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THE BASAL METAZOANS: SPONGES AND CORALS 283

Box 11.5 Rugosan life strategies

Despite the apparent simplicity of rugosan architecture, these corals may have pursued a number of
different life strategies (Fig. 11.24). A number of corals, for example Dokophyllum, probably sat
upright in the sediment rooted by fine holdfasts extending from the epitheca. Other taxa, such as

Holophragma, were initially attached to a patch of hard substrate but subsequently toppled over to
rest on the seabed. Grewingkia was cemented to areas of hard substrate. The small discoidal Pal-
aeocyclus, however, may have been mobile, creeping over the substrate on its tentacles. A number
of strongly curved rugosans, for example Aulophyllum, probably lay within the substrate, concave
upwards. Successive increments of growth were directed more or less vertically giving the coral
exterior a stepped appearance. Many other solitary corals exhibit a similar terraced theca, which
may be due to changes in growth direction associated with adjustments following toppling of the
corallum during slight turbulence or storms.

Attached to clast Recumbent

Fixosessile
Recumbent
Rhizosessile

Figure 11.24 Rugose solitary life strategies displaying attached, fi xosessile, rhizosessile and
recumbent life modes. (Based on Neuman; B.E.E. 1988. Lethaia 21.)

phaceloid Syringopora, with long, thin, cylin- sans. Secondary septa are inserted in all six
drical corallites, characterize the coral faunas spaces between the primary or cardinal septa.
of the period. By the Late Permian the group In further contrast to septal insertion in the
was very much in decline following a long rugosans, each cycle of six is fully completed
period of deterioration after the Frasnian before the next cycle of insertion commences.

extinctions; only five families survived to the Tabulae are absent, although dissepiments
end of the period. and dissepimentaria are developed. Moreover
the scleractinian skeleton, although relatively
Scleractinian corals light and porous, has the stability of a basal
plate which aids anchorage in the substrate.
The scleractinians are elegant zoantharian Additionally, the scleractinian polyp can
corals with relatively light, porous skeletons secrete aragonite on the exterior of the coral-
composed of aragonite (Fig. 11.29). Both soli- lite, often in the form of attachment struc-
tary and colonial modes exist with even more tures. Both adaptations provided a much
varied architectures than those of the rugo- greater potential for reef building than the less