still other mass extinctions

[John Alroy <jack@homebrew.uchicago.edu>]

        I was just asked by a member of the dino list (who may want  
to remain anonymous) what the "top folks" are saying about the  
Ordovician and Devonian extinctions. As you guys probably already  
know, the end-Pleistocene large-mammal extinction doesn't even rank  
as a mass extinction at all as far as the "top folks" in marine paleo  
are concerned, because basically nothing happened in the marine realm  
at that time (yet another reason to doubt climate-driven models for  
the the mammal extinctions). Instead, I think there's a consensus  
that about five or six extinctions during the Phanerozoic were true  
"mass extinctions" - end Ordovician, late Devonian, end Permian, end  
Triassic, end Cretaceous, maybe a couple of others. The K-T is  
clearly the best understood of any of these. Being a (relatively)  
humble mammal paleontologist and not one of said "top folks," I'm not  
prepared to speculate about any of the others.
        What I've got on hand is a nice 1987 coffee table book by  
Steve Stanley ("Extinction"), which is unfortunately quite out of  
date; "Extinctions: Bad Genes or Bad Luck?," by Dave Raup (1991:  
okay, so he's my advisor), which is written in an informal but very  
direct style with a minimum of jargon (it's informally known around  
here as "Bad Genes or Bad Vibes?"); a recent book on the  
Permo-Triassic event by Doug Erwin (1993: "The Great Paleozoic  
Crisis: Life and Death in the Permian"), which I haven't had a chance  
to look at yet but seems promising; a symposium volume edited by  
Stephen K. Donovan (1989: "Mass Extinctions: Processes and Evidence")  
that seems like a good review of the marine events but has an article  
on Pleistocene mammals that I think is sincere but deeply flawed (no  
point in arguing about it now); and another symposium volume called  
"Phanerozoic Diversity Patterns" (1985: James W. Valentine, ed.) that  
again is out of date but has some really classic articles with a more  
theoretical bent.
        Then there's always the technical literature, for people who  
really want to get into this. The best place to look is naturally  
Paleobiology, which specializes in this kind of thing, but there is  
plenty of other literature on mass extinctions in journals like  
Palae3, Lethaia, Palaeontology, Journal of Paleo, Palaios, etc., plus  
the more geological literature like Geology, Journal of Geology,  
Journal of Geophysical Research, etc., where you occasionally see  
something that bears directly on one mass extinction event or  
another.
        In response to the original question, P. Brenchley (in  
Donovan) thinks the Ordovician event (or events) was caused by  
glaciation and attendant climate/sea level changes. G. McGhee (again  
in Donovan) thinks the late Devonian event was caused proximally by  
cooling and anoxia in the oceans, but what was responsible for those  
changes (tectonic events, glaciation, bolide impacts, sea level  
changes) isn't clear. If someone wants to dig up more on this, I'd  
love to hear about it.
        My anonymous correspondent also made a point about the K-T  
event being size-selective - large animals having more trouble than  
small animals. I think we should remember that 1) from a global point  
of view, the K-T event is most important for its effect on marine  
organisms like forams, nannoplankton, snails, clams, cephalopods,  
echinoids, crinoids, and decapods, and I know of no work showing a  
size bias for extinction in these groups; 2) in the terrestrial  
realm, insects and angiosperms are and were the dominant groups, so  
what happens to terrestrial vertebrates is largely a side-issue; 3)  
even among terrestrial verts, there WERE demonstrable extinctions  
among mammals (all of which were small), turtles, squamates  
("lizards" and snakes), etc., not just dinosaurs; and 4) it's a  
really straightforward prediction of ecological theory that any time  
the world goes to hell (whatever the cause), large animals high up in  
the trophic pyramid are going to take a harder hit. Large animals  
have smaller absolute population sizes, slower rates of intrinsic  
growth, and greater dependence on the fates of other organisms  
because they tend to be high up on the food web. Some of the  
dinosaurs were "small" as large vertebrates go, but NONE of them  
(with the exception of the birds, which is a pretty significant  
exception) were as small as "true" small mammals, i.e., a couple of  
kilos or less. The vast majority of other tetrapods (frogs,  
salamanders, squamates, turtles, etc.) are also in this size range.