GEOL 104 Dinosaurs: A Natural History
Fall Semester 2005
The FORMAL Definition of Dinosauria
In class I use my preferred phylogenetic taxonomic definition for Dinosauria, namely:
Dinosauria = all descendants of the concestor (most recent common ancestor) of Iguanodon and
Megalosaurus.
I prefer this definition for a number of related reasons:
- It employs as the reference (anchor) taxa two of Owen's original three members of
Dinosauria (poor Hylaeosaurus is left out…)
- It uses taxa that have been universally regarded as dinosaurs since 1842
- It retains English genera as the anchor forms, honoring the birthplace of the concept
of “Dinosauria”
However, this is NOT the formal definition of Dinosauria in the scientific literature. In
fact, that definition is:
Dinosauria = all descendants of the concestor of Triceratops and
modern birds (Aves of Gauthier; Neornithes of other authors).
Okay, the taxa included by either definition remain the same: both definitions encompass
the same part of the Tree of Life.
However, I dislike this definition for a couple of reasons:
- Triceratops was unknown during the lifetime of Owen and the other founders of
the "dinosaur" concept (Mantell, Buckland, etc.)
- Birds have not been central to the historical understanding of the concept of
"Dinosauria"
- Furthermore, using a definition without birds as an anchor taxon more accurately
reflects the accomplishments of Huxley, Ostrom, Gauthier, etc.: we recognize birds as
part of Dinosauria because of discovery rather than because of definition
Nevertheless, because of the principle of priority, the “Triceratops + birds”
definition is the proper one.
So, students, please forgive me this little deception. For class purposes use the
"Iguanodon + Megalosaurus" definition of Dinosauria, but (should you delve
into the depths of recent dinosaurian paleontology) understand that the "Triceratops
+ birds" definition is the proper form.
For more information, read:
Padian, K. 1997. Dinosauria: Definition, pp. 175-179 in P.J. Currie & K. Padian (eds.),
Encyclopedia of Dinosaurs (Academic Press).
Incidentally, the formal definition of Saurischia is:
Saurischia = Modern birds and all taxa closer to modern birds than to Triceratops.
(Gauthier 1986)
And for Ornithischia is:
Ornithischia = Triceratops and all taxa closer to Triceratops than to modern
birds. (Padian and May 1993)
Main Syllabus for GEOL 104
Lecture 12 Notes page for GEOL 104
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Last modified: 22 August 2005
