Page 258 - Introduction to Paleobiology and The Fossil Record

ORIGIN OF THE METAZOANS 245

Ediacara
Dickinsonia
Sprigginia
Rangea

Charnia

Tribrachidium Praecambridium
Cyclomedusa Medusinites
(a) Radiata (b) Bilateria
Figure 10.8 Some typical Ediacara fossils: (a) the Radiata, which have been associated with the
cnidarians, and (b) the Bilateria, which may be related to the annelids and arthropods. Ediacaria (×0.3),
Charnia (×0.3), Rangea (×0.3), Cyclomedusa (×0.3), Medusinites (×0.3), Dickinsonia (×0.6), Spriggina
(×1.25), Tribrachidium (×0.9) and Praecambridium (×0.6). (Redrawn from various sources by Anne
Hastrup Ross.)

ognized based on multivariate biogeographic organisms. More importantly, the Ediacara
analysis (see p. 45) by Ben Waggoner (2003): body plan offered little defense against active
(i) the Avalon assemblage is from deep-water, predation. There is abundant evidence for
volcaniclastic settings in eastern Newfound- Cambrian predators: damaged prey, actual
land; (ii) the White Sea assemblage represents predatory organisms and the appearance of
the classic Vendian section in the White Sea, defense structures, such as trilobite spines and
Russia; and (iii) the Nama assemblage is a multielement skeletons. All suggest the exis-
shallow-water association from Namibia, tence of a predatory life strategy that was
West Africa. Unfortunately the distribution of probably established prior to the beginning of
these assemblages does not match any paleo- the Cambrian Period. The Proterozoic–
geographic models for the period and the Cambrian transition clearly marked one of
clusters may rather represent a mixture of the largest faunal turnovers in the geological

environmental and temporal factors (Grazh- record, with a significant move from soft-
dankin 2004). bodied, possibly photoautotrophic, animals
to heterotrophs relying on a variety of nutri-
Extinction of the Ediacarans ent-gathering strategies. It is, however, still
uncertain whether a true extinction, or the
The Ediacara biota, as a whole, became extinct slamming shut of a taphonomic window,
about 550 Ma. Nevertheless, in terms of lon- accounted for the disappearance of the Edia-
gevity, the ecosystem was very successful and cara biota from the fossil record.
a few seem to have survived into the Cam-
brian. The rise of predators and scavengers Cloudina assemblages
together with an increase in atmospheric
oxygen may have at last prevented the routine Although the Ediacara biotas were overwhelm-
preservation of soft parts and soft-bodied ingly dominated by soft-bodied organisms,