Ammonoid extinction (was RE: K/T revisited, was: Re: DINOSAUR digest 3382)

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Phil Bigelow
>
> > What about that survive/extinction pattern if we look at the marine
> > biota? Sharks survived, marine reptiles don't.

Well, sea turtles and dyrosaurids did.

> > Ammonites go
> > extinct,
> > squids don't.
>
> Peter Ward (and others) believes that ammonites went extinct because they
> bred/fed at shallow depths.  In contrast, he claims that nautaloids
> survived because they bred/fed in deeper water.  I'm don't know what his
> take is on squids and octopus.
>
> The hypothesis is that the deeper depths were "buffered" from the carnage
> that was occurring in the near-surface marine waters.  This idea is
> supported by the observation that Maastrichtian benthic biota tended to
> fare a little better than did their pelagic counterparts.

An additional factor: Dolf Seilacher has shown that ammonoids were likely more 
planktonivorous than carnivorous. The short-pathway
part of the food chain (sunlight -> phytoplankton -> zooplankton -> 
planktonivores -> upwards) would be much more immediately and
(in the short term) severely affected by a major decrease in sunlight than the 
detrivores and their prey (sunlight -> suface
dwelling organisms -> detritus in the sediment -> detritivores -> critters that 
eat detritivores). So organisms (like nautiloids and
coleoids) that fed on bottom-dwelling, bottom-feeding creatures (or predators 
of those) would have an easier time than those who fed
more directly from the surface.

                Thomas R. Holtz, Jr.
        Senior Lecturer, Vertebrate Paleontology
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
        Mailing Address:
                Building 237, Room 1117
                College Park, MD  20742

http://www.geol.umd.edu/~tholtz/
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