Re: Did non-avian dinosaurs guard nests?

[John Bois <jbois@umd5.umd.edu>]

On Tue, 21 Oct 1997, Thomas R. Holtz, Jr. wrote:

> This is a great case for the extant phylogenetic bracket.  A behavior is
> shared by both extant brackets (forms with very different ecologies and body
> plans) and is rare among outgroups.  There is a strong case that nest
> attendance is a behavioral synapomorphy of Archosauria, and suggestion that
> a particular extant or extinct archosaur did not practice said behavior
> would require additional evidence.

Thank you.  That is the answer I had hoped for.  But the following devil's
advocate argument occurred to me: each group, crocodilians and avians,
could have different selective pressures resulting in nest attendance.
How can we know that this isn't a convergent trait.  Birds tend nests
because their eggs are in the open air (and being able to lay eggs in air
gives them a huge selective advantage in that they have great nest site
choice!).  Crocodiles tend nests to protect them from predation.  Perhaps
predators, because they can concentrate their search to the immediate
river vicinity, have a higher success rate in finding croc eggs than they
might finding dinosaur eggs oviposited in some more remote
sandy area.  Having less predation pressure it might pay them to leave the
nest altogether to avoid its discovery.  In other words, if we don't know
what the selective pressure is, maybe we have a problem with this
technique.  
        On the other hand, if I knew that the selective pressure was the
same--eg., the need to provide long-term temp, humidity, and oxygen at
optimum--for both brackets then I would have better support.
        Also, nest guarding isn't that rare for snakes (an out group).

        I hope I haven't talked you out of it!