Re: Rahonavis....Both!

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 08:38 AM 2/24/99 -0500, Larry Febo wrote:

>Guess what I`m really trying to do is point out an inconsistancy. I mean, if
>Rahonavis is descended from Dromaeosaurs (according to BAMM),

No.

No.

NO NO.

NO NO NO NO NO NO NO NO NO NO NO NO.

(Okay, maybe, but I'll get to that in a bit!!).

The "standard model" (aka BAMM) does NOT suggest that dromaeosaurids are
bird ancestors!!  Instead, it suggests that they are the sister group to
birds: the closest known relatives, both diverging from a common ancestor.
Similar situation to mammals: few would suggest marsupials are the ancestors
of placentals, but instead suggest that marsupials and placentals are sister
groups: they both diverged from a common ancestor not shared by other mammal
groups.

So, if you check out Gauthier, or my work, or Sereno, or Makovicky & Sues,
or Forster et al., etc., etc., you'll find Dromaeosauridae or
(Dromaeosauridae + Troodontidae) as the sister group to birds, but not the
ancestor to birds.

(One interesting exception, though, was Norell's paper at the Ostrom
Symposium, which used individual specimens rather than grouping all
"dromaeosaurids" into "Dromaeosauridae".  He found that dromaeosaurids did
not form their own group separate from birds, but that there was an
uncertain relationship between individual dromaeosaurid specimens and birds:
this might actually suggest 'dromaeosaurids' as traditionally concieved ARE
bird ancestors.  However, many of the specimens and the data matrix he used
in this analysis are not yet available for study, so we'll have to wait
before we can interpret these results in more detail).

>then how could
>Archaeopteryx also be. (although I think I just got the answer by reading a
>Jeff Poling article claiming it to be a possible "holdover" from the
>Jurassic).

Okay, even given the premise that 'dromaeosaurids' ARE _Rahonavis_'s
ancestors, why COULDN'T _Archaeopteryx_ also be a descendant?

After all, you and all and all other humans are descendants of the first
primate, but so are gorillas, and so are baboons, and so are squirrel
monkeys, and so are bushbabies, and so lemurs ("monkey wannabes" as Homer
Simpson calls them...), and so was _Gigantopithecus_, and so was
_Dryopithecus_, etc.

An ancestor can produce many descendants.

>Also, it seems, that Rahonavois would then have to be an INTERMEDIATE
>between Archie and Dromaeosaurs, given its large pedal claw and bony rods in
>the tail etc. (here, I`m assuming it is closer to Dromies than Archie
>morphologically).

Aha!!  I see your misconception!!

_Rahonavis_ does NOT have the long bony rods in the tail that dromaeosaurids
have.  At present, only _Deinonychus_, _Velociraptor_, _Saurornitholestes_,
and _Utahraptor_ are known to have these among theropods.

The large sickle claw is certainly dromaeosaurid-like, but as most analyses
show dromaeosaurids and/or troodontids are bird outgroups, this feature is
probably basal to the bird-dromaeosaurid (& troodontid) group, and later
lost in more advanced birds (_Archaeopteryx_ and some other basal birds do
retain a hyperextensible digit II without keeping the sickle claw).

Otherwise, _Rahonavis_ doesn't have any "dromaeosaurid" features not also
found in _Archaeopteryx_, and does have some "bird" features not found in
any dromaeosaurid.

However, in general terms, I would agree that _Rahonavis_ does seem to be
intermediate between the dromaeosaurid form and _Archaeopteryx_.

>Personally, (despite the "holdover" explaination), I think the relationship
>of these forms is better indicated by BCF, given the stratigraphic evidence
>of where (when) these fossils occur.

Although you might think that, you do have some misconceptions about the
difference between ancestors and sister taxa, about the anatomy of
_Rahonavis_, and about ancestor-descendant relationships, so hopefully I can
help clear some of that up.

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