Page 293 - Introduction to Paleobiology and The Fossil Record

280 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

cerioid astraeoid thamnasteroid

aphroid meandroid hydnophoroid

coenostoid phaceloid halysitid
Figure 11.20 Terminology for the main modes of colonial growth in corals. (Redrawn from various
sources.)

Scleractinian corals may be highly integrated
because they have symbiotic zooxanthellae Rugose corals
(see p. 285). The relatively low levels of inte- Rugose corals are generally robust, calcitic
gration seen in the Rugosa and some Tabulata forms with both colonial and solitary life
colonies perhaps suggests a lack of algal sym- modes, more varied than those of tabulates.
bionts. There has been a great deal of argu- Rugosans have well-organized septal arrange-
ment about this. Some rugosans are in fact ments with six cardinal or primary septa. Sec-
quite highly integrated, and it is questionable ondary septa are inserted in four spaces
whether high integration should only be asso- around the corallum – between the cardinal
ciated with the presence of zooxanthellae. septa and the two alar septa and also between
Coral experts also use quantitative the two counterlateral septa and lateral septa
approaches in describing colony shapes. Key (Fig. 11.23a). Horizontal structures such as
measurements are made on the colony and the tabulae, dissepiments and dissepimentaria
these are plotted on a ternary diagram. A are also well developed across the order.

series of fields can be mapped out within the Undoubted rugosans, such as Streptelasma,
triangle – for example, bulbous, columnar, with short secondary septa and lacking a dis-
domal, tabular and branching colonies are sepimentarium, are not recorded until the
discriminated (Fig. 11.22). These different Mid Ordovician. By the Late Ordovician,
growth strategies may be ecophenotypic (see rugose faunas were well established with the
p. 123), commonly refl ecting ambient envi- development of a wide variety of morpholo-
ronmental conditions. gies (Fig. 11.23b; Box 11.5). For example, the