Page 284 - Introduction to Paleobiology and The Fossil Record
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THE BASAL METAZOANS: SPONGES AND CORALS 271
(a) (c)
(b) (d)
Figure 11.11 Archaeocyathan reef structures which, when preserved, become (a) boundstones, (b)
bafflestones, (c) bindstones or (d) bioherms. (Based on Wood et al. 1992.)
bilities of secure attachment on a soft sub- tains of the former Soviet Union, North
strate; moreover growth to large size was America and South Australia (Fig. 11.13).
enabled, together with a greater facility for Archaeocyaths were most common in the Mid
regeneration (Wood et al. 1992). The archaeo- to Early Cambrian (Botomian) when a number
cyaths were thus key elements of the fi rst reef- of distinct biogeographic provinces can be
type structures of the Early Cambrian (Fig. defined, but by the Lenian Stage the group
11.11), in intervals of high turbulence and was very much in decline. Few genera have
rates of sedimentation. However, although been recorded from the Middle Cambrian and
archaeocyathan reefs were probably not par- only one is known from Upper Cambrian
ticularly impressive, usually up to 3 m thick strata. Archaeocyath history demonstrates a
and between 10 and 30 m in diameter, they progressive move towards a more modular
were nevertheless amongst the fi rst animals to architecture in response to conditions of high
establish complex biological frameworks, turbulence. In general, solitary taxa domi-
processing large amounts of seawater through nated the Early Cambrian; but following the
their bodies (Box 11.2). Archaeocyathan reefs late Botomian, modular morphotypes contin-
were always associated with calcimicrobes ued after the extinction of most solitary forms
that may have been the main frame builders. (Fig. 11.14; Box 11.3). One advantage is that
There are also some examples of cryptic the abundance and diversity of the group in
organisms living within the reef cavities, some parts of the world, particularly in Lower
including other sponges. Cambrian rocks, has allowed its effective use
in biostratigraphic correlation when there
were few other organisms around that could
Distribution: Cambrian world of act as zone fossils (see p. 28).
the archaeocyaths
The first archaeocyaths are known from the
lowest Cambrian (Tommotian) rocks of the CNIDARIA
Siberian Platform and are represented by The bottom was absolutely hidden by a
mainly solitary regulars. During the Early continuous series of corals, sponges,
Cambrian, the phylum diversifi ed, migrating actiniæ [sea anemones] and other marine
into areas of North Africa, the Altai Moun- productions, of magnifi cent dimensions,
