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No aquatic dinos? (was Re: No dinosaurs in Quebec, Canada?)

Stan Friesen wrote (quoting Gautam Majumdar):

>  > Is there any generally agreed explanation why over their 160 MY reign
>  > dinosaurs did not venture to exploit the aquatic niches ?
> The crocodiles got in the way? :-)

One of the reasons why there are no (known) dinosaurian equivalents
of otters or any other aquatic mammals is that the mammals themselves
may have filled these niches in the Mesozoic.

The description of _Kollikodon_, a beaver-sized monotreme from the
Early Cretaceous of eastern Australia, suggested that it may have
been an aquatic mammal; the design of its teeth suggest that it fed
on shellfish.  The evidence is not compelling, I know, but how can we
be sure that numerous other Mesozoic mammals known from teeth and
jaws weren't aquatic (or even marine) with a lifestyle similar to the
modern otter.

Tim Williams