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amoebozoan clade, occur in the same rocks (Porter et al. 2003). Biomarkers ascribed to
alveolates have been found in shales as old as 1100 million years (Summons and Walter
1990), but animals are unknown before 600 million years ago (Xiao and Knoll 2000).
Save for the late appearance of animals, this paleontological picture of Proterozoic clade
divergence is consistent with recent molecular clock estimates (e.g., Wang et al. 1999;
Yoon et al. 2002).
The overall pattern, then, appears to be one of early eukaryotic clade differentiation,
with later—and in some cases much later—evolution of biomineralized skeletons within
clades. Preservational biases have undoubtedly influenced this pattern; skeletal fossils
can be recognized in the rock record only when both mechanical and solubility thresholds
for preservation and morphological thresholds for identification have been crossed.
Nonetheless, the inferred evolutionary pattern must be at least broadly correct, as detailed
in the following sections.
The earliest records of mineralized skeletons.
Today, calcium carbonate and silica
leave the oceans largely as skeletons. This could not have been the case early in Earth
history; nonetheless, as expected when chemical weathering continuously introduces
calcium and silica into the oceans, limestone, dolomite, and chert are common in
Proterozoic sedimentary successions. Proterozoic and Phanerozoic successions differ not so
much in the abundance of carbonates and silica as in the facies distributions of preserved
deposits. Prior to the radiation of skeleton-forming eukaryotes, CaCO
3
and SiO
2
both
accumulated preferentially along the margins of the oceans—in tidal flats and coastal
lagoons, where evaporation drove precipitation (Maliva et al. 1989; Knoll and Swett 1990).
Bacteria undoubtedly facilitated this deposition—the microscopic textures preserved in
silicified stromatolites include both encrusted filaments and radially oriented crystal fans
nucleated within surface sediments (e.g., Knoll and Semikhatov 1998; Bartley et al.
2000)—but the overall distribution of Proterozoic carbonates and silica reflects primary
physical controls on precipitation. In this regard, it is worth noting that surface textures of
Proterozoic sandstone beds record the widespread distribution of microbial mats in
environments where no stromatolites accreted (e.g., Hagadorn and Bottjer 1997; Noffke et
al. 2002); stromatolites formed where carbonate was deposited, not the reverse.
At present, the oldest inferences of protistan biomineralization come from vase-
shaped tests preserved in 742±6 million year old rocks of the Chuar Group, Grand
Canyon, Arizona (Porter and Knoll 2000; Porter et al. 2003). These fossils are veneered
by pyrite (or iron oxides after pyrite) with regularly arranged ovoid holes, interpreted as
insertion sites for mineralized scales in originally proteinaceous tests (Figure 4B). The
morphology, inferred organic construction, and scale distribution of these fossils closely
(and uniquely) resemble the tests of euglyphid filose amoebans (Porter et al. 2003).
Further evidence of silica biomineralization is provided by a remarkable assemblage of
small (10-30 µm) scales preserved in early diagenetic chert nodules from ca. 630-650
million year old rocks of the Tindir Group, northwestern Canada (Figure 4A; Allison and
Hilgert 1986). These variously ornamented microfossils resemble the siliceous scales of
chrysophytes, albeit at a larger size.
Horodyski and Mankiewicz (1990) described a possible instance of early carbonate
skeletonization in silicified dolomites from the Pahrump Group, California, that are at least
broadly correlative with Chuar rocks in the Grand Canyon. The fossils consist of sheet-like
cellular structures originally encrusted by finely crystalline calcium carbonate and now
preserved texturally in early diagenetic chert nodules. In some specimens, cell walls are
preserved as clear chert, sharply differentiated from the carbonate-textured silica within and
without. Horodyski and Mankiewicz (1990) proposed that the Pahrump fossils record
thalloid algae with carbonate impregnated cell walls. More likely, however, the fossils
