Page 259 - Introduction to Paleobiology and The Fossil Record

246 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 10.4 Vendobionts or the fi rst true metazoans

The apparently unique morphology and mode of preservation of the Ediacara biota has led to much
debate about the identity and origins of the assemblage. Adolf Seilacher (1989) argued that these
organisms were quite different from anything alive today in terms of their constructional and func-
tional morphology (Fig. 10.9). Apart from a distinctive mode of preservation, the organisms all share
a body form like a quilted air mattress: they are rigid, hollow, balloon-like structures with sometimes

additional struts and supports together with a signifi cant flexibility. Seilacher termed the Ediacaran
organisms vendobionts, meaning organisms from the Vendian, and he speculated about their unique
biology. Reproduction may have been by spores or gametes. The skin must have been fl exible,
although it could crease and fracture, and it must have acted as an interface for diffusion processes.
This stimulating and original view of the Ediacarans, however, remains controversial. Several members
of the Vendobionta have been interpreted as regular metazoans, suggesting a less original explana-
tion for the Ediacara group.
Leo Buss and Adolf Seilacher (1994) suggested a compromise. Their phylum Vendobionta includes
cnidarian-like organisms lacking cnidae, the stinging apparatus typical of the cnidarians. Vendobi-
onts thus comprise a monophyletic sister group to the Eumetazoa (ctenophorans + bilaterians). This
interpretation requires the true cnidarians to acquire cnidae as an apomorphy for the phylum.
The vendobiont interpretation has opened the doors for a number of other interpretations and the
understanding of Ediacaran paleobiology is as open as ever: some authors have suggested the Edia-
carans are giant protists, lichens, prokaryotic colonies or fungus-like organisms. However most agree
that the Ediacara assemblage includes some crown- and stem-group sponges and cnidarians, a conclu-
sion proposed by Sprigg in the late 1940s. This is supported by biomarker and molecular clock
data.

unipolar growth Parvancorina

Vendia
Rangea

Spriggina Phyllozoon Charnia

bipolar

Pneu
structure
Dickinsonia spindle-shaped form

radial

Ovatoscutum Cyclomedusa Tribrachidium Rugoconites Albumares

Figure 10.9 Vendozoan constructional morphology, recognizing unipolar, bipolar and radial
growth modes within the Ediacara-type biota. Scale bars, 10 mm. (From Seilacher 1989.)