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RE: Burrowing/hibernating mammals have lower extinction risk

> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] 
> On Behalf Of Jerry D. Harris
> >Robertson et al. (2004) highlight a burrowing habit as a likely 
> >survival
> trait in the first hours of the Cenozoic, while Smith & Botha 
> (2005) suggest burrowing therapsids (as organisms adapted to 
> dealing with periods of dysoxia and hypercapnia) may have had 
> a selective advantage over non-burrowers at the P/Tr event.
>      I must say I've never really understood this concept all 
> that well.  I can certainly see that burrowing would be 
> advantageous during the first few moments of a major 
> catastrophic event, like a bolide impact -- it provides a 
> nice shield from things like shock waves, heat pulses, etc. 
> that non-burrowers don't enjoy.  But these are not the parts 
> of catastrophic events that induce extinctions -- it's the 
> longer-term effects, like rapid climate shifts, food web 
> collapses, etc. that have those effects, and unless there are 
> burrowing vertebrates that never come to the surface and have 
> an entirely underground food web, I don't see how they're 
> rendered immune from these things.  For example, they still 
> have to come up to the surface to find food -- presumably 
> non-burrowing food -- and if that food source is severely 
> impacted by the longer-term effects of the event, then the 
> burrower is going to have just as hard a time subsisting as 
> anything else.  
>      And, of course, burrowing is DISadvantageous in many 
> aspects of catastrophic events, such as lahars, tsunamis, and 
> even massive fires (as exemplified by the 85% mortality rate 
> among burrowing desert tortoises in the large tortoise 
> preserve outside St. George during massive lightning-induced 
> fires in 2005).

In the case of Robertson et al., they consider a major driving factor in the
initial extinction event to be a thermal pulse experienced worldwide from
reentering ejecta. The argument here is that it is not primarily forest
fires and so forth, but a thermal load (calculated to be something like 8
times the normal IR felt at noon at the equator) experienced globally,
lasting for minutes to hours. Marine organisms and aquatic organisms are
protected from this by the water (the IR goes into vaporizing a few microns
depth of water), and the atmosphere itself only raises by about 10 C
degrees. Opaque terrestrial matter, however, takes the full load. Their
calculations are that anything at depths greater than 10 cm under the soil,
or under rocks and other solid objects, would be protected. This is
independent of the later, longer term effects (impact winter and food chain
collapse, impact greenhouse summer, long time to get the deep ocean warmed
again, etc.), but would represent the initial sorting factor for the
terrestrial realm.

As for the Permo-Triassic extinctions: some models put dysoxia and
hypercapnia on both land and sea as a major environmental factor as a
byproduct of the various triggering phenonema. In such a world, organisms
that had evolved physiologies capable of dealing with these conditions for
some other reason might preferentially survive. Research by Bambach &
colleagues show that ability to deal with hypercapnia is strongly correlated
with relative rates of survivorship among marine inverts at the P/Tr, which
lead Ward, Smith, and colleagues to examine if there is a correlation with
the terrestrial extinctions. That said, again this would represent only one
of several detrimental phenomena at the time.

Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216                        
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
Fax: 301-314-9661               

Faculty Director, Earth, Life & Time Program, College Park Scholars
Fax: 301-405-0796

Mailing Address:        Thomas R. Holtz, Jr.
                        Department of Geology
                        Building 237, Room 1117
                        University of Maryland
                        College Park, MD 20742 USA