Re: The Biggest Dinosaurs

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

At 02:44 PM 8/20/97 -0400, you wrote:

>> 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.
>
>Are there any best-guesses as to why this mean size increase
>occurred?

Standard explanation: competion between predator & prey or herbivore &
herbivore favored increased size.  Alternatives include: changes in climate
(more continental climate due to draining seaways) favored bigger forms.

>Was there a similar size increase among smaller dinosaurs of
>the same time?

Later hypsilophodonts/thescelosaurs are bigger than earlier forms.  The
sample size of Maastricthian dromaeosaurids and troodontids is not yet large
enough to make any conclusion as to whether Judithian forms were bigger,
smaller, or the same size as Lancian forms.

>Thanks in advance.
>John Bois. 
>
>

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