Re: attack on dinosaur--horrific video

[Michael Habib <mhabib5@jhmi.edu>]

> My point is that today, at least, this strategy is not 
> viable...otherwise we would, indeed, see it.  But crocs are the _only_ 
> oviparous species that "choose" defense over concealment and remote 
> laying.  Concealment carries selection on _small_ size; whereas remote 
> laying selects for long distance flying.

I'm not certain that we can use absence of modern presence as a test of 
viability in this case.  Historical constraint also can explain the 
observed pattern, which others have noted or hinted at already.  
Essentially, I think you have noted a very interesting pattern, but I 
see no reason to suspect that large-bodied nest defenders are not 
viable in the modern ecology in a "blanket" sense, because the pattern 
can be explained by historical constraint.

Archosaurs met with a major extinction event at the K/T boundary (along 
with all sorts of other critters), which disproportionately affected 
large-bodied, active, terrestrial forms within their clade.  Thus, the 
archosaurian groups that survived happen to be a semi-aquatic clade and 
a smaller-bodied (on average) and volant one.  The nest protection 
strategies we see present in archosaurs today are thus a product of 
over-representation of these two groups: both crocs and birds have been 
selected to utilize their respective "aces" with regards to nest 
defense.  Taxa that do not utilize nests protected by location 
(near-water, elevated, remote location) are probably at a disadvantage 
compared to their relatives.  I think you are absolutely correct that 
nest-protection strategies matter, I'm just not convinced that they 
matter in quite the way that you have argued above (in making whole 
morphotypes/ecotypes unviable)  And, of course, we have to consider 
what being viable means.  Do we mean that the animals would literally 
be eaten into oblivion, or that they would simply have a selective 
disadvantage next to oviparous species with flight and/or cryptic 
approaches on their side?  One could argue that if there are 
disadvantages of unprotected nests, then they are largely enforced by 
the presence of relatives that can fly into the treetops or hide in the 
darkest corners, rather than the specific nest-attacking faunas.

Cheers,

--Mike


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu