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Re: TETRAPODS, PHYLOGENETIC TAXONOMY, AND CLEAR DEFINITIONS
At 07:10 AM 7/26/98 -0400, Johnathon Woolf wrote:
>You say that the perforate acetabulum is a diagnostic feature for Dinosauria --
>that is, it's one of the apomorphies that show Dinosauria is a real group,
>one of the apomorphies that can be used to identify a member of that group.
>also say that ankylosaurs don't have this apomorphy. Yet, ankylosaurs are
>classified as dinosaurs. Why?
[*Sigh*] Because taxa are DEFINED by their ancestry, not their diagnosis.
Dinosauria = all descendants of the most recent common ancestor of birds and
_Triceratops_. All phylogenetic information demonstrates that ankylosaurs
are ornithischian dinosaurs, even though they have secondarily sealed off
>Is or isn't the perforate acetabulum diagnostic for _all_ Dinosauria?
It IS diagnostic for Dinosauria. It, however, is NOT present in all dinosaurs.
>If it isn't,
>why is it still in the list of diagnostic features?
Because it is found in the basal members of all dinosaur lineages, and is
not found in any of their immediate relatives. Same as complex teeth of a
certain form being diagnostic for therian mammals, even though dolphins have
secondarily transformed their teeth into simple spikes.
>If it is, then how can you be
>sure that ankylosaurs are dinosaurs, when they don't have all the diagnostic
>features for dinosaurs?
Because the most parsimonious distribution of features clearly places them
among ornithischian dinosaurs.
>Are you assuming ankylosaurs are dinosaurs because past
>workers said they were dinosaurs?
>Or are there perhaps other features that are
>more useful than the acetabulum for identifying something as a dinosaur?
Not as such. All the other diagnostic features (except for a few "loss"
characters) of Dinosauria can be found transformed to a different state in
one or the other lineages of dinosaurs.
>To me, saying "X is a dinosaur even though X doesn't have all the diagnostic
>features of a dinosaur" is as much a contradiction as saying "6 is a prime
>number." (A ^ ~A) --> B; in words, "from a contradiction, anything
>systematics will never make any logical sense as long as you allow casual
>violations of the laws of logic.
Okay, I talked with my wife (who's also a software designer), and maybe what
we came up with will help explain the difference:
Engineers are creators. Like artists, you are building fresh and anew
something that didn't previously exist. As such, within certain frameworks
(e.g., computer language, architectural constraints, etc.), you have greater
freedom to build things the way you see fit.
Scientists are discoverers. Like explorers, we are trying to find something
that already exists. (In the case of systematics, trying to recover the
historical pattern of descent with modification). We may use certain tools
to analyse, label, and describe these patterns of nature, but we do not have
the liberty of creating them out of whole cloth.
It would be great if diagnoses were invariant in all descendants: evolution
would have been recognized LONG ago, and cladistics would be trivially easy.
However, as part of the process of natural selection, features which existed
in earlier forms can be lost, modified, or otherwise transformed from the
ancestral state. (Good thing, too: I LIKE having a brain, an internal
skeleton, indeed having more than one cell!!). Unfortunately, this means
that any given feature which we use to recognize groups at some more
inclusive level (perforate acetabulum in Dinosauria, particular tooth
patterns in Theria, etc.) may be transformed in some more exclusive taxon.
That, as they say, is Life.
Thomas R. Holtz, Jr.
Vertebrate Paleontologist Webpage: http://www.geol.umd.edu
Dept. of Geology Email:email@example.com
University of Maryland Phone:301-405-4084
College Park, MD 20742 Fax: 301-314-9661