Page 306 - Introduction to Paleobiology and The Fossil Record

THE BASAL METAZOANS: SPONGES AND CORALS 293

(a) (b)
Figure 11.35 Devonian reefs of the Canning Basin, Australia: (a) main face, and (b) Windjana Gorge.
The fore-reef slope in the foreground has large blocks of unbedded reef material in the background; the
reef is prograding over the fore-reef toward the viewer. (Courtesy of Rachel Wood.)

building and non-reef-building groups in
Distribution: corals through time
shallow- and deep-water environments, respec-
Although some coral-like forms have been tively. Scleractinian evolution was marked by
described from the Cambrian, most lack a number of morphological trends: solitary life
typical zooantharian structures. Cothonion, strategies were eventually superseded by a
for example, with poorly integrated corallite- dominance of colonial forms that display tran-
like clusters and opercula was probably a sitions from low levels of integration in phace-
Cambrian experiment with coralization (Fig. loid growth modes to higher levels in meandroid
11.36). The first tabulates appeared during styles, common in modern reefs.

the Early Ordovician with cerioid growth Corals have been used effectively for the
modes; tabulae were rare and septa and mural correlation of Silurian (tabulates) and Devo-
pores were absent. Nevertheless, by the Mid nian (rugose) strata but they have been proved
and Late Ordovician the more typical charac- most useful for Carboniferous biostratigra-
ters of the Tabulata had evolved when they phy. During the early 1900s, Arthur Vaughan
dominated coral faunas. Some workers have studied in detail the distribution of Lower
removed the heliolitids, with individual coral- Carboniferous corals in Belgium and Britain
lites mutually separated by extensive coenos- and he argued they would be of great value
teum, from the Tabulata, as a distinct order. in Carboniferous biostratigraphy. Corals are
The group was common until the Early Silu- very common, often widespread, usually dis-
rian when the more open structures of the tinctive, and well preserved in the Lower Car-
favositids with massive cerioid colonies began boniferous rocks of Europe. However, more
to dominate, although they were already modern studies on Carboniferous biostratig-
abundant in the Ordovician. raphy using microfossils such as conodonts
The Rugosa appear during the Mid and foraminiferans, together with sequence
Ordovician. Many of the evolutionary trends stratigraphy, have shown that the occurrences
across the order have been repeated many of corals are controlled as much by rock facies
times in different families. In general terms the as by time, and so they cannot be used for
group evolved more complex, heavier skeletons global correlation. Nevertheless, many corals
prior to extinction at the end of the Permian. are still useful for local correlations, and

The first scleractinians were established by Lower Carboniferous stratigraphy (Fig. 11.37)

the Mid Triassic, derived from multiple ances- has been refined on the basis of Vaughan’s
tors among the sea anemones. The Triassic pioneer work and more modern techniques
taxa were probably photosymbiotic, forming (Riley 1993).
patch reefs in parts of the Tethyan belt. The But was coloniality amongst the bilaterians
group, however, expanded signifi cantly during a derived condition or, more controversially,
the Jurassic with the radiation of both reef- a primitive state (Box 11.10)?