Tempo and Mode in Evolution Genetics and Paleontology 50 Years After Simpson (1995) / Chapter Skim
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Proterozoic and Early Cambrian Protists: Evidence for Accelerating Evolutionary Tempo
Proterozoic and Early Cambrian Protists: Evidence for Accelerating Evolutionary Tempo
Pages 63-84
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From page 63... ...
In this paper, I examine the early fossil record of eukaryotic organisms, asking whether or not this longer record is amenable to the types of investigation used to estimate tempo in Phanerozoic evolution. Even though analysis is limited by incomplete sampling, patchy radiometric calibration, and taxonomic uncertainty, a robust pattern of increasing diversity and accelerating evolutionary tempo is evident.
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From page 64... ...
~ 10 Z I I ~ ~Oo5~Oo~Oo~ 50 Oo Oo So Oo sO6`oos~ss6~os~~ss~~ ski, sit, sit, ~~ ~0 (Ma) FIGURE 1 Species richness of selected protistan microfossil assemblages in 17 stratigraphic intervals running from the latest Paleoproterozoic era (P)
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From page 65... ...
) Formation Location Reference Late Paleoproterozoic- and Mesoproterozoic M1 (1700-1400 Ma)
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From page 66... ...
Interval includes Varanger ice age. path (Harland et al., 1990; Semikhatov, 1991; Knoll and Walter, 1992; Compston et al., 1992; 13owring et al., 1993~.
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From page 67... ...
of protistan microfossil species for the 17 intervals recognized in this study (Tables 1 and 2~. V marks the Varanger ice age.
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From page 68... ...
The time scale along the abscissa is linear, underscoring the long initial interval of low diversity and the rapidity of later radiations. V marks the Varanger ice age.
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From page 69... ...
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.
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From page 70... ...
Page 70
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From page 71... ...
71 e Q to 4~ ,~< ~ ~ ~ ~ > ~ ~ U ~ ~ C U ~ ..
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From page 72... ...
The fossils are morphologically simple, but sedimentological distributions, size frequency distributions, and inferred excystment structures ally these remains to unequivocally eukaryotic microfossils that extend upward from this interval to the present. Independent evidence for the occurrence of late Paleoproterozoic to early Mesoproterozoic eukaryotes comes from significant sterane concentrations in bitumens (Summons and Walter, 1992)
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From page 73... ...
Red algae that display clear cellular differentiation are locally abundant in silicified peritidal carbonates of the Hunting Formation, arctic Canada (Butterfield et al., 1990~; probable chromophytic algae are beautifully preserved in Lakhanda mudstones (German, 1990~; and several green algal taxa occur in the younger Svanbergfjellet Formation of Svalbard (Butterfield et al., in press)
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From page 74... ...
Species richness began low and rose only slightly during the first 70~800 Ma of the acritarch record an interval longer than the entire Phanerozoic Eon. A burst of first appearances 90~800 Ma nearly doubled both assemblage and total diversity, bringing them to a new level that would persist with limited change until the Varanger ice age.
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From page 75... ...
Interestingly, after an increase during interval N2, per taxon rates of origination returned to levels comparable to earlier intervals; both total and per taxon extinction rates increased toward the Varanger ice age. Another order of magnitude increase in origination and extinction v
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From page 76... ...
Thus, relative to earlier intervals, Cambrian acritarch assemblages contain more and more species that survive for shorter and shorter periods of time. Mean species duration and half-life for Cambrian acritarchs is similar to values computed for both younger protistan groups with good fossil records and Phanerozoic animal taxa (Table 3~.
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From page 77... ...
Since then, even more diverse assemblages have been recognized in rocks of this age (Figure 1~. The N7 spike cautions us that despite the overall stability of Proterozoic and early Cambrian acritarch diversity trends, short-lived diversification and extinction episodes may be missed at current levels of temporal sampling density.
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From page 78... ...
As noted above, the increase in acritarch diversity and tempo near the Mesoproterozoic-Neoproterozoic boundary coincides with the appearance of identifiable red, green, and probable chromophyte algae in the record. Branching patterns in molecular phylogenies of the eukaryotes suggest that these algal taxa, along with stramenopiles (ciliates, dinoflagellates, and plasmodia)
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From page 79... ...
These patterns suggest that on the time scale of eukaryotic evolution as a whole, evolutionary tempo has increased episodically. Morphological diversity and turnover rates were low for the earliest recorded period of early protistan evolution, an interval that lasted longer than the entire Phanerozoic Eon.
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From page 80... ...
Evidently, the Cambrian diversification of animals strongly influenced evolutionary rates within clades already present in marine communities, implying an important role for ecology in fueling a Cambrian explosion that extends across kingdoms. I thank Zhang Yun, Yin Leiming, Kathleen Grey, Zang Wenlong, Malcolm Walter, Tadas lankauskas, Nina VoLkova, Tamara German, Alexei Veis, Vladimir Sergeev, Nicholas Butterfield, and~specially Gonzalo Vidal and Malgorzata Moczydlowska for access to and discussions about Proterozoic and Cambrian microfossils.
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From page 81... ...
(1990) A Geologic Time Scale 1989 (Cambridge Univ.
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From page 82... ...
(1985) Major features of the fossil record and their implications for evolutionary rate studies, in Rates of Evolution, eds.
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From page 83... ...
(1992) Late Proterozoic and Cambrian microfossils and biostratigraphy, Amadeus basin, central Australia.
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