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Re: Nocturnal crocs?



On Thu, 23 Aug 2001 18:34:14  
 John Bois wrote:
>As many good people of this list may know, I am investigating the
>possibility that relaxation on mammal size constraints toward the very end
>of the Cretaceous, brought them over the dinosaur nest/juvenile predation
>threshold.  One of the many criticisms leveled at this hypothesis is the
>following: if mammals were such a problem for non-concealing dinos, why
>didn't they take out non-concealing crocs as well?  A prediction for this
>hypothesis is that there should be some critical  difference between these
>archosauran clades.  It seems likely that most dinos were diurnal
>(see HP Rowe's paper at _Palaeontologia Electronica_ Vol 3, Issue 1).  It
>seems likely that most/many mammals were nocturnal.  Small jackals today
>cannot approach ostrich nests in the day time but have no trouble at
>night.  But what about crocs.  A prediction of this hypothesis would be
>that most/all crocs making it through the Cretaceous would be nocturnal
>(i.e., they survived because they could defend their nests at
>night).  Paul Willis seemed to feel that no Cenozoic crocs possessed
>sclerotic rings (a structure linked by Mickey and others to the diurnal
>habit).  I searched for
>about three days and could find none.  But I did find this from Underwood
>writing in _Biology of the Reptilia Vol 2.  "It may be significant that
>the only (crocodilians) that survived the Cretaceous were nocturnal."  So,
>I just wanted to see if this agreed with what this esteemed body
>knows; and, if so, whether it would be considered a nice piece of
>supporting evidence.

Maybe.  I don't really know, as I don't have nearly enough expertise in the 
area of crocodiles to begin to answer your question.  However, in addition to 
sclerotic rings, maybe you should look at orbit size, and compare with extant 
crocs and other reptiles.  Many types of animals do possess sclerotic rings, 
and I'm not convinced that they're indicative of a nocturnal animal.  For 
instance, some marine vertebrates possess them.  

I do believe that somebody is working on orbit size in dinosaurs, and trying to 
determine whether or not some species may have been nocturnal.  To my 
knowledge, though, no such study has been conducted with crocodiles.  

>From what you say, though, John, I think it is a possibility.  It is certainly 
>something interesting to consider.  I would talk to Chris Brochu, though, 
>because my expertise is nowhere near the level needed to answer such a 
>question.  

Steve

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