Chemistry Reference
In-Depth Information
Figure 4
. Evidence for skeletal biomineraliza-
tion in Neoproterozoic rocks: (A) chrysophyte-
like scales from ca. 650-630 million year old
cherts of the Tindir Group, northwestern
Canada; (B)
Melicerion
, test of a scale-forming
filose amoeban from rocks just below a 742±6
million year ash bed in the Chuar Group, Grand
Canyon, Arizona; (C)
Cloudina
, thin section
photograph showing skeletal structure, from
548-543 million year old reefs of the Nama
Group, Namibia; and (D)
Namacalathus
, also
from the Nama Group, Namibia—note the
goblet shaped individual in the upper right part
of the image. (See text for references.) Scale
bar (found in C) = 35 µm for (A), = 40 µm for
(B), = 600 µm for (C), and = 1.5 cm for (D).
record unmineralized thalli whose relatively decay resistant walls were cast by encrusting
carbonates soon after burial. Cyanobacterial sheaths preserved in similar fashion occur
widely if sporadically in Neoproterozoic rocks around the world (e.g., Knoll and
Semikhatov 1998). Ca. 600 million year old rocks of the Doushantuo Formation, China,
contain extraordinary fossils of multicellular red and green algae preserved in anatomical
detail, and these include florideophyte reds interpreted as stem group members of the (now)
skeleton-forming Corallinales. Despite their superb preservation, however, these fossils
show no evidence for the precipitation of CaCO
3
skeletons (Xiao et al. in press).
Early animals are best known from so-called Ediacaran fossils, problematic remains
found globally as casts and molds in latest Proterozoic (575-543 million year old) storm
deposits and turbidites (Narbonne 1998). Ediacaran remains and associated trace fossils
contain little evidence for mineralized skeletons, prompting the widespread belief that
pre-Cambrian animals were exclusively soft-bodied. Coeval carbonates, however, tell a
different story. Microbial reefs in Namibia and elsewhere contain abundant and
moderately diverse assemblages of calcified metazoans.
Cloudina
is a broadly cylindrical
fossil built of funnel-like, apically flaring tubes set one within the next—ontogenetically,
but not necessarily phylogenetically, reminiscent of pogonophoran worm tubes (Figure
4C; Grant 1990). The systematic relationships of these fossils are poorly known, but
occasional evidence for budding supports a broadly cnidarian interpretation. Two distinct
size classes, 0.5-2 and up to 7 mm in cross-sectional diameter, have been distinguished
as different species (Germs 1972). Preserved walls are thin (a few tens of microns), but
they commonly retain their rounded cross-sections in sediments, displaying only minimal
evidence of compaction. Shell hash sometimes shows evidence of brittle fracture,
