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Re: My Gr. 9 Science Fair Project: Questions



Stephen Throop wrote:

>That brings up a question: how much did an edmontosaur weigh?  L B
>Halstead's reference book showed it at nine meters and implied a weight
>of three tons.  Lately I've been hearing it was 13 meters long.  I don't
>know how the length of a fossil could change that much.  If it was so
>much bigger than Halstead said, did it weigh nine tons instead of three?
>
>I wasn't wondering if a croc would eat an edmontosaur.  I was wondering
>if a croc would hold his ground when he heard an animal of unknown size
>thundering toward him.  I think even grizzly bears often retreat in the
>face of threatening noise.  I think the footsteps of a large running
>biped could hit a croc hard enough to kill it.

Although Deinosuchus material is incomplete and size estimates are hairy at
best, they were certainly larger and heavier than any hadrosaur around
them. In similar situations today, big croc do not get out of the way of
smaller herbivores.

>I speculate that duckbills were different from zebra, wildebeast, and
>buffalo.  I've read that, in their time, duckbills accounted for 75% of
>dinosaur herbivores.  They must have been doing something right.

Are you assuming that duckbills were occur is large numbers because they
had a system for avoiding being eaten by crocodiles? If so, you need to
seriously consider the plethora of other factors that could be at play in
the differential sucess of hadrosaurs as indicated by thier occurance in
the fossil record. It is far more likely that they were out breeding their
competitors or that, because they moved in hers, they are disproportionatly
represented in fossil deposits or that their favoured habitats were closer
to areas where fossilisation was likely to occur or that their breeding
season more closely tied to environmental cycles that would favour
fossilisation or that their are hydrodynamic factors that favour the
preservation of hadrosaurs over other herbivores etc, etc, etc,.
Interpreting apparent competative advantage based on relative occurances of
species in the fossil record is a very tricky business.

>Don't humans regularly go into the waters of the Nile, the Everglades,
>and the Louisiana Bayous with very little chance of being eaten?  Aren't
>there animals that frequent crocodile waters but have little chance of
>being eaten?  How about panthers and manatees?  Are these waters
>inhabited only be crocs?

For get humans for a start. We have complex behavioural structures lacking
in other animals that can be used to avoid crocodile attacks (eg, Communal
calls of "Get out of the water you bloody idiot, there are crocs in
there"). Further, in the cases you bring up (the nile, everglades etc) the
habitats have been radically altered in the last 40,000 years or so that
they do not represent a true natural environments (Nile crocs are no longer
seen along most of the length of the Nile River below Karthoum). Yes, there
are other animals that live along crocodiles in water (fish, turtles,
invertebrates etc) and these can form part of the crocs diet. All animals
that come to the waters edge are fair game for the crocs (with one or two
notable excaptions such as hippos). Carnivores such as various cats and
monitor lizards have been seen to be taken by crocs. If these observations
are consistent with croc feeding preferences in the late Mesozoic, there is
no reason to assume that any dinosaur, including hadrosaurs, ceratopsians
and even tyrannosaurs) may have been taken by a large croc such as
Deinosuchus.

>> >Don't crows drive hawks and owls from an area by harassment?  Couldn't
>> >duckbills  also have used commotion and tooting to encourage big crocs
>> >to relocate?
>>
>> Again, this doesn't seem to be the case with many modern prey items of
>> modern crocodiles.
>
>It doesn't seem to be the case with many modern prey items of owls and
>hawks, either.  It's probably why crows aren't favorite prey items.

While it is possible that hadrosaurs, or any other dinosaurs for that
matter, may have had warning systems to alert others of danger and that
these systems could have been used to warn of crocodiles, it will remain
unknown exactly how effective such systems were. Modern animals eaten by
crocs also have communication systems for danger but still they get eaten.
Do not underestimate the stealth of a croc on the hunt. There are numerous
documentaries that show zebras and wildebeast quietly drinking at a river
only to find that there is a croc lying in wait in the middle of them. If
modern mammals have trouble seeing a hidden croc, I fail to see how a
dinosaur would have faired any better.

[ Sorry to interject, but dinosaurs probably had better color vision
  than most mammals and may have been sensitive to optical polarization
  as well.  The latter would be particularly helpful in allowing an
  animal to see under the surface of the water because the polarization
  of reflected light would be different from the polarization of light
  transmitted from under the water's surface.  -- MR ]

Cheers, Paul


Dr Paul M.A. Willis
Consulting Vertebrate Palaeontologist
Quinkana Pty Ltd
pwillis@ozemail.com.au