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Re: Aquatic dinosaurs

Dalmiro Maia wrote:

>I've seen often stated, I think even in this mailing list, that there
>were no aquatic dinos, yet some exist even today (penguins). To what
>time can we trace the first "aquatic dinos"? are they in anyway related
>to ichtyosaur extinction or loss of diversity? Why do people generally 
>fail to recognize the existence of aquatic dinosaurs? I've just found
>such a claim in one post at sci.antropology.paleo

The ref on sci.anthropology.paleo may be a systematic mistake by the
anthropologists (in that they may think ichthyosaurs, plesiosaurs, etc. are
dinosaurs, which they are most emphatically not!).

Except for pengiuns, auks, hesperornithifomrs, and other aquatic birds, there
is no strong evidence for dinosaurs specialized for marine habits.  It has been
suggested that spinosaurids (such as Baryonyx) were ecologically a cross
between grizzly bears and crocodiles: catching large fish (such as the giant
coelacanth Mawsonia) with both their big manual claws and their croc-like
snouts.  Bakker suggested that Carcharodontosaurus was a fish eater (its
teeth do look like those of great white sharks, hence the name), but the
rest of the anatomy shows no aquatic specializations, and shark teeth cut
through the flesh on land as well as in water.

However, it should be noted that some marine forms (e.g., the marine iguana
of the Galapagos) do not show much in the way of skeletal adaptations for
marine life.

As to why there were no marine nonavian dinosaurs, we could speculate that the
assemblage of ichthyosaurs, plesiosaurs, marine crocs, and mosasaurs held
such a lock on their niches that there was no ecological space for the
evolution of marine dinos.  Unfortunately, the fact that marine crocs and
mosasaurs invaded the sea well into the Mesozoic shows that there was
ecological opportunities for new groups of secondarily aquatic large
carnivores during the age of dinosaurs.

To make a long story short (too late :-), I don't know.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661

"There are some who call me...  Tim."