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Phyl tax misunderstandings (was Re: tetrapoda) (LONG)



At 06:37 AM 11/14/97 -0800, Johnathon Woolf wrote:

>OK, time for another of those stupid questions for which I am probably
>becoming infamous: what is the point of the arguments over different
>cladogram-based definitions of taxa?

Actually, what us systemicists are trying to do is to establish explicity
definitions for taxon names so that there WON'T be any arguments over them.
However, these are the formative years, which (we all no) is the time when
most of the tantrums take place.

>As I understand it, "crown-based"
>definitions say "all animals closer to modern X than to modern Y." 

Actually, no.  "Crown-based" taxon definitions are the a special type of
node-based definitions, where the 'anchor' taxa are currently living: as in
Mammalia as all descendants of the most recent common ancestor of
Monotremata and Theria.

(Note that some people have misapplied the term "crown-based" and refered
to, for example, Hadrosauridae as a "crown group" within Iguanodontia...).

>"Node-based" definitions say "the last common ancestor of groups X and
>Y, and all descendants of that ancestor." 

Yes.

>"Stem-based" definitions say
>"X and all organisms closer to X than to Y," where X and Y may be living
>or extinct species.

Yes.

>All depend on cladograms, which means the
>definition is only as good as the cladogram is.

Nope.  Time to do the mind-shift.  Accepting, for the moment, the reality of
evolution, there will ALWAYS be some clade which can be described by these
different definitions.  That clade might already have a name, or it might
not be a useful taxon (Camptosaurus and all taxa sharing a more recent
common ancestor with Camptosaurus than with Iguanodon), but it really
existed.  Different cladograms might hypothesize different member taxa in a
given taxon, and different cladograms might yield different diagnoses for
these taxa, but the taxa will always describe SOME real clade.

Or, there will always be a clade (Maniraptora) comprised of birds and all
taxa sharing a more recent ancestor with birds than with Ornithomimidae.
Whether dromaeosaurids, troodontids, tyrannosaurids, coelophysoids,
crocodylians, wombats, or shell fungi belong in Maniraptora, and what the
diagnosis for such a group will be, are dependent on the particular cladogram.

>All also depend totally on having a good fossil record to work with.  

Definitely not true!!  Most invertebrate workers can produce useful
phylogenetic taxonomies for groups with little or no fossil record.

>For example, we can't build a proper crown- or node-based definition of
>mammals because nobody knows when monotremes branched off from the basal
>mammal line, so we don't know the last common ancestor of modern
>mammals.

Sure we can, and it has been done!  (I still think people on this list think
cladistics is some kind of dinosaur paleo plot, despite the fact that
Novacek & McKenna & others were doing mammal cladograms, Estes with lizards,
Gaffney with turtles, etc. long before Sereno, Gauthier, et al. got into the
act...)

We don't have to know exactly WHEN the split was.  Unless monotremes and/or
therians were either specially created or are alien intruders, they share a
most recent common ancestor at some point.  The goal of systemicists then
becomes to "map out" the distribution of features in known organisms, so
that we can hypothesize as to the ancestral mammalian condition.

>They
>depend on cladograms to tell them relationships -- but cladograms never,
>ever say "X is descended from Y."

But phylograms might...

>I don't understand why people got away from the plain old-fashioned idea
>of basing definitions on physiology:  if X and Y have features 1, 2, and
>3 in common, then X and Y are in the same taxon.  And taxa are defined
>as "the first organism to have features 1, 2, and 3, and all of its
>descendants."

Okay, actually there ARE such definitions (the "apomorphy-based" taxon
definitions), but they have a failing.  If you use multiple characters to
define a taxon, what happens when you get to the origins of that clade and
some (but not all) of the features are present.

Real World Example: Dinosauria was once defined by upright limbs,
asymmetrical hands, three or more sacral vertebrae, and an open acetabulum.
This worked fine when Pisanosaurus, Thecodontosaurus, and Coelophysis were
among the most primitive dinosaurs known.  Now we have Lagerpeton,
Lagosuchus, etc. (which have upright limbs), Eoraptor (with upright limbs,
asymmetrical hand, three sacrals, but uncertain acetabulum), and
Herrerasaurus (upright limbs, asymmetrical hands, only two sacrals, and an
open acetabulum).  Which ones are dinosaurs?

OR, you could choose a single feature for the definition.  The problem,
however, is what happens when that feature turns out to be convergent?

Real World Example: Artcometatarsalia was once defined as the first taxon
with the arctometatarsus and all of its descendants.  However, contra my
analysis in 1994, this structure is almost certainly convergent in
caenagnathids, mononykine alvarezsaurids, and true arctomets.  How could I
resolve which lineage to use for such a definition?

>Tetrapoda then would be the first four-legged partially 
>land-dwelling animal and all of its descendants.

Ichthyostega & company's legs are a lot more fin-like than previously
believed.  Panderichthys fins are a lot more leg-like than previously
believed.  Ichthyostega & company were a LOT more aquatic than once thought.
How "leggy" does the limb have to be for such a definition?  How partial
does the "land-dwelling" have to be?

>Amniota would be the
>first animal that laid hard-shelled eggs, and all of its descendants. 

Assigning eggs to fossil organisms is EXTRAORDINARILY difficult.  In fact,
dinosaurs are one of the few groups for which eggs are well studied (others
are some lizards and turtles).  There are Permian eggs known, but nobody
knows who laid them.  How could such a definition be used to determine if
Dimetrodon, Diadectes, or Limnoscelis is or isn't an amniote?

>Tyrannosauria would be the first theropod to have the shrunken forelimbs
>and whatever other autapomorphies distinguish tyrannosaurs, and all of
>its descendants.

D-shaped premax teeth and the characteristic squamosal-quadratojugal
protrusion are found in all known tyrannosaurid skeletal taxa, but the
shrunken limbs may occur much later in their evolution.

>Aves would be the first animal to have flight
>feathers, and all of its descendants.

But feathers are only preserved in a VERY limited environmental setting.
(We are damned lucky Archaeopteryx lived near lagoons!!).

>Ceratopsia would be the first
>animal to have the parrot beak and neck frill, and all of its
>descendants.

"Neoceratopsia", actually,  although a simple apomorphy-based definition for
Ceratopsia would just be "the first taxon to possess the rostral bone and
all of its descendants".

>And so on.  This is close to the "node-based" definition,
>but it's based on hard facts, not theorizing about "relationships."
>
The "hard facts" comment is ironic, as many of the features you suggested
are soft-tissue structures, and thus rarely preserved in the fossil record.

I hope this helps.

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