Harry Williams, Historical Geology1 HISTORICAL GEOLOGY LECTURE 3. THE FOSSIL RECORD. Paleontology: "The study of the remains or traces of ancient life" - or fossils. Mold of a tree trunk in a lava flow (igneous rock) marine fossils in marble (metamorphic rock) Where are fossils found?

Harry Williams, Historical Geology2 It is possible to find fossils in igneous and metamorphic rocks, but it is very rare because fossils would normally be destroyed. Fossils are far more abundant in SEDIMENTARY rocks, which is one of the reasons sedimentary rocks are so important in historical geology. (Pass hand sample around class). How do fossils get to be fossils?...

Harry Williams, Historical Geology3 Fossilization It is rare that a fossil is made of the original organic matter. More often, the organic matter is destroyed and replaced by a mineral – this is the process of petrification (turning to stone), by: Mosquito in amber - the basis for Jurassic Park.

Harry Williams, Historical Geology4 i) Permineralization - minerals precipitate into pore spaces Modern cow’s femur. Permineralized dinosaur bone

Harry Williams, Historical Geology5 ii) Replacement - soft tissue replaced by harder minerals eg. wood replaced by silica; calcium carbonate replaced by pyrite.

Harry Williams, Historical Geology6 Wood in Arizona’s petrified forest. Silica has replaced the wood.

Harry Williams, Historical Geology7 iii) Carbonization - soft tissue decomposed, leaving carbon film e.g. bee, fern leaf.

Harry Williams, Historical Geology8 iv) Molds - Applies especially to shells; shell dissolves away leaving a void that fills with a mineral precipitate -> a CAST; impression of the outside of the shell in sedimentary rock = EXTERNAL MOLD.

Harry Williams, Historical Geology9 Marine gastropod (snail) mold and cast.

Harry Williams, Historical Geology10 v) Trace Fossils - Evidence of animals in the form of trails, tracks, burrows, borings etc. Dinosaur footprint Grazing trails

Harry Williams, Historical Geology11 Trace fossils tell you something about the environment e.g. clam burrows -> intertidal; dinosaur tracks = terrestrial. Fossils are, of course, extremely useful, but it should be remembered that we are often dealing with an.... crawling resting grazing feeding dwelling

Harry Williams, Historical Geology12...Incomplete Record of Life As discussed previously, most sequences of sedimentary rocks contain GAPS in deposition or UNCONFORMITIES. These may represent millions of years and fossils belonging to the missing period will not be found. Some fossils have been removed in places by erosion; some rocks are barren of fossils due to unsuitable environmental conditions (i.e. it is rare to find fossils in coarse river deposits, because organic remains get worn away).

Harry Williams, Historical Geology13 Classifying Organisms The modern system of classification is referred to as TAXONOMY. The smallest taxonomic unit is the SPECIES - “a group of organisms basically alike in their structural and functional characteristics; can interbreed and produce fertile offspring (proves genetically related)”. The rest of the system is hierarchical levels of kinship:

Harry Williams, Historical Geology14 TAXONOMIC UNIT EXAMPLE Species lupus Genus (group of species) Canis Family (" " genera) Canidae Order (" " families) Carnivora Class (" " orders) Mammalia Phylum (" " classes) Chordata Kingdom (" " phyla) Animalia Domain (“ “ kingdoms)Eukarya An organism is usually identified by genus and species e.g. Canis lupus (Wolf). Note that there can be subdivisions of the main categories above e.g. subclass, subphylum, superorder, etc.). (Note: you are not required to memorize taxonomic names for the lecture exams).

Harry Williams, Historical Geology15 Use Of Fossils In Stratigraphy: 1. Relative dating (covered). 2. Correlation (covered). 3. Paleogeography - Species -> environment -> geography, e.g. = distribution of land and sea, based simply on presence of marine/terrestrial fossils. Mapping the location of fossils of intertidal species such as clams, can locate a former coastline - this is very important in historical geology, since many sea level changes have occurred in the past and coastlines have frequently shifted around. 4. Paleolatitude - fossils may also give an indication of LATITUDE, e.g. coral reefs usually form in low latitudes (under warmer climates).