February 20. Paleopopulations

life assemblage (biocoenosis)

• only lagerstätten come close

catastrophic assemblage (thanatocoenosis)

• sometimes preserved in obrution events; a single sediment event during which an existing life assemblage is smothered in place and only the most mobile elements escape

death assemblage (taphocoenosis)

• most fossil beds are an accumulation of several generations of inhabitants
• necropolis assemblage: biased toward juvenile and aged forms

census populations

• come from one bedding plane and tend to include relatively few species; a geological instant

normal populations

• time-averaged; consists of years or decades, depending on sediment accumulation rate

ANALYSIS OF POPULATION

size-frequency analysis

• starting point of many population studies; choose class intervals of size; create frequency table for all intervals; each class is equated with an absolute age unit (or arbitary units are applied)
• right skewed curves
• indicate high infant mortality and subsequently lower mortality
• normal (Gaussian) curves
• indicate high mortality in mid-life group; rare in nature
• artifactual causes: mechanical sorting and other diagenetic processes
• actual causes: selective predation on small size specimens; steady state growth: rapid initial growth followed by slower rate
• left skewed curves
• high senile mortality preceded by relative low mortality in youth

multimodal populations

• most frequency curves will have one high point (unimodal); multimodal curves caused by:
• seasonal spawning with perioidic recruitment
• molting causes quantum increases in size; downward trend in successive peak as older individuals are culled

AGE OF FOSSIL SPECIMENS

• without growth rings or known number of molt stages, a relationship between size and age is assumed
• linear growth: D=S*[T+1]
• log growth: D=S*ln[T+1]
• D=size; S=constant; T=time

SPATIAL DISTRIBUTIONS OF FOSSILS

1. random: each individual situated without respect to others; suggests lack of biotic or environmental control on spacing
2. regular: spacing controlled by competition either among individuals or for resources
3. clumped: most common distribution

OPPORTUNISTS VS. EQUILIBRIUM SPECIES

dN/dt = r*N*[K-N/K]

K=carrying capacity; N=actual population size; r=intrinsic rate of increase; t=unit of time

r-strategists: mature early; small, numerous offspring; die young; high "r"; density-independent conditions

• opportunistic species are abundant, widespread; dominate a variety of facies; variable morphology
• trade resources for reproductive capacity at expense of longevity

K-strategists: long-lived; low-"r"; density-dependent conditions

• equilibrium species are facies dependent; moderately abundant; have specialized morphology without much variation
• put energy into physiological (homeostatic) processes at expense of reproduction

SAMPLE STUDY

A.I. Miller, "Counting fossils in a Cincinnatian storm bed: spatial resolution in the fossil record", in Brett, C.E. and Baird, G.C. (eds.) Paleontological Events: New York (Columbia).

• studies of modern assemblages reveal that biota reflect changes in physical environmental conditions at scales <10 m
• such fine scales are rarely studied in the fossil record; time-averaging is assumed to remove the fine spatial structure
• Miller's study consists of a 150 m transect along a single outcrop, sampled at 10 m intervals and a 40 m transect similarly sampled

Stratigraphy and Localities

• Upper Ordovician; Cincinnati Arch
• storm deposits; basal packestone w/grainstone lags; 3-5 series of regressive sequences (shoaling upward); Fairview and Kope Formations of Maysvillean Series
• deposited on a north-dipping ramp; shale-dominated Kope facies; limestone-dominated Bellevue facies; Fairview mixed; all move north during regressions
• smaller cycles superimposed on above; caps of cycles are laterally persistent grainstone/packestones
• study examined one capping stone in Fairview; 30 cm thick; rich in bryozoans and brachiopods; consists of several storm deposits

Methods

• bulk samples 2-10 liters
• brachs and bivalve abundance determined by counting valves w/ intact umbo
• bryozoan abundance determined by point counting
• data analyzed using 3-axis polar ordination program using correlation of individual variables with each ordination axis (axes represent some emergent property of the environment)

Results

• Rafinesquina has large (+) correlation coefficient with axis 1
• Platystrophia and Parvhallophora have large (-) coefficient with axis 1
• Platystrophia and Dekayei have a large (+) coefficient with axis 2
• at location 1: 0-40 m rich in Rafinesquina; 50-90 m rich in Platystrophia and Parvhallophora; 100-150 m intermediate
• main point: composition does not vary randomly; clear zonation

Discussion

• taphonomic explanation: patterns generated by post-mordem transport
• but transition is neither continuous nor monotonic
• inter-biota explanation: Platystrophia attach to bryozoans, so spatial pattern in bryozoans is controlling factor
• no clear-cut association is apparent from these data
• biota-environment explanation: faunal transitions mirroring environmental transitions on Ordovician seafloor
• no obvious petrographic differences among samples; but he didn't look that closely
• random pattern: faunal transitions reflect random faunal variability on Ordovician seafloor in spite of time averaging
• not testable

Next step: examine contents of individual storm events; do better job of counting bryozoans