Abstract:
Preservational bias in the fossil record presents difficulties when knowledge of
biodiversity in paleoecosystems is needed. In such cases paleobiodiversity must be
assessed without having direct knowledge of undiscovered taxa. This study presents a
new method to estimate paleobiodiversity (= species richness) in paleoecosystems,
allowing 1) a reassessment of previous studies in which diversity affects conclusions; 2)
future studies to be placed in the context of estimated paleobiodiversity figures; 3) fossil
record completeness to be calculated; and 4) fossil collecting efforts to be tailored
appropriately when known paleobiodiversity approaches projected figures.
The species-area curve expresses the observation that taxonomic diversity
generally increases with geographic area. When anthropogenically introduced species
are removed from consideration, the insular nature of islands and other refugia offer an
uncontaminated picture of faunal richness within discrete boundaries. Log-log plots of
species richness against land area of islands with similar climatic regimes yield
regressions with slopes of approximately 0.30.
The reptile and amphibian faunas from nearly one hundred modern islands with
tropical, seasonal climatic regimes were compiled to create a species-area curve with a
slope of 0.28. A second set of twenty-eight paleoisland faunas from analogous climates
was compiled which ranged in age from 65 million years (Ma) to 500 years before
present (BP). The species-area equation was applied by using the slope of the analogous
modern island species-area curve, and the resulting expected species richness ranged
from seven species on the smallest paleoisland to more than one hundred species on the
largest.
Completeness indices (C.I.) for each paleoisland were calculated as a ratio of the
diversity of known fossil taxa to expected diversity based on the species-area
relationship. The completeness of the paleofaunal record on the selected paleoislands
was found to decrease with island age, thereby quantifying a general paleontological and
geologic trend of poorer data with age. Additionally, the C.I. is inversely correlated to
island size, likely indicating a collecting bias caused by the prohibitive nature of
exhaustive collecting over large areas, i.e. the fossil record is generally poorer with age
and island size.
This technique is also applied to geographic or political subdivisions of
continents, termed provinces. A dataset of species richness on modern provinces was
compiled as a baseline by which to assess diversity in a paleoprovince. The definition of
a modern province implies that it does not represent a significant (i.e. geologic) span of
time. In the absence of such “true” provinces in the geologic record, a lithostratigraphic
unit which contains an areal as well as a temporal component, was used. The Upper
Cretaceous Hell Creek Formation of the northern Great Plains was surveyed, both in the
literature and through the active collection of fossil material. Three previously
unrecognized taxa - at least two teleost fish and one solemydid turtle - were recovered
and are described herein, for a total of 132 recovered taxa in the Hell Creek Formation.
In order to test the accuracy and applicability of the species-area method, faunal
paleobiodiversity was assessed by several other means, including the discovery curve,
rarefaction, and jackknifing. These other methods are not able to predict undiscovered
paleobiodiversity over a wide geographic area. In this study the Hell Creek Formation is
estimated to have had 63 species of freshwater fish and 136 species of reptiles and
amphibians, with C.I.s of 0.40 and 0.32, respectively. Although most of the species in
these groups remain undiscovered, the law of diminishing returns will be invoked, and
the effort required to find them will increase dramatically in the future.