Page 256 - Introduction to Paleobiology and The Fossil Record

ORIGIN OF THE METAZOANS 243

Ma δ13c Cambrian-
Paleozoic Cambrian ? shelly
500 –10 0 +10 type
biota
550 Ediacara C ?
Ediacaran Gaskiers
biota
600
Marinoan
Neoproterozoic
650

700 Cryogenian

Sturtian
750

Figure 10.7 Stratigraphic distribution of the Ediacara biota. Solid triangles, glaciations; C, calcifi ed
metazoans; T, position of the Twitya disks. (Based on Narbonne 2005.)

shallow-water siliciclastic sediments, consist- tered areas of the Late Precambrian seafl oor;
ing of clasts of silicic-rich rocks, or volcanic predators and scavengers had yet to evolve in
ash, more rarely carbonates or even turbi- sufficient numbers to remove it.

dites. The sediments were deposited during
specific events, such as a storm, and are usually Morphology and classifi cation

termed event beds. Deep-water biotas are also
known such as those from Mistaken Point in Traditionally the Ediacaran taxa, a collection
Newfoundland. The style of preservation of disks, fronds and segmented bodies, have
plays an important role in understanding been assigned to a variety of Phanerozoic
these organisms (Narbonne 2005). The wide- invertebrate groups on the basis of apparent
spread development of algal mats, prior to the morphological similarities. In many cases
Cambrian substrate revolution (see p. 330), considerable speculation is necessary and
suggests that these too aided preservation, many assumptions are required to classify
sometimes providing “death masks”, of these these impressions. Most of the species have
non-skeletal organisms. been assigned to coelenterate groups, although
Although morphologically diverse, the Edi- some taxa have been identified as, for example,

acaran organisms have many features in arthropods or annelids. Michael Fedonkin
common. All were soft-bodied, with high (1990), however, suggested a form classifi ca-
surface to volume ratios and marked radial or tion based on the morphology and structure
bilateral symmetries. These thin, ribbon- of these fossils. Key areas of his classifi cation
shaped animals may have operated by direct are summarized in Box 10.3 and typical
diffusion processes where oxygen entered examples illustrated in Figure 10.8. The
through the skin surface, so gills and other bilateral forms were probably derived from
more complex internal organs were perhaps an initial radial body plan. The concept

not required. Most Ediacaran organisms have and classification of the Ediacara biota is in a

been studied from environments within the state of flux and Fedonkin’s classifi cation is
photic zone; many collected from deeper- one of a number of attempts to rationalize the
water deposits are probably washed in. Pro- group, assuming the majority are in fact
vincialism among these Upper Proterozoic animals. Some have argued, nevertheless, that
biotas was weak with many taxa having a the Ediacarans are organisms unrelated to
nearly worldwide distribution. It is possible modern metazoans (Box 10.4), or are even

that the flesh of the Ediacaran organisms lit- Fungi.