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Leg Bones of Dinos and Non-dinos



In their booklet "101 Questions and Answers about Dinosaurs" Phil Currie 
and Eva Koppelhus answer the question "What is a dinosaur?" by pointing 
out that a critical factor in determining what a dinosaur is involves 
examining the structure of the hind legs. The specific hind leg traits to 
look for are:
1. ball-like joint fitting into hip socket
2. fibula much thinner than the tibia
3. two of the ankle bones have "functionally become part of the lower leg"

Does this mean that pterosaurs and plesiosaurs lack one or more of these 
traits? If so, which ones? Does this also mean that modern birds DO have 
all these traits, which would justify calling them dinosaurs? (In their 
answer to the question, Currie and Koppelhus indicate that yes, birds do 
have these traits.)

----- Amado Narvaez