Re: attack on dinosaur--horrific video

[Michael Habib <mhabib5@jhmi.edu>]

> Birds--and probably non-avian dinosaurs--are/were limited re night 
> vision by their eye structure--sclerotic rings restrict light 
> gathering--owls, for example, must have huge eyes to enhance night 
> vision--crocs lack sclerotic rings, as do we.

I'm not sure this is correct; do you have a citation?  Owls, granted, 
have huge eyes and the overall shape maximizes sensitivity.  However, 
owls have 1) much greater sensitivity than many other nocturnal animals 
(so the fact that they have larger eyes than crocs, proportionately, 
does not mean that crocs have an advantage) and 2) owls also cannot 
sacrifice too much acuity (they do still sacrifice acuity to a 
substantial degree, but have limits).  Because acuity and sensitivity 
have conflicting shape requirements, the eyes of animals that maximize 
one parameter must generally have huge eyes if they do not minimize the 
other.

> I hate to use such fait accomli arguments such as: placentals are the 
> dominant form of mammal; their characteristics may have had something 
> to do with this.

Indeed; and so one might suspect that reproductive mode has made a 
major difference >within< mammals.  Arguing across major clades with 
other differences, however, is more difficult.  In fact, what is really 
striking about modern placentals (comparing them to their Mesozoic 
ancestors, as well as other living clades) is their major presence as 
medium to large-bodied, terrestrial forms.  Given that dinosaurs fell 
into the same category in the Mesozoic, we might expect that key to 
being viable in that morphotype lies in the similarities between 
placentals and dinosaurs, rather than the differences.  In any case, 
you and I have discussed this particular issue at length in the past.  
We don't seem to agree, but I do find the life history discussion 
lively and fun.

One point I would like to toss in, which I don't think has come up thus 
far, is that the nest predation lines are really rather blurred.  Many 
mammals have rather altricial young with high mortality (often confined 
to burrows or dens), and quite a few squamates utilize viviparous life 
histories.  No archosaurs do this, of course, but many birds have quite 
precocial offspring, while others have very short incubation periods 
(such that their offspring probably have similar predation rates to 
altricial mammal dens).  To really make the point that there is a major 
difference in predation-based mortality for placental mammals, we need 
some data for these various life history strategies.  Put simply, we 
need to first demonstrate with some confidence that being a viviparous 
mammal vastly increases overall juvenile production (after mortality) 
over oviparity.  I think you make a strong case that nest protection is 
very important.  Certainly, we would expect that oviparous species that 
do not protect nests effectively will have lower reproductive success 
than those that do.  That's very different from arguing cause for a 
mass extinction event, however.

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