[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: attack on dinosaur--horrific video
Yes, and hen turkeys (in the wild), when disturbed on their nests, are known to
destroy their own eggs by pecking them. I mention this total non-relevance
because I had an idea, but it disappeared as I hit the "reply" button.
Ah, now I remember. Not that it would provide proof of nest predation by
mammals, but are there dinosaur eggs w/ tooth marks that can be identified as
mammalian? Is there a smoking gun re dinosaur egg biting/gnawing by mammals?
And the distantly related question also appears in the fog; personal
observation and a wealth of lit. indicates the characteristic chisel marks of
rodent-gnawing are common on bone as old as Late Pleistocene -- does anyone out
have a ref for what the oldest rodent-gnawed (ie, chisel-tooth marks) fossil
bone might be?
GOOG isn't helping much this time. Stressing again my agnosticism -- Don
----- Original Message ---From: Michael Habib <firstname.lastname@example.org>
Sent: Friday, November 23, 2007 7:18:42 PM
Subject: Re: attack on dinosaur--horrific video
> 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
> 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.
Michael Habib, M.S.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181