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

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.

> Having giant crocs, pliosaurs and mosasaurs in the oceans and waterways 
> probably didn't help
> much either. Large theropods would have had a hard time competing with them 
> in deep water -
> especially if they were stuck bobbing like corks at the surface and making 
> appetising targets of
> themselves. However spinosaurs probably had the 'mega-wader' niche largely to 
> themselves.

Hesperornitheans were nevertheless quite common in Cretaceous
waterways.  Unsurprisingly, hesperornitheans did fall prey to aquatic
reptiles, based on the stomach contents of some mosasaur specimens.