Re: Did non-avian dinosaurs guard nests?

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 10:17 PM 10/22/97 -0400, John Bois wrote:
>
>
>On Tue, 21 Oct 1997, Thomas R. Holtz, Jr. wrote:
>>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?

We can't "know" that it isn't convergent, just as we can't "know" that the
feathers of crows are homologous to feathers in ravens (to pick an extreme
example).  In evolutionary biology, similar derived features are assumed a
priori to be homologuous, and the concordance of distribution of other data
will support or reject an hypothesis of homology.  In this case, given the
known distribution of this trait among living amniotes, it is more simply
explained by a single evolutionary event (at the base of Archosauria) rather
than two (once each for Crocodylia and for Aves) or more often (e.g., once
for gavials, once for crocodylids, once for alligatorids, once for tinamous,
once for ratites, once for neognaths).

>       Also, nest guarding isn't that rare for snakes (an out group).

But it is quite uncommon (if found at all) in the basal members of that
outgroup (i.e., in non-varanoid lizards and in tuataras).

Similarly, humans vocalize at their prenatal young, but as far as we know
this is not a common trait among other mammals.  On the other hand, birds
and crocs commonly vocalize to their pre-hatchlings.  The shared presence of
this trait between birds and crocs is most simply explained as a
synapomorphy; the shared presence of this trait in archosaurs and humans is
most simply explained as convergence.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661