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Re: attack on dinosaur--horrific video



----- Original Message ----- From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>
Sent: Friday, November 23, 2007 7:18 PM
Subject: Re: attack on dinosaur--horrific video


John Bois said:
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.

What is that quiz show where you can ask a friend to bail you out--a lifeline--I think? I haven't been able to find a ref. that deals precisely with our issues. And I am more than a little confused. But...

http://palaeo-electronica.org/2000_1/retinal/issue1_00.htm
...contains citations for the claim (hypothesis, really) that animals
w/sclerotic rings are generally diurnal; and that secondarily
sclerotic-ringless animals (crocs and mammals) inherited this condition due
to a period of ancestral nocturnal life. The above also claims that
sclerotic rings are reduced in owls--so I put two and two together:
sclerotic rings interfere with light gathering. However, while the owl's rings
may be reduced in proportion to the size of the eye (I'm not sure of this),
they seem to have a different function: in most birds the ring anchors
muscles that allow for rapid
focusing (http://books.google.com/books?id=8R8zpn8w54EC&pg=PA46&lpg=PA46&dq=sclerotic+rings+owls&source=web&ots=gQf0UcVQfv&sig=ZyQao3beQnmm3mjp21ZSXMVurjA#PPA19,M1
for that); but in owls they seem to serve to merely hold the eye in a long
tubular shape (the better to extend focal distance, i.e., obtain a larger
image on the retina). And they don't look that small to me. In any case,
it seems that one can make not entirely unreliabe inferences about diurnal
v. nocturnal lifestyle based on 1. presence/absence of rings; and 2.
structural arrangement of the rings. For our purposes, this evidence--and
more in the first citation--supports dinosaurs as mostly diurnal.


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.

OK...but the ways in which they were different may have had far reaching consequences. The big one, in my view, was that the dinosaurs had to become sedentary one or more times a year. If they were unable to outdistance other dinosaurs, then they would have to defend their nests. This by itself provided a selective advantage for even larger size, and all the ecological effects that came with it.


...we need to first
demonstrate with some confidence that being a viviparous mammal vastly
increases overall juvenile production (after mortality) over oviparity.

Such comparisons are famously hard to come by. They are so dependent upon all other life-history/ecosytem-structure factors. Also, as you know, there many ways to be viviparous, i.e., many baby's exit mother ready to go, but in comparison with other babies (viviparous or not) they are relatively half-baked. Indeed, even the selective pressures _for_ vivparity may be different from one clade to another (example, value for predation protection in lizards, vs., predator protection _plus_ producing a better-baked baby in wilderbeest).


And...my head hurts. I know we go back and forth on these issues...I, too, enjoy the discussions...and I have aired my views too many times as is. My excuse this time is the amazing video of the subject line...it graphically demonstrates the problem of a non-concealed, non-remote nest.