[Coral-List] Have massive scleractinians been getting more hardy over geologic time?

Charles Birkeland charlesb at hawaii.edu
Thu Dec 17 18:43:05 EST 2015

Bambach et al. (2004 Paleobiology 30: 522-542) found the overall survival
of marine genera for both the Triassic/Jurassic and the
Cretaceous/Paleogene (K-Pg) extinctions to be about 53%. The scleractinian
genera did much worse than marine life in general in the Triassic/Jurassic
extinction with about 29%, i.e., 20 out of 69 genera surviving (Lathuilière
and Marchal 2009 Terra Nova 21: 57-66).   In contrast, according to
Keissling and Baron-Szabo (2004, Palaeogeography, Palaeoclimatology,
Palaeoecology 214: 195-223) scleractinians did much better than marine life
in general in the K – Pg  with 70% scleractinian genera and over half the
species surviving.  In fact, according to Wells (1956 In: Moore (ed)
Treatise on Invert paleontology, Part F, Coelenterata. Geol Soc America)
and Ma (1959) Oceanographica Sinica Special Vol 1) about two dozen of the
important genera of massive or encrusting corals today were in the
Cretaceous and have survived the K-Pg, the Paleocene-Eocene Thermal Maximum
and turnovers of species in the western Atlantic. These genera must be from
hardy lineages.

Bambach et al. believe that a part of the problem at the T/J was little
speciation while the K-Pg was a full-on mass extinction. There were major
extinctions of many other calcareous organisms in the K-Pg such as
planktonic foraminifers and coccolithophores and complete extinctions of
some calcareous molluscs such as rudists and ammonites. This would make the
relative hardiness of the more recent massive Cretaceous corals even

Most of the coral genera surviving the T/J mass extinction went extinct by
the end of the Early Jurassic when conditions became very favorable for
reef-building and 100 new genera appeared (Wood 1999 Reef evolution.
Oxford).  Once again, the “Lazarus” corals (genera coming back to life
after the K-Pg extinction) did better. Although some did go extinct when
the environment became good for reef-building and new genera originated in
the late Oligocene – early Miocene, a couple-dozen genera of Lazarus corals
are still important today.

All this does not seem to apply to fast-growing erect corals which are
mostly wiped out with each mass extinction. New genera of fast-growing
corals originate after mass extinctions and become prevalent when the
environment gets good for reef-building.   For example, in the acroporids,
the encrusting or somewhat massive *Astreopora* survived the K-Pg mass
extinction and is still with us, but the branching genera have originated
in the more recent Paleogene or Neogene.

So the fossil record suggests that some lineages of the massive and
encrusting genera might be getting more resistant through time, but the
fast-growing genera have their 15 minutes (OK, 10s of millions of years) of
glory when environmental conditions are good, but may be terminated when
things get tough. Perhaps growing fast has tradeoffs with resistance to
stress. During the good times of the Neogene, it may have been so rewarding
to be a fast grower that natural selection disregarded insurance for future
climate changes as it seems to have done in previous periods.

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