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Buitreraptor and the Phylogeny of "Dinobirds"



  In the recent paper by Makovicky, Apesteguía and Agnolín, the authors
describe a new theropod skeleton and a referred skeleton as the types of a new
species of dromaeosaurid, *Buitreraptor gonzalezorum*, associated by skeletal
apomorphies with *Unenlagia* and *Neuquenraptor*, both considered synonyms but
unlikely in the direct examination of their features, similarites and
differences, as well as the Malagasy "avialan" *Rahonavis*. In addition, Calvo,
Porfiri and Kellner published in 2004 a new species of *Unenlagia*, *U.
paynemili*, on the basis of a partial pelvis and hindlimb with associated
dorsal vertebra. The "Unenlagiinae" as used by Makovicky et al., or the
"Unenlagiidae" of Bonaparte, is not represented by five species, and the
phylogenetic analysis by Makovicky et al. places them at the base of
dromaeosaurids proper, more basal indeed than Microraptoria (the name used,
"Microraptorinae," is a lapsus calami, Makovicky pers. comm.). However, some
features in the anatomy of *Buitreraptor* are unique, not just as a species,
but also relevant to its placement in the "unenlagiine" and dromaeosaurid
groups.

  Mickey Mortimer has recently presented the topology of his study on
coelurosaurs which, while not detailed in it's pelvic and hindlimb characters,
is substantial in its cranial and pectoral features, both of which figure here
in important details. At my behest, Mickey removed a host of "large,
terrestrial" taxa from his analysis in order to test an hypothesis of mine,
which is that basal taxa will have more relevance in a phylogeny than derived
forms when linking multiple groups, which in coelurosaurs appear to develop
terrestrial, larger forms independantly, from what are essentially small,
semi-cursorial forms with a suite of "avian" features. Contra Paul (2002), the
"avian flight apparatus" which he uses to infer secondarily flightlessness may
in fact be more basal to the origin of birds, and rather than support a
derivation of "dinobirds" from avian stock, as dromaeosaurids from an
*Archaeopteryx*-like ancestor, the features are developed not only uniquly
before flight-capable forms, but multiple times. Indeed, in Mickey's study, I
also had him use the same taxa when possible in Makovicky's analysis, and came
up with some interesting data. General topologies were retained, as from Holtz,
the Theropod Working Group's various analyses, and Mickey's own major analysis,
barring the few "inconsistencies" that we've been discussing.

  The constrained Makovicky analysis went something like this:

--+--Ornitholestes
  `--+--+--Sinosauropteryx
     |  `--Compsognathus
     |--+--Avimimus
     |  `--Caudipteryx
     |--Alvarezsaurus
     `--+--+--Archaeopteryx
        |  `--+--Confuciusornis
        |     `--Rahonavis
        `--+--+--Byronosaurus
           |  `--+--Mei
           |     `--Sinovenator
           `--+--Buitreraptor
              `--+--Sinornithosaurus
                 `--Microraptor

  Obviously, without larger taxa, one cannot test this topology against the
whole, such as ornithomimosaurs, dromaeosaurids proper, and so forth. In a
divergent turn of events, Mickey's topology reads like this:

--+--Dilong
  `--+--+--Coelurus
     |  `--Tanycolagreus
     `--+--+--Sinosauropteryx
        |  `--Compsognathus
        `--+--Ornitholestes
           `--+--+--+--Incisivosaurus
              |  |  `--Avimimus
              |  `--+--Alvarezsaurus
              |     `--+--Protarchaeopteryx
              |        `--Caudipteryx
              `--+--+--Yixianosaurus
                 |  `--+--Sapeornis
                 |     `--+--Confuciusornis
                 |        `--Ornithothoraces
                 `--+--+--Scansoriopteryx
                    |  `--+--Archaeopteryx
                    |     `--Jinfengopteryx
                    `--+--+--Buitreraptor
                       |  `--+--Rahonavis
                       |     `--+--Shenzhouraptor
                       |        `--+--Jixiangornis
                       |           `--Yandangornis
                       `--+--+--Sinovenator
                          |  `--+--Mei
                          |     `--Byronosaurus
                          `--+--Graciliraptor
                             `--+--+--NGMC 91
                                |  `--Sinornithosaurus
                                `--+--Bambiraptor
                                   `--+--"Cryptovolans"
                                      `--Microraptor

  Notice that I had him pointedly exclude Unenlagia from the picture, in order
to test it's large, terrestrial nature as an effect on topology. In both
Mickey's results, you will notice that Deinonychosauria is retained, and that
troodontids and dromaeosaurids are explicitly split, but that *Buitreraptor*
and *Rahonavis* are not part of either but are basal to the group. In
Makovicky's results, *Buitreraptor* is still a dromaeosaurid, and the most
basal one, but *Rahonavis* is not. That Mickey get's birds split in two,
finding a third "bird" group of Archie, *Scansoriopteryx* (I would prefer
*Epidendrosaurus* as the senior synonym), and *Jinfengopteryx* which is a
particularly avian, yet short-armed, "troodontid". The rest of the trees
largely conform to the arrangements of Maniraptora found by other workers in
the last five years by almost unparalleled concensus for such large groups of
taxa.

  But that was only the beginning. Apart from Makovicky et al.'s data recoded
not supporting *Buitreraptor* as a dromaeosaurid, some other, more avian
features appeared to pull it toward birds. A more thourough look at the
skeleton reveals some oddities that require explanation. Makovicky et al.
diagnosed *Buitreraptor* by the following: "*Buitreraptor* differs from other
theropods in the following
unique combination of traits: skull long, exceeding femoral length by 25%;
teeth small, unserrated, without root-crown constriction; quadrate with large
lateral flange and pneumatic foramen; posterior cervical centra with
ventrolateral ridge; furcula pneumatic; brevis shelf expanded and lobate,
projects laterally from caudal end of ilium." (pg. 1008).

  Importantly, several of these features are diagnostic of larger clades, or
are found to some variation among the "avian" form of maniraptorans, including
the Microraptoria, which are distinctive in their anatomy yet still remarkably
dromaeosaurid.

  *Buitreraptor*, in a odd sort of Forster et al.-esque way, appears to have
features that are not only avian (as in the narrow angle between coracoid and
scapular distal shaft, U-shaped furcula, and strongly developed acrocoracoid
process in the proximal 1/3 of the coracoid) but also several troodontid
features (dental foramina in a groove, antorbital fossa nearly flush to the
lateral maxillary margin without a distinct ridge, extremely large maxillary
fenestra, postorbital process of the frontal separated from the parietal by a
long caudal corpus, and the subsequent isolation of the postorbital
articulation away from the parietal and carried on the frontal (as well as the
unrevealed laterosphenoid, I am sure) alone). The elongated jaws, apparent
large external mandibular fenestra, tiny and numerous unserrated teeth, may
also link it to troodontids. So, as in Forster et al.'s analysis linking
*Rahonavis* to birds and not only them but Troodontidae as well, may in fact
*Buitreraptor* link troodntids and avians and separate Deinonychosauria? Or
will it show the scansorial qualities of basal dromaeosaurids are in fact
present in troodontids, as well? And why are these taxa in Gondwana, millions
of years after their origin, appearing un-modified from ancestral
"deinonychosaurian" stock? Many good questions, perhaps. The ilium appears to
link *Buitreraptor* to the "unenlagiines", while the ischium is both like that
of *Sinornithosaurus*, as well as *Rahonavis*. But basal troodontid ischia, as
shown in *Jinfengopteryx*, are remarkably similar to both *Rahonavis* and
*Archaeopteryx* in having a small distal process and a large,
distally-positioned obturator process. *Buitreraptor* has long ridges on the
lateral surfaces of it's caudals, a troodontid feature, but appears to have a
dorsal crest as a neural spine, rather than a sulcus, making it tit for tat.

  But we do know that more stories and questions can arise on basal
maniraptorans than just from NW China, and that South America has not yielded
all of its secrets so far.

  Cheers,

Jaime A. Headden

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


                
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