Page 298 - Introduction to Paleobiology and The Fossil Record

THE BASAL METAZOANS: SPONGES AND CORALS 285

tabula

septal
spines

mural
pores

(a) (b)
Figure 11.27 Tabulate morphology: (a) transverse and (b) longitudinal sections of Favosites. The insets
on (a) show the lateral and upper surfaces of the entire Favosites colony.
stable rugose and tabulate corals of the Paleo- The zooxanthellae are endosymbionts living
zoic. Finally, scleractinians have a distinctive in the tentacles and mouth of the cnidarian
ultrastructure composed of aragonite and a where they recycle nutrients, accelerate the
widespread development of coenosarc. rate of skeletal deposition and convey organic
Although scleractiniomorph corals are carbon and nitrogen to the cnidarian in return
now known from both the Cambrian and for support and protection from grazers.
Ordovician (Box 11.7), the scleractinians Hermatypic corals are commonly multiserial
first appeared in the Mid Triassic with forms forms, with small corallites displaying a high

such as Thamnasteria becoming quickly degree of integration. Ahermatypic corals,
widespread throughout Europe. lacking algal symbionts, are commonly soli-
The scleractinians developed a wide range tary or uniserial compounds with large, poorly
of morphologies (Fig. 11.31). For example, integrated corallites.
Montlivaltia is a small, cup-shaped coral Some have suggested that coral morphol-
common from the Early Jurassic to the ogy may help predict the presence of symbi-
Cretaceous. Thecosmilia is a small, dendroid onts in fossil coral communities. It is probable
to phaceloid colonial form with similar coral- that many tabulates were zooxanthellate
lites that ranges from the Middle Jurassic to whereas the rugosans were not. In broad
the Cretaceous; the massive cerioid Isastraea terms, there may be parallels between the
has a similar range. Scleractinians are now the platform and basin associations of rugose and
dominant reef-building animals in modern tabulate corals of the Paleozoic and reef-
seas and oceans where they form reef struc- building and non-reef-building scleractinian
tures in a variety of settings, usually in the corals of the Mesozoic and Cenozoic.
tropics. Reefs are biological frameworks with sig-
nificant topography (Box 11.8). Three main

types of structure occur in tropical shallow
Synecology: corals and reefs
water: (i) fringing reefs develop directly adja-
Virtually all fossil corals were benthic. Two cent to land areas; (ii) barrier reefs have an
ecological groups have been recognized among intervening lagoon; and (iii) atolls completely
Recent scleractinians. Hermatypic corals are surround lagoons and are usually of volcanic
associated with zooxanthellae (dinofl agel- origin. The last will continue to grow as the
lates) and are restricted to the photic zone to volcanic island subsides until eventually only
maintain this symbiosis. Symbiosis between a barrier reef, enclosing a lagoon, remains.
the dinoflagellates and cnidarians is wide- Paleozoic corals were not particularly suc-

spread across the living representatives of the cessful reef builders; many preferred fi rm sub-
phylum, with algae associating not only with strates and lacked structures that allowed
corals but also anemones and gorgonians. anchorage and aided stability; calcareous