Geological Applications of Fossils Sue Rigby 1. An introduction to palaeontological data 2. Biostratigraphy 3. Palaeobiogeography 4. Palaeoenvironments 5. Taphonomy + practicals and computer assignments Assessment by practical folder and exam
1. An introduction to palaeontological data a. Types of fossil b. The potential of the fossil record c. How good is the fossil record? d. Taxonomy and phylogeny e Data from a single fossil f. Data from a set of fossils Practical: A crash course in fossil identification Computer assignment: Manipulating data from fossils
a. Types of fossil Body fossils Trace fossils Biomarkers, eg dinosterane Isotopic signatures eg light C isotopes Rocks - reefs, coal, oil, black shale etc. Temporal information in DNA/RNA
b. The potential of the fossil record. Secular changes in organisms Secular changes in Earth system Local changes in environment Local changes in palaeogeography Short term records of events - ontogeny, population dynamics Relative timescales Post-mortem events
c. Is the fossil record good enough? For some things but not for others. Fossil record is of about 250 000 species About 2 million species known today. Probably less than half the total number (But most of these are insects and their fossil record is terrible) Fossil record lasts 3.5 billion years Hence a small sample. In addition it is a biased sample
Bias in the fossil record 1. To skeletonised organisms, and within this group to organisms with robust skeletons. 2. To common organisms - this carries ecological implications. 3. To organisms living in environments of net deposition, or at times of the year when deposition occurs. And in areas of rapid deposition. 4. To more recent fossils. 5. Towards higher taxonomic groups 6. To particular sea level stands/ systems tracts.
d. Taxonomy and phylogeny Organisms have a name and are assigned a place in a hierarchy - eg Phylum Vertebrata Order Primates Genus Homo Species sapiens This may also reflect an evolutionary succession, but it may not. Eg Climacograptus Phylogeny is the study of evolutionary relationships, usually undertaken with cladistic methods. Naming a fossil gives it utility and implies the possibility of deducing its family tree.
e. Data from a single fossil Complete/ damaged Ontogeny? Phylogeny Characteristics relative to its group - big, small, odd? Unique indicator of conditions? Eg Kodonophyllum from Torquay Limestone. f. Data from a set of fossils Variations within and between sets of fossils Univariate Bivariate Multivariate measures of form data Explored in computer assignment.
2. Biostratigraphy a. Stratigraphic procedure b. Correlating with fossils - evolution vs ecology c. Calibration of the fossil record d. Confidence limits on stratigraphy e. Sequence stratigraphy and fossils Practical: A crash course in microfossil identification Computer assignment: Producing a functional biostratigraphic solution from outcrop data.
a. Stratigraphic procedure ‘Stratigraphy is the study of the geometry, composition and time relations of stratified rocks.’ Biostratigraphy addresses these questions via fossil data. Three aims: 1. Lithostratigraphy - define and describe individual rock units in the area of study. 2. Correlation - correlate these units with units elsewhere and with standardised time scales. 3. Chronostratigraphy - to define and refine a standard global timescale.
Lithostratigraphy Hierarchy: Supergroup Group Formation Member Bed Formation is the basic mappable unit. Usually named after a locality and a rock type, eg. Speaton Clay Formation. Formation boundaries need not by isochronous. Definition of formations is the choice of the individual mapper.
Correlation Primary unit: Biozone (or zone) This is visualised as a bed or set of beds characterized by a particular fossil, not a time period. Different types of zone include: Total range zone Partial range (Overlap) zone Assemblage zone Acme zone Should always define which is being used, as each had advantages and disadvantages. Most people don’t do this.
Chronostratigraphy Establishes age relationships based on rock sequences to build up a globally applicable timescale. Divides time equivalent rock packages into: System, series and stage. Time equivalents are Period, epoch and age.
b. Correlating with fossils - evolution vs ecology Why it works: Evolution changes morphology Species disperse Why it doesn’t work Morphology may not reflect phylogeny Local variation may not be an evolutionary signal Species don’t completely disperse Bioturbation Good zone fossils Evolve rapidly Have a wide geographic range Are common
c. Calibration of the fossil record By isotopic dating, usually U-Pb for best accuracy. About 32 dates for the period 580-380 Ma. Correlation is often imprecise because sections with appropriate lavas may lack good zone fossils. The Precambrian/Cambrian boundary is a good example of this. By contrast there are around 50 graptolite biozones for the Silurian (443-417 Ma), so many of these, and their relative lengths remain unconstrained.
d. Confidence limits on stratigraphy Fossils disappear in most places before they really went extinct. If fossiliferous horizons are randomly distributed then - Rc=ar where Rc is the confidence interval for the true end-point of a range and a is given by a = [(1-C)-1/(H-1) -1] where H is the number of fossiliferous horizons that contain the fossil and C is the desired confidence level (say 95%). In other words, real extinction is more likely to be close to the last occurrence in fossiliferous sections (not rocket science but useful)
e. Sequence stratigraphy and fossils But most sections are not randomly fossiliferous. Many reasons for this including sea level changes. These often interpreted in the light of sequence stratigraphy. Sequences are built up from parasequences which are shallowing-upward sedimentary cycles bounded by flooding surfaces. These packets can be erosional on on another. Apparent extinctions cluster at flooding sequence boundaries, because they represent times of non-deposition. Also, sequences reflecting changing sea level juxtapose facies deposited at different water depths and this can lead to multiple repeats of faunas, and apparent extinctions of the more diverse shallow water forms.
3. Palaeobiogeography a. Modern biogeography b. Moving organisms - dispersal and vicariance models c. Identifying fossil provinces d. Examples Practical: Fossil identification test Brookes Parsimony analysis
a. Modern biogeography The modern distribution of organisms is controlled by geographical parameters. In modern world six main biogeographical regions on land and in shallow water. Boundaries in constant flux. (In addition diversity is usually higher at equator than at poles) Two models for how organisms move around the world - dispersal theories and ideas of vicariance.
b. Dispersal Organisms evolve in one place and migrate to others. Maps of modern diversity suggest that it remains highest close to the point where organisms originate. Migration away from this site is via corridors of dispersal, and is impeded by barriers to dispersal. Corridors often take the form of islands. Barriers can be permanent, ephemeral or intermittant. Barriers are recorded by separate evolution in two areas. Barrier removal is recorded by faunal interchange.
b. Vicariance Organisms stay on one place, but it becomes two or more - that is barriers form. This is a plate tectonic scenario, but works on smaller scales as well, for example river dissection of uplands. When two areas are separated, evolution will produce new species in each. Species evolution therefore mirrors geological evolution of a region. Offers a powerful tool for geoscientists.
c. Recognizing fossil provinces Recognizing fossil provinces depends on faunal analysis and having a well constrainted biostratigraphy. Vicariance techniques can use biotas to explore the geological evolution of a region. Broader assessments of similarity and difference can identify ancient provinces and hence ancient continents. This only useful on a small scale or in deep time. d. Examples North American Cordillera Lower Palaeozoic continental dispositions.
3. Palaeoenvironments a. Temporal constraints b. Bathymetry c. Oxygen d. Atmospheric CO2 e. Climate from plants Practical: Practical test Web-based analysis of Cretaceous climate
a. Temporal constraints Particular organisms or ecosystems reflect particular palaeoenvironments, but these change through time. Eg. Colonisation of land extended utility of fossils to terrestrial environments. |Reefs and their inhabitants - Specific implications of modern reef distribution. Only applicable to zooxanthellae-bearing reef builders? Or generally true?
b. Bathymetry Relative depth indicators - not a direct depth control but indirect changes in eg salinity, O2, temperature, energy) eg Welsh Basin ichnofacies Photic zone - absolute indicator, but turbidity is a factor. CCD - eg agglutinated forams become dominant over calcareous forms. But oxygen also a control at depth.
Depth controls on fossil distribution 1. Modern photosynthetic organisms 2. Silurian Welsh basin
c. Oxygen Oceans have been oxygenated at depth since the late Proterozoic, but levels have increased with time. Secular variations are related to icehouse/greenhouse states. Local conditions can contribute to low oxygen levels. Palaeontology mainly contributes by providing a recognition of adaptations to aerobic respiration in low oxygen tolerances.
c. Animals designed for low O2 Low metabolic rates - slow, long lived, small. Efficient blood circulatory systems. Eg. Blood arks (bivalves). Flattened or thread like bodies eg paper clams Tolerance of H2S and symbiosis woth sulphur reducing bacteria eg olenid trilobites Planktic larvae eg larval beds in Kimmeridge Clay. General faunal characteristics can be useful as well. Eg presence of deep burrows.
Characteristics of assemblages from specific oxygenation states
d. Atmospheric CO2 Secular changes in CO2 have occurred throughout the Phanerozoic, and probably most of the Precambrian Some of these are caused by biota, most are recorded by fossils. Most accurate record is in the number of stomata, compared with nearlest living relative (NLR)
Estimates of palaeo-CO2 from stomatal indices plotted against a calculated curve.
e. Fossil climate from plants Most useful feature is leaf morphology - controlled by temperature and water availability ie local climate. Univariate methods mainly entire/non-entire margins Multivariate methods eg CLAMP (climate leaf analysis multivariate program) also factor in things like leaf size and presence of diagnostic features like drip-tips. Also wood anatomy - ring porousity, presence of annual growth rings