GEOS 26400/36400/BIOS 23255/EVOL 32300: Introductory Paleontology

Nature of the Fossil Record. II.
I. Spatial distribution of known fossiliferous rocks
      -mostly from relatively shallow water
      -sediments best known near centers of civilization
      
II. Long-term changes in nature/quality of fossil record
      -increase in bioturbation (Droser and Bottjer 1993)
      -increase in abundance & diversity of aragonitic vs. calcitic skeletons
      -increase in shell-bed thickness (Kidwell and Brenchley 1994)
      -increase in time-averaging
      -latitudinal distribution of sedimentary rock (Allison & Briggs 1993)
      -extent of cratonic flooding (Ronov 1994)
      -decrease in relative extent of carbonate vs. terrigenous clastic sediments
      *later in course: long-term changes in quantity of fossil record

III. Paleontological completeness: general
      --working definition: proportion of taxa preserved
      --factors that contribute to completeness include:
            --intrinsic preservability
            --temporal duration 
            --geographic range
            --inclusiveness of taxa (taxonomic rank)

IV. Global completeness at species level
      -comparison between estimated number of species that ever have lived
("total progeny") and known number of fossil species
      -total progeny dominated by taxonomic turnover rather than increase
 	in standing diversity
      -completeness estimates on order of 1%-10% for well-preserved marine
	animal species

V. Local completeness at species level
      -Valentine (1989) study: some 77% of living clam and snail species
 	from California have a Pleistocene fossil record
      -Discrepancy between local and global completeness suggests global
incompleteness may reflect subsequent loss of fossiliferous rock (through
erosion, metamorphism, etc.) rather than failure of species to enter the
fossil record in the first place.

VI. Probability of preservation per unit time
      -All else equal, longer lived species have greater chance of leaving
some fossil record, so we'd like an approach that corrects for this.
      -Gap analysis allows estimate of preservation probability per unit time.
      -Gap analysis requires detailed information on occurrences within
stratigraphic ranges, not just the range endpoints.










VII. Estimating preservation probability per unit time when we know only the range endpoints (the "FreqRat")
      -Basic rationale: the poorer the fossil record, the greater the
proportion of species (or other taxa) confined to single stratigraphic
intervals.
            -This intuition can be formalized mathematically.
      -This approach allows us to show, for example, that, in agreement with common view, the mammal fossil record is spottier than that of clams, but also that the apparently short stratigraphic ranges of mammals are real, not just an artifact of preservation.

VIII. Comparison between independent estimates of completeness at different
taxonomic levels and using different methods
      -Correlation between (i) estimated probability of preservation per
stratigraphic interval for genera and (ii) proportion of families with fossil record. [Exceptions to this rule easily understood.]
      -Agreement between "FreqRat" and Gap Analysis

IX. Phylogenetic estimates of completeness
	-comparison between lineages implied by phylogenetic relationships
	and lineages actually sampled
	-properties of such measures still need to be worked out
		-e.g., relative contribution of uncertainty
		in phylogenetic relationships
		-e.g., assumption that most closely related taxa share
		common ancestor (rather than one being ancestral to other)