils two-dimensional

Trails and burrows (feeding and habitation), footprints, coprolites (feces), castings of sediment-feeders, borings, bite-marks, and other Lebenspuren (German, life traces) provide what Seilacher has called "fossil behavior." They provide important paleoecological information to stratigraphers. Spores, pollens, seeds, egg cases, reproductive cysts, burrow-linings, and other such nonskeletal secretions or constructions of an organism provide useful information about life habits and reproduction.

The terms mold and cast have the same meaning in paleontology that they do in metal work. A cast is a replica of the original thing, formed by filling a mold. A mold is an impression of the original thing, a negative. A external mold records the external features in negative. An internal mold (steinkern, rock heart) records the internal features in negative. A cast may have the external appearance of the original thing, but it has a different composition and lacks the microstructure. An impression is an informal term for an external mold that is flat, essentially two-dimensional.

Insects trapped by tree resins are entombed in the resulting amber. Microscopic cells trapped in colloidal silica gels are now preserved in chert. Mummification of an entire animal, including soft tissues, by freezing or drying is extremely rare. Examples include Ice Age elephants in permafrost, vestiges of dinosaur skin, and the 5,000-year old man frozen into an Alpine glacier.