Fossil lessons

Mike Risk riskmj at
Thu Sep 20 09:44:22 EDT 2001

Hi Rick (-list).

It's hard to concentrate on academic debates with the world in disarray,
my office in cardboard boxes, my wife in recovery and my department in
ruins. But I will stop whining.

Yes, I could not agree more-the fossil record has a great deal to say
about survival and extinction.

We hear a lot about how "resilient" corals are. They aren't.

In general, Phyla are extremely robust. Now that Paleo has done the
sensible thing and folded the Archeocyatha into the Porifera, we can
observe that no phylum extant in the Cambrian has ever died out. So the
trunks of the trees remain, while branches come and go.

Corals have contributed to reefs in varying proportions, from the
Ordovician on-but how many Rugosa and Tabulata have you seen on reefs? The
real survivors among the Coelenterata are the gorgonians, virtually
unchanged since the Ordovician. Along with nereid polychaetes. Perhaps the
largest barrier reef in the history of the planet (Guadalupian, W. Texas)
is virtually devoid of corals.

Most of our view that corals are robust and omnipresent stems from our
experience with Cenozoic reefs, which are well-exposed and preserved in
many classical outcrops. Cenozoic reefs experienced three major extinction
events: Eocene/Oligocene, Oligocene/Miocene, and Plio/Pleistocene. (See
work by Stan Frost, Ann Budd, etc.) The Plio/Pleistocene event was a
freeze-out, and not very relevant to what looms. Examination of the
Oligo/Mio event, however, is illuminating.

This extinction event was likely caused by a shelf-edge upwelling,
bringing in conditions of turbid water and high nutrients. These are the
conditions that reefs face now-and I point out that grazing in the
Oligocene was unaffected by people. Not even Alley Oop.

Half the corals in the Caribbean died (Edinger and Risk, 1994: PALAIOS 9:
576-598). Some other bad news: bioeroders, primarily filter-feeders,
sailed through unchanged: so the balance was severely upset. (I have to
point out here that any reef "model" that ignores bioerosion is dealing
with less than 50% of the carbonate balance, and hence deserves less than
50% of our confidence.) I suggest that what we are seeing now precisely
parallels what the record tells us: massive regional extinctions, shifting
of the carbonate balance equation...This event remade the Caribbean coral
fauna, reducing it to a fraction of previous biodiversity levels. Although
Indo-Pacific representatives escaped the Caribbean event, they have yet to
recolonise the Caribbean.

So I suggest that the fossil record allows us to estimate recovery times
of reef coral faunas: between 1,000 years (Adey) to >25 million years. You
and I won't see it!

Another view from SE Asia: Edinger et al., 2000: Diversity and
Distributions 6: 113-127: " pollution was the primary
determinant of coral species diversiity and species occufrrence on reefs."

I continue to be pessimistic. I feel that present fixation of the
biological research community is at least partly driven by a reluctance to
deal with the real problems: coastal development associated with
population increases.


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