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Re: definition of dinosaur



>I believe I read somewhere (I think it was here) that we should define 
>dinosaurs as all the descendants of the most recent common ancestor of 
>_Megalosaurus_ and _Iguanodon_.

A brief history.

Gauthier (1986) formally  defined "Saurischia" as birds and all taxa closer
to birds than to Ornithischia.

Padian and May (1993) formally defined "Ornithischia" as Triceratops and and
all taxa closer to Triceratops than to Saurischia.

At the 1994 meeting of the SVP, while talking to Kevin Padian, I mentioned
the fact that I regreted the above authors not using _Megalosaurus_ and
_Iguanodon_ instead of birds and _Triceratops_.  This would have preserved
two of the original three (the last is _Hylaeosaurus_) of Owen's 1842 taxon
"Dinosauria".  It would also reflect an important historical point:  birds
are NOT central to our original understanding of dinosaurs, and are
considered dinosaurs only because they fall within that monophyletic clade.

Padian agreed it would have been a better idea, but it is too late for
that.  I use _Meg_ and _Ig_ as the "type" dinosaur taxa in my class
informally.

>I can provide that definition to my students (but I will have to follow 
>up with an explanation as to how they can recognize a dinosaur when they 
>see one).  I predict that these questions will be asked about the 
>cladistic definition:
>
>Why are those two genera singled out?  Isn't it true that after all these 
>years we still do not have even one complete composite skeleton of 
>_Megalosaurus_?  Why not choose _Allosaurus_, instead?

These taxa were singled out because they were two thirds of the original
concept of Dinosauria.  _Megalosaurus_ is more complete than most think
(work by Freidrich von Heune and grad student Laura Canning have shown/are
showing that), and although not as complete as _Allosaurus_, is clearly a
theropod and not some non-dinosaurian creature.

>Do we really know when ornithischians branched off (separated?, 
>whatever--what would the correct cladistic term be?) from saurischians to 
>know that we're not leaving behind something like _Eoraptor_ or a 
>herrerasaurian?  Let me rephrase that so no one here will jump on me for 
>misunderstanding _Eoraptor_ and herrerasaurs, which has nothing to do 
>with the point.  Might we be leaving something behind that many people 
>would label as a dinosaur based upon its over-all anatomy (hips, sacrum, 
>limbs, feet, shoulder, neck, etc.)?

As for the last: yes, yes, yes.  We (i.e., phylogenetic taxonomists) WANT to
leave behind grade-based thinking of taxa.  Taxa in this system are NOT
defined by characteristics; they are soley defined by ancestry.  An animal
is a dinosaur if and only if it descends from the common ancestor of the two
taxa picked.

Or, to go to the paper by Holtz and Padian at last year's SVP meeting,
_Eoraptor_ and Herrerasauridae are dinosaurs only if they are shown (by the
most parsimonious distribution of derived characters) to fall within the
clade joining Ornithischia and Saurischia (or _Triceratops_ and birds, or
_Iguanodon_ and _Megalosaurus_ informally).  In some of our cladograms, they
did; in others, they did not.  Kevin presented the most divergent cladogram
(i.e., the one in which both _Eoraptor_ and Herrerasauridae are excluded
from Dinosauria) to illustrate the extreme condition.

Of course, very close sister groups will be morphologically very similar to
the most primitive members of a clade (because of descent with modification,
and all that stuff).

>What is the closest thing we have discovered to that most recent common 
>ancestor of _Megalosaurus_ and _Iguanodon_?

_Eoraptor_.  Whether it is within the clade or just outside of it, it
clearly represents the closest thing we know to a common dinosaurian ancestor.

(Incidentally, this aspect of the Holtz and Padian talk at SVP seems to have
gotten lost in all the discussions of the symposium (as has the fact that
there WAS a second author to the paper, who presented it...)

Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661
Phone:301-405-4084