Page 257 - Introduction to Paleobiology and The Fossil Record

244 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 10.3 The Ediacaran animals: a form classifi cation

RADIATA (RADIAL ANIMALS)

Three main classes are defined. Most colonial organisms in the fauna, for example Charnia, Char-

niodiscus and Rangea, are assigned to coelenterates and were part of the sessile benthos. The affi nities
of these animals have been debated in detail, but their close similarity to the sea-pens suggests an
assignment to the pennatulaceans.
Class CYCLOZOA

• These animals have a concentric body plan with a large disk-shaped stomach and the class
includes mostly sessile forms such as Cyclomedusa and Ediacaria. About 15 species of jellyfi sh-
like animals have been described and in some, for example, Eoporpita tentacles are preserved
Class INORDOZOA
• Medusa-like animals with more complex internal structures, for example Hielmalora
Class TRILOBOZOA

• Characterized by a unique three-rayed pattern of symmetry. Tribrachidium and Albumares are
typical members of the group

BILATERIA (BILATERAL ANIMALS)
This division contains both smooth and segmented forms.
Smooth forms

• These morphotypes are rare. They include Vladimissa and Platypholinia, which may be turbel-
larians, a type of platyhelminthes worm
Segmented forms

• Much of the Ediacara fauna is dominated by segmented taxa inviting comparisons with the
annelids and arthropods. Dickinsonia, for example, may represent an early divergence from the
radial forms whereas Spriggina, although superficially similar to some annelids and arthropods,

possesses a unique morphology

Mark McMenamin (1986), although this
Ecology
model has its opponents. McMenamin con-
There is little doubt that the Ediacara biotas sidered that the ecosystem was dominated by
dominated the latest Precambrian marine eco- medusoid pelagic animals, and that attached,
system, occupying a range of ecological niches sessile benthos and infaunal animals were
and pursuing varied life strategies probably sparse; the medusoids have been reinterpreted
within the photic zone (Fig. 10.10). There is as bacterial colonies or even holdfasts. Food
no evidence to suggest that any of the Edia- chains were thus probably short and the
caran organisms were either infaunal or trophic structure was apparently dominated
pelagic, thus in contrast to the subsequent by suspension and deposit feeders.
Cambrian Period, life was restricted to the
seabed. It is also possible that these fl attened Biogeography
organisms hosted photosymbiotic algae,
maintaining an autotrophic existence in the Although provincialism was weak among the
tranquil “garden of Ediacara” as envisaged by Ediacara biotas, three clusters have been rec-