Page 292 - Introduction to Paleobiology and The Fossil Record

THE BASAL METAZOANS: SPONGES AND CORALS 279

discoid
scolecoid
trochoid

patellate

turbinate ceratoid cylindrical calceoloid pyramidal

Figure 11.19 Terminology for the main modes of solitary growth in corals. (From Treatise on
Invertebrate Paleontology, Part F. Geol. Soc. Am. and Univ. Kansas Press.)

arrangement. At the circumference, the septa grams (Fig. 11.20). Moreover colonies with
are joined to the theca or skeletal wall, which imperforate walls may exhibit phaceloid,
extends longitudinally from the apex of the cateniform, cerioid and meandroid forms,
corallum to the calice where the polyp is whereas those with perforate walls have only
attached. During growth the polyp may secrete phaceloid and cerioid growth modes together
a series of horizontal sheets, or tabulae, with coenenchymal structures in some taxa,
together with smaller curved or angled plates such as the sarcinulids. These growth modes
or dissepiments. The columella, usually arising are variably developed across the rugosans,
from the fusion of the axial edges of the septa, tabulates and scleractinians – but meandroid
occupies the core region of the corallum. The and hydnophoroid modes were developed
vertical walls or septa radiate outwards from during the Mesozoic and are thus restricted
the columella and divide the corallite. Despite to the scleractinians.
the apparent simplicity of the coral skeleton, Colonial integration usually involves a loss
there is a great deal of variation in both soli- of individuality. Many organisms display a
tary and colonial growth programs and the transition from solitary growth modes,
end result is a remarkable array of shapes and through morphologies with asexually budded
sizes of corals. modules, to a fully integrated colony with the
The three main subclasses of stony corals growth or astogeny of the compound struc-
have colonial or compound growth modes ture showing little variation across the indi-
whereas only the Scleractinia and Rugosa vidual corallites. The degree of integration of
have solitary skeletons. The solitary growth a colony is usually measured by the amount
forms include conical, ceratoid or horn- of cohesion between the individual skeletal
shaped, calceoloid, cylindrical, discoid, patel- parts and soft tissues and by the range of form
late, scolecoid, trochoid and turbinate observed between individual components.
skeletons (Fig. 11.19). Colonial corals with Clearly there is a spectrum from phaceloid
corallites have adopted either fasciculate or modes with little or no integration to thamn-
massive growth modes. Fasciculate styles asteroid and meandroid (and coenenchymal)
exhibit either dendroid or phaceloid strategies modes with high levels of integration. Indi-
with either no or poor integration. The hal- vidual polyps are no longer separated by cor-
ysitid or cateniform chain-like growth strat- allite walls and may share a common enteron
egy is a further variation on this pattern. The and nervous system. This suggests a high
massive colonies are much more varied, with degree of integration where the colony
cerioid, astraeoid, aphroid, thamnasteroid, approaches the body plan of a typical meta-
meandroid and hydnophoroid together with zoan. These modes have varied through time
coenenchymal or coenostoid growth pro- (Fig. 11.21).