[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Dinosaur Discussion List Dictionary

In a message dated 95-10-25 11:31:58 EDT, Thomas_R_HOLTZ@umail.umd.edu (th81)

>>In a message dated 95-10-24 13:34:07 EDT, rowe@lepomis.psych.upenn.edu
>>(Mickey Rowe) writes:
>>>> OUTGROUP--n. A clade considered primitive to a group of organisms in
>>>> a cladistic analysis of that group.
>>>I think this one is downright wrong because of the inclusion of the
>>>word "primitive".  While it's true that you look for primitive
>>>character states by comparisons with outgroups, it is a fallacy to
>>>presume that the character state retained by an outgroup is the
>>>primitive character state.  An outgroup is just a clade whose most
>>>recent common ancestor with the ingroup lived before the common
>>>ancestor that defines the ingroup as a monophyletic clade. 
>>I don't say that the characters and character states are primitive in the
>>outgroup, just that the outgroup itself is.
>An outgroup is not necessarily more primitive than the ingroup,
>unfortunately.  However, it is best to work with the most primitive members
>of the outgroups, under the assumption that they are most likely to retain
>the primitive features. To use Dinogeorge's phylogeny, Phytodinosauria as a
>whole is the outgroup to the theropod clade, but is not necessarily more
>primitive (i.e., Brachiosaurus and Triceratops, relative to Coelophysis).
>In this example, it would be best to use the basal phytodinosaurs (such as
>Lesothosaurus or Thecodontosaurus) for character polarization.

I think I see the problem: my use of the phrase "primitive to" (which is NOT
the same thing as "more primitive than"!) By this I mean something like
"having a common ancestor with the group earlier than the common ancestor of
the group itself"--sister grouping within the more inclusive clade that
comprises the outgroup and the clade. The phrasing may be weird and rather
mathematical. There may already be (nay--there MUST be, given the propensity
of cladists to name everything) a term that describes this particular
relationship, but I couldn't come up with it when I was writing the
definition. So until someone clues me in, outgroups of a clade are "primitive
to" that clade--strictly a relational term, not intended to convey the notion
that the outgroups are "more primitive than" that clade. Examples:
Phytodinosauria is primitive to Theropoda (and they are sister groups within
Dinosauria). Protoceratopidae is primitive to Ceratopidae (and they are
sister groups within Neoceratopia). Psittacosauridae is primitive to
Neoceratopia (and they are sister groups within Ceratopia). And so on. And
incidentally, if we were examining the clade Protoceratopidae (for example),
then its closest outgroup is Neoceratopia, so Neoceratopia is primitive to
Protoceratopidae. "Primitive to" must be taken in the context of the clade
being analyzed.