It is hard to define what an acritarch is. The typical statement in the literature is: "Acritarchs [are] a group of decay-resistant organic-walled vesicular microfossils ... . Most acritarchs from the Proterozoic and Paleozoic are interpreted as unicelled photosynthetic protists ... ." Huntley et al. (2006).
The Proterozoic includes over half the history of life on this planet, but the available data about Proterozoic organisms are exceedingly sparse. We usually have a little discussion about plankton at about this point on the Time pages, but we know practically nothing about plankton in the Proterozoic. Consequently, we'll talk mostly about acritarchs instead. And what exactly is an acritarch? No one is quite sure. Acritarchs are like sharks' teeth from the Phanerozoic or cigarette butts from the 1960's. That is, they are found everywhere, are practically indestructible, and come in a variety of interesting shapes and sizes. They look informative, but ultimately tell us rather little about the organisms or ecosystem which produced them -- other than the fact that it contained sharks or smokers, as the case may be. Acritarchs tell us even less because we don't know what produced them. Although acritarchs have been probed and prodded with almost every sort of device known to man, they have yielded relatively little detailed structural information. They are clearly unmineralized, organic-walled structures. After sitting around for one or two billion years, almost all nano-scale molecular organization has been lost. What is left is kerogen, amorphous platelets of polycyclic aromatic hydrocarbons with no obvious resemblance to any familiar cell wall material. Kempe et al. (2002).
No acritarchs are yet known from the earlier Paleoproterozoic. Biomarker evidence (i.e. the presence of long-chain 2-methylhopanes) suggests that a phytoplankton population was present, consisting mainly/entirely of cyanobacteria. Summons et al. (1999); Falkowski et al. (2004); Canfield (2005).
Acritarchs are present in the later Paleoproterozoic; but they are rare and consist of morphologically simple spheres (i.e. sphaeromorphic acritarchs). Javaux et al. (2004). They sometimes look a good deal like prasinophytes (basal green algae), and perhaps that's what they are. More likely, they're below the split between red algae and green algae, like glaucophytes, or even basal to all crown eukaryotes. Many or most acritarchs from the Paleoproterozoic and earliest Mesoproterozoic are probably akinetes or other bacterial remains. Golubkova & Raevskaya (2005). Akinetes are inactive resting stages of cyanobacteria induced by cold, lack of food, or similar environmental conditions unfavorable for growth -- the sorts of stimuli that generally lead organisms to seek metabolic stasis and/or admission to graduate school. Meeks et al. (2002) (review).
Other types of fossils from near the Paleoproterozoic-Mesoproterozoic boundary include coiled or worm-like forms such as Grypania, and possibly related structures resembling beads on a string. Porter (2004). Some of the Grypania-like fossils approach 1 mm in width, which seems unreasonably large for a bacterium or even a colonial bacterial structure. Yet the earliest Grypania are about a billion years too old to be actual worms. So perhaps -- for lack of any other hypothesis -- these are very early Eukarya, possibly outside the crown group.
By the early Mesoproterozoic (Calymmian), the evidence for a eukaryotic grade of organization becomes more definite. This judgment is based on: "(1) wall structure and surface ornamentation (2) processes that extend from vesicle walls (3) excystment structures (openings through which cysts liberate their cellular contents) (4) wall ultrastructure and (5) wall chemistry." Javaux et al. (2004). In particular, large cells with processes extending beyond the wall (i.e. acanthomorphic acritarchs) are thought to be impossible without a eukaryotic cytoskeleton. Id. Oddly enough, this agrees reasonably well with recent "molecular clock" work, which likewise places the primary radiation of the Plantae in the Calymmian. Yoon et al. (2004). Mesoproterozoic acritarchs include specimens with new morphological features: ellipsoidal shape, vesicle pores, and a multi-celled or colonial appearance. Huntley et al. (2006).
Javaux et al. (2001) studied well-dated samples from the Roper Group of northern Australia. They recovered specimens of the controversial, but almost certainly eukaryotic, Tappania. Note the relatively large size and the long, irregular processes which penetrate the outer wall. A bacterial origin isn't completely impossible, but the more likely explanation is a eukaryote with a well-developed cytoskeleton. In any case, some of the acritarchs known from this era are relatively enormous, such as Chuaria circularis (better known from the Neoproterozoic), which can approach 1 mm in diameter. Golubkova & Raevskaya (2005).
Another key finding of this study was that the Roper acritarchs showed clear ecological zonation, with different populations characteristic of inshore, nearshore, and distal shelf environments. The authors speculate that communities were limited by nutrient mineral runoff, since abundance and diversity seem higher in marginal marine settings. However, there is no guarantee that acritarch diversity reflects biotic diversity generally.
Slightly later (Ectasian) communities from the Ruyang Group of North China are dominated by Dictyosphaera, an acritarch also found in the Roper Group. Kaufman & Xiao (2003). This cosmopolitan distribution is typical of Proterozoic acritarchs. The authors performed ion microprobe isotopic analysis of individual specimens, and were able to make a rough estimate that Mesoproterozoic CO2 levels were between 10 and 1000 times higher at present.
It isn't clear that these fossils are crown eukaryotes (i.e. descendants of the last common ancestor of all living eukaryotes). It seems likely, if only because all acritarchs are assumed to have been photosynthesizers; but a good deal of room for doubt remains. However, there is a modest consensus that crown eukaryotes had appeared by the Stenian, if not earlier, since these latest Mesoproterozoic acritarchs more closely resemble modern algae. Javaux et al. (2004); Porter (2004). In particular, Bangiomorpha looks like an extant alga and shows a very unusual "intercalary" pattern of cell division that is characteristic only of living bangiacean red algae. Porter (2004). Some acritarchs from this period can be found with surface ornamentation, and not simply processes. The Stenian also marked the first appearance of stalked cyanobacteria. Golubkova & Raevskaya (2005).
Fossils from even the earliest Neoproterozoic (Tonian) include forms commonly identified as fungi and recognizable modern orders of green algae. Falkowski et al. (2004); Golubkova & Raevskaya (2005). Morphological features new to the Neoproterozoic include "polyhedral vesicles, bulb-shaped vesicles, barrel-shaped vesicles, triangular and hair-like processes, funnel-tipped processes, processes that fuse at the tips, and flange ornamentation about the vesicle equator." Huntley et al. (2006)
Generally speaking, the data from the Cryogenian is poor, but suggests continuity with the Tonian forms without dramatic changes in morphology or even diversity. Porter (2004). The first fossil remains of testate amoebae appear late in the Cryogenian -- the first good evidence of heterotrophic Eukarya. Porter (2004). The Ediacaran introduced two new communities, one associated with the Ediacaran animals (or whatever they may have been), followed by one associated with the transitional metazoans of the Doushantuo type. The latter include recognizable modern orders of red algae. Xiao et al. (2004). Many acritarchs from this period bore regular processes and surface ornamentation, e.g. Appendisphaera, Ericiasphaera. Golubkova & Raevskaya (2005).
A number of efforts have been made to quantify the pattern of acritarch diversity across the Proterozoic. Knoll (1994); Porter (2004); Huntley et al. (2006). In general they verify that after an initial burst of diversification in the later Paleoproterozoic, development was gradual or even static until the Neoproterozoic. Surprisingly, evidence for a diversity or even abundance bottleneck as a result of the Cryogenian "snowball earth" episodes is weak to almost non-existent. See also Olcott et al. (2005) (biomarker evidence). It might be fairer to say that the diversity curve began to take off in the Tonian and was, at most, slowed down in the Cryogenian.
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