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RE: What exactly IS a dinosaur?



> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] 
> On Behalf Of Brandon Pilcher
> Sent: Sunday, April 13, 2008 10:18 PM
> To: dinosaur@usc.edu
> Subject: What exactly IS a dinosaur?
> 
> 
> Hello all,
>  
> I am currently working on a Powerpoint for a high school 
> project due around the beginning of June. The subject of my 
> presentation will be dinosaurs; it will be a general FAQ on 
> dinosaurs and various debates about them (for example, 
> warm-bloodedness, theropod foraging behavior, relationship to 
> birds, etc.). The first question that I will answer in my 
> presentation will be, "What is a dinosaur?" The problem is, 
> I'm not 100% sure how to answer that question. What 
> distinguishes dinosaurs from, say, pterosaurs, plesiosaurs, 
> or saber-toothed cats? What are some characteristics shared 
> universally among dinosaurs that other animals do not have?
>  
> P.S. Since my presentation in its entirety (which will be 
> between 30 mins and 1 hour, I'm guessing) will have a mostly 
> adolescent audience, I would also appreciate knowing how to 
> communicate this technical sort of information in an 
> easily-grasped manner.

A dinosaur is the most recent common ancestor of Iguanodon and Megalosaurus
or any of its descendants. If an animal is a descendant of that ancestor, it
is a dinosaur; if it isn't, it is not.

Dinosaurs are distinguished from most of the closest relatives by:
* Hindlimbs that were directly underneath the body and feet standing "on
their toes", allowing for a striding gait
* An "open" hip socket (one where the inside was lined only with cartilage,
not with bone)
* More than 2 hip vertebrae
* A grasping hand with a semiopposable thumb and shortened ring and pinky
fingers.

Note that some of these traits were modified in later evolution in each of
the major dinosaur groups.



Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216                        
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
http://www.geol.umd.edu/~tholtz/
Fax: 301-314-9661               

Faculty Director, Earth, Life & Time Program, College Park Scholars
http://www.geol.umd.edu/~jmerck/eltsite/
Fax: 301-405-0796

Mailing Address:        Thomas R. Holtz, Jr.
                        Department of Geology
                        Building 237, Room 1117
                        University of Maryland
                        College Park, MD 20742 USA