Page 300 - Introduction to Paleobiology and The Fossil Record

THE BASAL METAZOANS: SPONGES AND CORALS 287

1 4 3
4
2
4
a 3 4
1 1
a'
a a' 4 3
4
b b' mesentery septum 2 4
3
4
1 1
septum 4
b 3
4
2
b' 1 4 3 4
epithecal wall
(a) (b) (c)
Figure 11.29 Scleractinian morphology: (a) longitudinal and (b) transverse sections, and (c) mode of
septal insertion.

algae and stromatoporoids were usually more Devonian successions (Fig. 11.34). The scler-
important. Nevertheless, frameworks domi- actinians gradually became the dominant reef
nated by colonial tabulates, and to a lesser builders during the Mesozoic and Cenozoic.
extent rugosans, do occur, particularly during Modern coral reef associations have been
the Mid Paleozoic. Growth bands on the latter documented in detail from eastern Australia,
have provided us with a Paleozoic calendar the eastern Pacific and the Caribbean.

(Box 11.9). The Great Barrier Reef on the continental
Pioneer and climax communities have been shelf of eastern Australia is the largest coral
described from a number of Silurian and structure on Earth, approaching 3000 km

Box 11.7 Kilbuchophyllida and iterative skeletalization

Did the scleractinian corals have a long cryptic history through the Paleozoic? When the coral Kil-
buchophyllum (Fig. 11.30a) was described from the Middle Ordovician rocks of southern Scotland,
it caused a sensation, at least amongst coral workers. Kilbuchophyllum seemed to have patterns of
septal insertion and a microstructure identical to those of modern scleractinians, and quite unlike

the contemporary rugosans and tabulates. At first, some paleontologists said this was an aberrant
local form, but specimens have been found in the Silurian too. It is unlikely that Kilbuchophyllum
was the stem group for the scleractinians; however, clearly other groups of soft-bodied anemones
with the potential of skeletalization were around early in the history of the group. Following the
end-Permian mass extinction, when the rugose and tabulate corals fi nally disappeared, calcifi cation
of other scleractinian-type morphs during the Triassic marked a new start of another highly success-
ful calcified coral group. Similarly calcified, scleractinian-type polyps are known from the Permian,

implying that this skeletal type re-evolved iteratively, that is time and time again. But what did the
naked scleractinian-type polyps look like? Hou Xian-guang (Yunnan University) and his colleagues
(2005) have described the sea anemone-like Archisaccophyllia from the Early Cambrian Chengjiang
fauna (Fig. 11.30b; see p. 386). This organism may well have been one of a group of naked polyps
that generated various scleractiniomorph corals during the Paleozoic and probably were responsible
for seeding the Mesozoic radiation of the most successful reef builder in the oceans today.

Continued