Faunal diversity in paleoecosystems: A model for using the species-area relationship to analyze paleoenvironments

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.