Introduction

Acritarchs, a group of decay-resistant organic-walled vesicular microfossils, dominate the fossil record of Proterozoic (2500-542 Ma) and Cambrian (542-488 Ma) protists. Most acritarchs from the Proterozoic and Paleozoic are interpreted as unicelled photosynthetic protists, though some may represent multicellular algae (Mendelson, 1987; Butterfield, 2004), and a few have been tentatively interpreted as fungi (Butterfield, 2005). Acritarchs are among the oldest eukaryotes in the fossil record (Zhang, 1986; Yan, 1991) and offer the earliest adequate data to assess the history of protistan biodiversity (Knoll, 1994; Vidal and Moczydlowska-Vidal, 1997).

Acritarchs Proterozoic

Figure 1. Estimates of acritarch taxonomic diversity during the Phanerozoic and early Paleozoic. Bars are adapted from Knoll (1994). Black circles adapted from Vidal and Moczydlowska-Vidal (1997). Vertical black lines represent Era boundaries. The dashed vertical line to the left of the Neoproterozoic/Paleozoic boundary represents the first appearance of Ediacara organisms. The gray box represents the time of Neoproterozoic global glaciations or Cryogenian. P1, M1, M2, N1, N2, N3, C1, C2 and C3 represent the geochronological bins of our study.

Figure 1. Estimates of acritarch taxonomic diversity during the Phanerozoic and early Paleozoic. Bars are adapted from Knoll (1994). Black circles adapted from Vidal and Moczydlowska-Vidal (1997). Vertical black lines represent Era boundaries. The dashed vertical line to the left of the Neoproterozoic/Paleozoic boundary represents the first appearance of Ediacara organisms. The gray box represents the time of Neoproterozoic global glaciations or Cryogenian. P1, M1, M2, N1, N2, N3, C1, C2 and C3 represent the geochronological bins of our study.

Previous estimates of acritarch diversity suggest that the number of acritarch species was low from the first occurrence in the Paleoproterozoic to as late as the early Neoproterozoic (Fig.1). Acritarch taxonomic diversity began to increase through the Neoproterozoic, but suffered a decline during mid-Neoproterozoic glaciation events. An unprecedented, though shortlived, diversification occurred after these glaciation events, and was then followed by another extinction, concurrent with the rise of macroscopic Ediacara organisms, some of which clearly were metazoans (Fedonkin and

Waggoner, 1997). Acritarch taxonomic diversity subsequently increased in step with animal radiation in the early Cambrian (Knoll, 1994; Vidal and Moczydlowska-Vidal, 1997).

Taxonomic inconsistencies have caused some to question the validity of taxic measures of protistan biodiversity (Butterfield, 2004). The problem is common in paleontology, and can be acute in the study of acritarchs. Evolutionary convergence among simple protists can lead to taxonomic deflation, or an underestimation of diversity, whereas the heteromorphic alternation of generations can lead to taxonomic inflation, or an overestimation of diversity (Butterfield, 2004). However, the problem of taxonomic inconsistency can be partly alleviated by a complementary and concurrent analysis of morphological disparity, as demonstrated by morphometric studies of Phanerozoic plants and animals (Foote and Gould, 1992; Boyce, 2005). The usefulness of morphometric tools in the analysis of Phanerozoic organisms encouraged us to use such strategies to independently address the question of the evolutionary history of acritarchs. In this chapter we present the results of our literature-based investigation of the first 1.3 billion years of morphological evolution in the Group Acritarcha.

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