Page 249 - Introduction to Paleobiology and The Fossil Record

236 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
in oxygen levels in the deep ocean (Canfi eld
et al. 2007).
Body fossil evidence
Body fossils of basal metazoans in the Edia-
caran Period are few and far between. The
morphology of an early metazoan fossil must
be clearly described and convincingly illus-
trated, different organs and tissues identifi ed,
and comparisons drawn with other extant
and fossil organisms. Many Upper Precam-
brian successions have been subjected to
intense metamorphism and tectonism (see p.
48) and are now located in some of the Earth’s
mountain belts. The chances of fi nding ade-
quately preserved fossils are slight. Neverthe- Precambrian of Australia, showing
less, the earliest undoubted metazoans occur
within the widespread Ediacara biota (see p.
242) dated at approximately 600–550 Ma.
Moreover the fact that a relatively advanced
metazoan, the mollusk Kimberella, possibly
equipped with a foot and radula (see p. 330),
occurs within the Ediacara biota from south-
ern Australia and Russia could suggest a
history of metazoan evolution prior to the
Ediacaran. But although a strong case can be
made for a significant Proterozoic record for
the cnidarians and sponges and perhaps some
other metazoans, the Cambrian explosion still
marks the arrival, center stage, of the bilateri- noan glaciation (635 Ma), the second main
ans (Budd 2008).
Trace fossil evidence
Trace fossils are the behavior of organisms
recorded in the sediment (see p. 510). By their
very nature they occur in place and thus
cannot be transported or reworked by cur-
rents. Nevertheless these too must be convinc- bacteria or that they are not embryos at all
ingly demonstrated as biogenic and the age of
their enclosing sediments accurately deter-
mined. If and when metazoans developed
locomotory organs, such as the molluskan
foot, and digestive systems, we might expect
to find burrows and trails together with fecal
pellets. Records of trace fossils from rocks
older than 1 Ga in India (Seilacher et al. 1998)
and over 1.2 Ga in the Stirling biota of Aus-
tralia (Rasmussen et al. 2002) generated
considerable excitement (Fig. 10.2). Both
suggested metazoan life older than 1 Ga but
both are now considered questionable (Jensen
2003). The oldest undoubted locomotory
Figure 10.2 Putative trace fossils from the
Myxomitodes, a presumed trail of a mucus-
producing multicellular organism about 1.8–2
billion years old from Stirling Range, Western
Australia. (Photo is approximately 65 mm wide.)
(Courtesy of Stefan Bengtson.)
trace fossils are from about 550 Ma (Droser
et al. 2002) from northwest Russia, whereas
fecal strings have been reported from rocks
some 600 Ma (Brasier & McIlroy 1998) sug-
gesting the existence of an ancient digestive
system. In fact no convincing trace fossils are
known from successions older than the Mari-
icehouse event associated with snowball Earth
(see p. 112).
Embryo fossil evidence
Fossil Neoproterozoic embryos are now
known from a number of localities, although
claims that they represent sulfur-oxidizing
have their advocates. Some of the best studied
examples are from the Doushantuo Forma-
tion, South China. The part of the formation
yielding the embryos was fi rst dated at approx-
imately 580 Ma, predating much of the Edia-
caran but postdating the Marinoan glaciation.
Revised dates seem to suggest that the faunas
are younger and that they overlap with the
older Ediacaran assemblages. Cell division
and cleavage patterns are obvious although it
is difficult to assign the material to distinct
metazoan groups in the absence of juvenile
and adult forms. There are, however, a lack
of epithelia even in clusters of over 1000 cells