Paleoecology 11/22-11/26/02

New assigned reading: http://www.geo.arizona.edu/ceam/Hecold/hecolcd.htm

Paleoecology is, as you might suspect, simply the study of the inter-relationships of fossil organisms and their environment.

So, all we gotta do is figure out

1. Paleoenvironmental reconstruction. the environment in which the fossil organism lived,

2. Paleobiology of the fossil organism,

3. Biotic interactions of the fossil organisms,

4. Distribution of the organisms; their biotic communities; the species they co-occur with.

1. Paleoenvironmental reconstruction

Getting at this involves the use of a variety of approaches. They involve examining the rocks and fossils for

Environmental indicators - physical, chemical, biotic, or taphonomic features that are diagnostic of one or a few environmental conditions.

Understanding environmental indicators really requires understanding Recent environments. Interpreting features in the rocks and fossils usually requires understanding those features in Recent sediments and organisms. That's why a lot of paleoecological research involves examining Recent environments. .

Consider some common environmental indicators

a. Physical environmental indicators. Sediments and sedimentary structures that can reveal evidence for wave and current energy, subaerial exposure (exposed to air or not and for how long) and other environmental features.

Some examples of physical environmental indicators, from a long list of possible examples:

- mudcracks: subaerial exposure

- raindrop imprints - subaerial exposure

- grain size and sorting: It's a major generalization, but sediment grain size is a rough

indicator of water energy. Coarse high energy; fine is low energy, quiet water.

- ripple marks: evidence of currents

types of ripple marks: wave ripples- symmetrical

current ripples- asymmetrical

- cross-bedding: current energy and direction; found in sand dunes, river channels, tidal

channels

b. Chemical environmental indicators: minerals, chemicals, trace elements and isotopes that may be indicators of: availability of oxygen, amount of evaporation, sedimentation rate, temperature and other features.

Some examples of chemical environmental indicators are

Diagnostic minerals:

- oxidation state of Fe in the rocks.

Red beds, or red colored sediments or rocks are often indicators that the sediments were deposited in an environment where oxygen was available. In many terrestrial habitats for example.

Reduced iron disseminated in rocks may give them a greenish or dark gray color. This usually indicates deposition where little or no oxygen was available, such as in lakes or in marine environments.

- evaporite minerals or deposits. The presence of minerals that form when sea water or lake water is evaporated, such as halite (NaCl) or Gypsum (CaSO₄) indicates high evaporation rates.

- presence of disseminated sedimentary pyrite (Fe₂S) is often an indication of low oxygen conditions. Smell the rock or sediment

- the mineral glauconite, is a mineral that forms in sedimentary environments (a complex K Mg Fe Al silicate) in the presence of organic matter. Usually indicates low sedimentation rates.

Trace elements - slight impurities in composition are often environmentally controlled.

For example, Mg (the trace element) often substitutes for Ca in calcium carbonate, the common constituent of many hard parts. Turns out that in many cases, the amount of Mg varies with temperature. handout:

Thus, fossil clams could be used to establish what the temperatures were, and/or what the temperature gradient was in some region.

Isotopes: elements that differ in the number of neutrons in their nucleus are said to be different isotopes. Some isotopes are unstable and undergo radiometric decay. Others are stable, and do not decay with time. Such stable isotopes have proven to be very useful in environmental reconstructions. Example here from oxygen isotopes will illustrate the approach

-oxygen isotopes. A long but important story:

Oxygen comes in three isotopes, the most common being ¹⁶O. (99.76%); Only 0.2% is in the form of ¹⁸O. O from water (not from respiration) is incorporated into shelly material CaCO3. The isotopic composition of that O can be measure with an instrument called a mass spectrometer.

Measured and expressed as a deviation from a standard ratio:

Del (difference)¹⁸O = 1000 x ¹⁸O/¹⁶O of the sample - ¹⁸O/¹⁶O of the standard

¹⁸O/¹⁶O of the standard

del ¹⁸O of zero means the value equals the standard

del ¹⁸O that is positive means the value is greater than the standard (relative more ¹⁸O, less ¹⁶O)

del ¹⁸O that is negative means the value is less than the standard (relatively less ¹⁸O, more ¹⁶O

units are per mil, or parts per thousand. A per mil increase of +1 means that for every 1000 atoms of oxygen measured, there was one additional atom of ¹⁸O

Controls on oxygen isotope composition

Global ice volume

Local fresh water mixing or evaporation

temperature

1. Ice volume

When seawater (e.g. H₂O) evaporates, relatively more water with ¹⁶O evaporates than does water with ¹⁸O (e.g. more than 99.76% of the water molecules that evaporate are the isotopically lighter water). The water that evaporates is the stuff that makes rain and snow, right?, so there's more ¹⁶O in rain and snow than in the seawater.

If there's a big enough pile of ice in the polar regions, as during glacial times, the ice caps are enriched in ¹⁶O and the oceans have got relatively more (e.g. more than .2%) ¹⁸O. These large-scale climatic changes control the isotopic composition of sea water.

Organisms with CaCO₃ hard parts “sample” the water as they grow. Fluctuations in the ratio of ¹⁸O to ¹⁶O record the changes in ice volume. Relatively more ¹⁸O means high ice volume; relatively less ¹⁸O means low ice volume and inferred warmer climates.

2. Local effects:

a. fresh water mixing

Local effects of fresh-water mixing; mouths of rivers, deltas, etc. More fresh water, more ¹⁶O, makes the water more negative

b. Evaporation removes more ¹⁶O, so enriches the remaining water with ¹⁸O, positive deviation, more positive water.

Recording the signal

Meanwhile, organisms secreting calcareous skeletons have been busy. They remove oxygen from seawater to form CaCO₃. In other words, they sample the isotopic composition of seawater.

If the ¹⁸O/¹⁶O ratio of the oceans is constant (as it would be, more or less within the lifetime of an animal) the ratio can be used to estimate changes in temperature, and if the composition of the water is known, can be used as a direct temperature estimator because. A one per mil shift in del¹⁸O represents a ~4.5 degree C change in temperature.

Grossman and Ku experimental work:

Grossman and Ku equation

Isotopic approach mostly limited to Mesozoic and Cenozoic rocks because re-crystallization will obliterate the signal. Some efforts in Paleozoic though.

- other isotopes (Carbon, Sulfur) used, but I can't get into those: this is complex enough.

c. Biotic environmental indicators. Inferring environmental conditions from the fossil organism found. Broad range here

-one approach involves what has come to be called transfer ecology - a uniformitarian assumption that the ecology of the living representative is an indication of the ecology of the fossil.

This approach can be used with some precision with relatively young rocks and fossils. For example, the use of forams to infer precise paleo-oceanographic conditions during the Pleistocene.

Globorotalia menardii in the Caribbean.

Or interpreting Pleistocene deposits in the Gulf of California using by assuming that the Pleistocene species had the same environmental tolerances as the ones living there today.

Under these circumstances, the same species found alive may be found as a fossil.

As one goes back farther and farther in the record, less and less precision can be expected of this approach. A species' ecology may change without any obvious change in its hard parts. The fossil species may be extinct, with only a distant relative alive in the recent.

examples:

Dosinia found today only in outer flats/shallow subtidal; probably the same 120,000 years ago.

coral reefs today; warm and shallow - fossils?

brachiopods today only marine - fossils?

algae indicate photic zone

As "taxonomic distance" increases, precision of interpretation goes down.

As "temporal distance" increases, precision of interpretation goes down.

-leaf margins and the Jack Wolfe story:

Taxon-free approach.

Most of the time, an inference on paleoenvironmental conditions is based on several different lines of evidence -- on several different environmental indicators

-diversity gradients: diversity here is a measure of the number of different kinds of species (some very complex measures possible). Diversity gradient indicates an environmental gradient of some sort.

Scale matters

On a local scale, diversity may be high where conditions are relatively benign and normal; dropping off when conditions are bad.

On a regional scale, diversity drops as salinity drops, going into the Baltic.

On a global scale, diversity is highest in the tropics, lowest near the poles.

-trace fossils. trace fossils are the sedimentary structures made by the activity of organisms. Tracks, trails, footprints, burrows. Whole assemblages of trace fossils today are indicative of different marine habitats -- ichnofacies. Then when seen in the record, that recent habitat is inferred.

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d. Taphonomic indicators. Uses the taphonomic condition of fossils to make inferences about water energy, sedimentation rate, and the like.

Taphonomic differences indicate environmental differences

- all broken up shells - high energy conditions

- shells encrusted by other organisms - low sed rate

- fossils oriented, current lineations,

concave up/down.

2. Paleobiology of the fossils (focuses on the ecology of the particular species;

Here, what many paleoecologists are interested in doing is figuring out the substrate and trophic (feeding) adaptations of the fossils in question.

A. Substrate classification (you've heard much of this vocabulary already)

mobile

epifauna

sessile (attached)

benthic

mobile

infauna

sessile

planktic (or planktonic) = floating passively (some forams, rads)

nektic (or nektonic) = swimming under own power (fish. sharks)

B. Trophic (feeding) classification

primary producer - photosynthetic

suspension (or filter) feeding

deposit (or detritus) feeding

herbivore- eats plant material

carnivore - eats other animals

omnivore - eats most anything

scavenger/ carrion-feeder - eats dead stuff (vultures, some snails)

How to figure these out?

1. transfer ecology - if same species or close relative living today

2. direct evidence - from the actual mode of occurrence in the rocks: oysters attached to rocks, bivalves found in life position; stomach contents (ichthyosaurs, mammoths), dung contents (fossil ground sloths), fossil found at end of a trail (horseshoe crab)

Example of fossil ground sloth dung:

Richard Hansen’s research on the Shasta ground sloth, Nothrotheriopos shastense, a large ground sloth (250kg) that went extinct (along with about 33 other genera of large mammals) about 11,000 years ago. Bones known in many localities, often caves, including Rampart Cave, in the Grand Canyon; lower end of canyon, 535 m elevation; 200 m above river level.

Rampart Cave full of dung, thought to be sloth dung because of associated sloth bones, and the dissimilarity of the dung from horse, mountain lion, sheep, or goat.

Dung can be dated, from 36,000 to about 10,500 ybp

Plant fragments in the dung analysed (typical range management practice)

Young layers:

Desert globemallow 50%

Nevada mormontea 40%

saltbush 1%

catclaw acacia 3%

various cactus 2%

others

Average digestible energy 1800 cal/g

Old layers

Desert globemallow 50%

Nevada mormontea 4%

saltbush 18%

catclaw acacia 15%

various cactus 5%

others

Average digestible energy 1650 cal/g

Results indicate:

vegetation present when sloths were extant

dietary preferences

minimum feeding range (down to river for some reeds)

change in vegetation/diet not likely a cause for its extinction

III. Biotic 3. Biotic interactions. Many biotic interactions leave little or no direct evidence in the fossil record Some exceptions:

-naticid and muricid gastropods drill boreholes into the shells of their prey

-shell repair in gastropods

-coprophagous Platyceras on crinoids

-rodent teeth in owl pellets

-mosasaur tooth marks on ammonites

-tooth marks on bones

4. Distribution of species and communities. Beyond these relatively rare instances where biotic interactions have left their marks (so to speak) on hard parts, most of what can be inferred in paleosynecological studies in inferred from patterns of co-occurrence.

Assemblages of species that commonly co-occur (i.e., are often found together) are often said to make up a community.

Species co-occurrences can result from

1. biotic interdependence (monkeys in trees), or more simply and probably more commonly, from

2. shared environmental tolerances, e.g., both species happen to do well under similar conditions of temperature, salinity, substrate, whatever.

Read example of paleoecology of the Colorado delta: http://www.geo.arizona.edu/ceam/Hecold/hecolcd.htm