Page 291 - Introduction to Paleobiology and The Fossil Record

278 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Table 11.1 Features of the main coral groups.

Feature Rugosa Tabulata Scleractinia

Growth mode Colonial and solitary Colonial Colonial and solitary
Septa 6 prosepta; later septa in Septa weak or absent 6 prosepta; later septa in all 6 spaces
only 4 spaces
Tabulae Usual Well developed Absent
Skeletal material Calcite Calcite Aragonite
Stability Poor Poor Good with basal plate
Range Ordovician to Permian Ordovician to Permian Triassic to Recent

forms have been collected from the Late Juras- during fi shing, overfishing of important her-

sic Solnhofen Limestone of Bavaria. Living bivores and predators, and even harvesting
members of the group include Aurelia, the for jewelry. There seems little hope for this

moon jellyfish, and the compass jellyfi sh, spectacular habitat unless more attention is
Chrysaora. Although the anthozoans include paid to conservation.
the sea anemones, sea fans, sea pens and sea The anthozoans are the most abundant
pansies, the class also includes the soft and fossil cnidarians, pursuing a polypoid life-
stony corals. Following a short, mobile, style. The class Anthozoa contains two sub-
planula larval phase, all members of the group classes with calcareous skeletons. Whereas

pursue a sessile life strategy as polyps. the Octocorallia have calcified spicules and
axes, the Zoantharia include the more famil-
iar fossil coral groups, the orders Rugosa,
Corals
Tabulata and Scleractinia (Table 11.1). The
Corals are probably best known for their Octocorallia, including the Alcyonaria, have
place in one of the planet’s most diverse but eight complete mesenteries and a ring of eight
most threatened ecosystems, the coral reef. hollow tentacles; the skeleton lacks calcifi ed
Shallow-water coral reefs form only in a zone septa but calcareous or gorgonin spicules and
extending 30˚ degrees north and south of the axes comprise solid structures in the skeleton.
equator and reef-forming corals generally do Although the group is only sporadically rep-
not grow at depths over 30 m or where the resented in Silurian, Permian, Cretaceous and
water temperature falls below 18˚C, although Tertiary rocks, the octocorals are important
certain groups of corals can also form struc- reef dwellers today. Some familiar genera
tures in deep-water environments. Corals are include Alcyonium (dead men’s fi ngers), Gor-
not the only reef-forming organisms but gonia (sea pen) and Tubipora (organ-pipe
throughout geological time they have con- coral).
structed three main types of reefs: fringing
reefs, barrier reefs and atolls. These structures Morphology: general architecture
formed the basis for Charles Darwin’s then
cutting-edge analysis Coral Reefs published in There are four main elements to the zoanthar-
1842. Unfortunately, such structures are under ian coral skeleton: radial and longitudinal
current threat, including damage from structures, together with horizontal and axial
increased bleaching, coastal development, elements. Corals have planula larvae. Follow-
temperature change of seawater, tourism, ing the planula larval stage the coral polyp
runoff containing agricultural chemicals, initially rests on a basal plate or disk termed
abrasion by ships’ hulls and anchors, smoth- the holotheca and begins the secretion of a
ering by sediment, poisoning or dynamiting series of vertical partitions or septa in a radial