Re: The Biggest Dinosaurs

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 02:02 PM 8/20/97 -0400, you wrote:
>I checked the archives and found some messages about size extremes in 
>dinosaurs, but none that address my question....which is, does anyone 
>have any thoughts about why dinosaurs in Patagonia were growing so large 
>during the Cretaceous, while it seems that the trend was the opposite 
>elsewhere in the world?  I just read a popular article called "Land of 
>the Giants" by Don Lessem (Aug. 95) and am very interested in learning 
>more about this.  Thank you very much.

I don't think that the evidence really supports the idea that dinosaurs
*were* getting smaller in the rest of the world.  Very large sauropods were
present throughout much of the Cretaceous (albeit known only from remains
more fragmentary than those of the Morrison Formation) in much of the world

The disappearence of the sauropods in western North America at the beginning
of the Late Cretaceous seems to have caused a drop in the mean size of
dinosaurs of that region.  However, within most of the lineages which
survived there is no clear pattern of size change positive or negative.  The
size of dromaeosaurids seems to shrink, but most everything else stays the
same (at least until the late Campanian) or grow (in the case of
iguanodontians, generally getting bigger the closer to Hadrosauridae you get).

In fact, during the last eleven million years or so of the Cretaceous of
western North America, there seems to be a net size increase in most of the
dinosaur groups.  The latest Maastrichtian tyrannosaurids, ceratopsians,
pachycephalosaurians, "hypsilophodontians", ankylosaurids, and nodosaurids
are the biggest known for each of those lineages.  Furthermore, the average
hadrosaurids of the latest Maastricthian are pretty big, too, but the
largest known are from either the early Campanian of Baja California or the
early Maastrichtian of China.

Still, the fact that the Rio Limay Formation has produced such giant giants
is an interesting situation, ecologically speaking.

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