Aretz et al_2011.pdf - ORBi - Université de Liège

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Kölner Forum Geol. Paläont., 19 (2011)
M. ARETZ, S. DELCULÉE, J. DENAYER & E. POTY (Eds.)
Abstracts, 11th Symposium on Fossil Cnidaria and Sponges, Liège, August 19-29, 2011
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164
The naked Lazarus effect and recovery of corals after the end-Permian
mass extinction
George D. STANLEY, Jr.
The University of Montana Paleontology Center, Missoula 59812 USA
The collapse of corals and reef systems at the end of the Permian is recorded as a gap which extends
through the Early and part of the Middle Triassic and is well known to mark an absence of metazoan reefs.
Perplexing problems of the origin of modern corals and the delayed recovery of corals and reefs can be
explained by an extended Early to Middle Triassic geochemical perturbation of the world’s oceans. Ocean
acidification and the “naked coral” hypothesis help explain some aspects of the Middle Triassic recovery.
Through the Late Permian, reefs constructed by corals, Tubiphytes, algae and a variety of other
organisms collapsed suddenly at the end of the Permian and did not recover until the Middle Triassic
(FLÜGEL & STANLEY 1984; WEIDLICH 2002). What caused the extinction is being discussed and the delayed
recovery is among the unresolved issues (BOWRING et al. 1999). STANLEY (1988) proposed that protracted
sea chemistry perturbations of the ocean held back reefs from recovering and this was borne out of
subsequent research (PAYNE et al. 2006). The perturbation of the marine carbon cycle continued throughout
Early Triassic time, and corals, reefs and many skeletonizing metazoans virtually disappeared.
All corals of the Paleozoic died out (SCRUTTON 1999) so the great coral and reef gap of FLÜGEL &
STANLEY, (1984) appears to be real and not an artifact of incomplete sampling. Lazarus taxa have bearing on
this issue but the dynamics of survival and the idea of refugia are not well resolved. Anisian skeletal
metazoans marked the beginning of the recovery, and carbonate mounds of late Anisian age in the Tethys
and South China are the first examples of Mesozoic reef-like deposits (FLÜGEL 2002). They contain smallscale
framework, similar to bioconstruction of the Late Permian, with Tubiphytes, calcimicrobes, calcareous
red algae, foraminifers, sponges, bryozoans, serpulids, crinoids and problematica. Scleractinians did not
appear at the onset but appear later in the Middle Triassic, some 8 million years after the end-Permian.
Corals functioned as dwellers in these early reef-like mounds and also into the subsequent Middle Triassic
where spinctozoid sponges and red algae dominated. Middle Triassic corals were not simple and were high
in diversity, being differentiated into 6-8 superfamilies. Guizhou province, south China, shows four
superfamilies as well as six families displaying highly integrated corallites (QI & STANLEY 1989).
The idea that Scleractinia evolved from surviving Paleozoic Rugosa is not supported by the evidence
(OLIVER 1996). Rare sporadic occurrences of Permian and Ordovician aragonitic scleractinian-like
scleractiniamorphs (SCRUTTON 1999) provided new insight into the problem. Unlike Paleozoic Rugosa,
scleractiniamorphs has 6-fold septal insertion and favored aragonite biomineralization. EZAKI (1998)
viewed scleractiniamorphs as Paleozoic progenitors of modern corals, including them in the clade
Scleractinia. Scleractiniamorphs as Paleozoic “Scleractinia” make sense as molecular clocks pinpoint a
scleractinian origin deep in the Paleozoic (ROMANO & PALUMBI 1996). A possible polyphyletic Scleractinian
tree, also resolved, complicates the issue of stem and crown groups, especially if clades of “Scleractinia”
evolved more than once from different “naked” ancestors. STANLEY (2003) viewed biocalcification
responses to seawater chemistry between Early and Middle Triassic time, as triggers for sudden
appearances of corals during the recovery. The “naked coral” hypothesis (STANLEY 2003) helped answer
some aspects of the delayed recovery. The Early Triassic was a time of anoxia, injection of greenhouse
gases, high pCO2 and possible introduction of hydrated sulfuric acid. Such geochemical conditions would
lowered pH and adversely affected calcifying organisms, shutting down most of the carbonate factories
and, of course, strongly discouraging mineralization of skeletons.
Results from aquaria experiments assisted understanding survival and recovery. FINE & TCHERNOV
(2007) tested survival of living coral in marine aquaria where pH was elevated to simulate a much higher
CO2 world. After one month, all corals in the lower pH aquaria experienced complete skeletal dissolution
but continued to live as soft polyps. Even after 12 months in such low pH water, the polyps continued to
metabolize and reproduce without protective skeletons. After a year, pH was changed back to normal and,