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Paleozoic Scleratinia: Progenitors or Extinct Experiments?

Yoichi Ezaki
Paleobiology
Vol. 24, No. 2 (Spring, 1998), pp. 227-234
Published by: Paleontological Society
Stable URL: http://www.jstor.org/stable/2401240
Page Count: 8
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Paleozoic Scleratinia: Progenitors or Extinct Experiments?
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Abstract

The Scleractinia, which are one of the most important builders of modern reefs, have been considered to have first appeared in the Middle Triassic. Recently, Paleozoic scleractiniamorphs have been reported from both the Ordovician and the Permian, suggesting that the scleractinian-like body plan was already established in the Paleozoic. Those Paleozoic scleractiniamorphs are considered either unsuccessful skeletonized offshoots (extinct experiments) or Paleozoic progenitors of the post-Paleozoic Scleractinia. Permian scleractiniamorphs are characterized by "ancestral" features and have no specific morphologies that deny scleractinian affinities. Molecular phylogenetics also indicate that extant scleractinians are monophyletic and originated long before their Triassic appearance. A Paleozoic origin for the Scleractinia is supported by morphological and molecular phylogenetic data. On the other hand, there is no positive evidence to show that different groups of scleractinians had separate soft-bodied precursors. The Paleozoic scleractinians evolved within the framework of their basic body plan, and a direct derivation of the Scleractinia from the Rugosa is not probable. The Anthozoa are characterized by a bilaterally symmetrical body plan, which is traditionally considered to have been derived from other radially symmetrical Cnidaria. The problem of the origin of scleractinian body plan may provide a key for deciphering the early anthozoan radiation within the Bilateria. Other examples of Paleozoic Scleractinia and scleractiniamorphs will be found, probably in shallow-water reefal facies or deeper-water communities, bridging the stratigraphic gaps in occurrence and elucidating the origin of the Scleractinia and their body plan.

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