lem is well understood by invertebrate paleontologists who enjoy excellent preservation for only a few of the more than 30 phyla of invertebrate animals. Estimates of evolutionary tempo and the timing of diversification and extinction events are broadly similar across phyla for Phanerozoic invertebrates and protists with good fossil records. Therefore, the limited clade diversity of Proterozoic and Early Cambrian acritarchs may reflect a broader pattern of early eukaryotic evolution.

A second type of biological uncertainty concerns the interpretation of paleospecies. In studies of acritarchs, analyses are necessarily done at the species level, because biologically meaningful higher taxa have not been defined. The uncertain phylogenetic relationships of most forms exacerbate the common paleontological problem of relating paleospecies to biological species. While the paleontological use of the term species is convenient and accurate in the sense of "most inclusive diagnosable units," what we can really measure is diversity of morphology. Estimates of tempo are, therefore, to be viewed as rates of morphological diversification and turnover within a preservable subset of early eukaryotes.

The Paleontological Data Base: Sampling Quality. Several hundred Proterozoic and Lower Cambrian formations are known to contain protistan microfossils (Schopf and Klein, 1992); however, many assemblages are indifferently preserved, poorly described, and/or accompanied by inadequate stratigraphic and paleoenvironmental information. Assessment of sample quality is better based on those assemblages that are well preserved, meticulously monographed, and well buttressed by stratigraphic and sedimentological data (Figure 1).

Older assemblages are both less numerous and less diverse than those of younger intervals. One might, therefore, suppose that low observed diversity is a product of poor sampling or poor preservation; however, sampling adequacy is not simply a function of assemblages per interval. It is also dependent on quality of preservation, facies and/or paleogeographic heterogeneity, and rates of taxonomic turnover. If fossils are well preserved, cosmopolitan, and slowly evolving, a limited number of samples may be sufficient to characterize the paleobiology of an interval.

This appears to be the case for the Mesoproterozoic acritarch record. The quality of fossil preservation in Mesoproterozoic mudstones (e.g., Peat et al., 1978) and silicified carbonates (e.g., Sergeev et al. , in press) matches the best seen in Neoproterozoic rocks, but the acanthomorphic (process or spine bearing) and other ornamented acritarchs seen in younger rocks of comparable environmental setting are not seen in these or any other rocks older than ca. 1100 Ma. In contrast, even metamorphosed Neoproterozoic rocks may contain ornamented acritarchs



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