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Re: What exactly IS a dinosaur?



On Sun, Apr 13, 2008 at 7:17 PM, Brandon Pilcher <trex_kid@hotmail.com> wrote:
>
> 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?

This is probably more information than you need, but....

Here is Owen's original diagnosis, naming, and composition list:

"This group, which includes at least three well-established genera of
Saurians, is characterized by a large sacrum composed of five
ankylosed vertebrà of unusual construction, by the height and breadth
and outward sculpturing of the neural arches of the dorsal vertebrÃ,
by the twofold articulation of the ribs to the vertebrÃ, viz. at the
anterior part of the spine by a head and tubercle, and along the rest
of the trunk by a tubercle attached to the transverse process only; by
broad and sometimes complicated coracoids and long and slender
clavicles, whereby Crocodilian characters of the vertebral column are
combined with a Lacertian type of the pectoral arch; the dental organs
also exhibit the same transitional or annectent characters in a
greater or less degree. The bones of the extremities are of a large
proportional size for Saurians; they are provided with large medullary
cavities and with well-developed and unusual processes, and are
terminated by metacarpal, metatarsal, and phalangeal bones which, with
the exception of the ungual phalanges, more or less resemble those of
the heavy pachydermal mammals, and attest, with the hollow long bones,
the terrestrial habits of the species.

"The combination of such characters, some, as the sacral ones
altogether peculiar among reptiles, others borrowed, as it were, from
groups now distinct from each other, and also manifested by creatures
far surpassing in size the largest of existing reptiles, will, it is
presumed, be deemed sufficient ground for establishing a distinct
tribe, or suborder, of Saurian reptiles, for which I would propose the
name of Dinosauria.

"Of this tribe the principal and best-established genera are the
Megalosaurus, the HylÃosaurus, and the Iguanodon, the gigantic
crocodile lizards of the dry land, the peculiarities of the
osteological structure of which distinguish them as clearly from the
modern terrestrial and amphibious Sauria as the opposite modifications
for an aquatic life characterize the extinct Enaliosauria, or marine
lizards."

Note that Owen did not, strictly speaking, define the name, but just
diagnosed the taxon and provided with a composition list. (Also, he
excluded the sauropod _Cetiosaurus_, which he thought was aquatic.
Other non-avian dinosaurs had been named at the time, but I'm not sure
if he was familiar with them.)

"Dinosauria" was not defined until 1992. You can see a history of
phylogenetic definitions of _Dinosauria_ here:
    http://www.taxonsearch.org/dev/taxon_edit.php?Action=View&tax_id=2

Pretty much everyone agrees that the definition should be node-based,
but there is some discrepancy on the matter of specifiers. The two
basic camps are "_Triceratops_ and _Aves_ (or some avian)" vs.
"_Megalosaurus_ and one or both of the ornithischians included by
Owen". The current draft of the PhyloCode (Rec. 11A) points out that
using an avian as a specifier trivializes the question of whether
birds are dinosaurs, and recommends using specifiers from Owen's
original composition list:
    http://www.ohiou.edu/phylocode/art11.html#rec11a

What's the upshot? Well, you can decide for yourself how to simplify
this for a general audience, but perhaps the best way to present the
definition would be something like, "the last common ancestor of
_Megalosaurus_, _Iguanodon_ and _Hylaeosaurus_, and all descendants of
that ancestor." Pterosaurs, plesiosaurs, mosasaurs, mammoths,
trilobites, etc. are not considered dinosaurs because they are not
thought to be descended from that ancestor. Birds, on the other hand,
are thought by most (nearly all) researchers to be descended from that
ancestor, so they are considered dinosaurs.

The question of posture, bone histology, and other osteological
characters is not about the definition, but about the diagnosis. That
is, characters what we use to infer a phylogenetic hypothesis. The
definition of a name, applied to a phylogenetic hypothesis, yields the
hypothesized content.

Let me know if any of that made sense.
-- 
T. Michael Keesey
Director of Technology
Exopolis, Inc.
2894 Rowena Avenue Ste. B
Los Angeles, California 90039
http://exopolis.com/
--
http://3lbmonkeybrain.blogspot.com/