Re: What did Spinosaurus eat? New species of Lepidotes found

[Dann Pigdon <dannj@alphalink.com.au>]

On Tue, Feb 22nd, 2011 at 5:00 PM, Tim Williams <tijawi@gmail.com> wrote:

> Dann Pigdon <dannj@alphalink.com.au> wrote:
> 
> 
> > I imagine that non-avian theropods would have had similar problems where 
> > diving was 
concerned,
> > with their avian-style respiratory systems. Perhaps large theropods were 
> > prevented from
> becoming
> > habitual divers because of their inherant bouyancy.
> 
> Apparently _Hesperornis_ overcame its inherent buoyancy by
> pachyostosis (in this case, developing thicker bone walls).  Same for
> the putative loon _Polarornis_.  For example, in _Polarornis_ the
> average bone wall thickness is 37% of its diameter, which is even
> higher than the emperor penguin (_Aptenodytes forsteri_).  The
> red-throated loon (_Gavia stellata_) has a relative bone wall
> thickness of only 15%.

> So the natural buoyancy of non-avian theropods could similarly have
> been overcome by making bones heavier (such as by being
> thicker-walled).  Pachyostosis (including osteosclerosis) is a
> strategy adopted by many tetrapods that returned to water.

Given that the vast majority of non-avian theropods were dedicated cursors 
(even at large sizes), 
would the thickening of bones effect terrestrial locomotion? Would it make the 
bones more liable to 
breakage, for instance, or would it significantly slow the animal down? Most 
aquatic avians don't do 
much in the way of running (a brisk waddle is usually their limit).

Birds also had the initial advantage of two separate locomotory systems. Some 
aquatic birds use 
their forelimbs for propulsion, while a few primarily use their hindlimbs. 
Early aquatic birds could 
have coopted one means of locomotion for aquatic propulsion, while still having 
another to fall back 
on when out of the water. Many non-avian theropods didn't have this luxury - 
and even those 
with 'wings' probably weren't using them as a primary means of locomotion. 
Perhaps the transition 
to a largely aquatic lifestyle in theropods required well developed powered 
flight first (even if it was 
later lost)?

-- 
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Dann Pigdon
Spatial Data Analyst               Australian Dinosaurs
Melbourne, Australia               http://home.alphalink.com.au/~dannj
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