Re: Bambiraptor paper published

[Nick Longrich <nrlongri@midway.uchicago.edu>]

>Nick Pharris wrote-
>
>> Any discussion on why this is not _Velociraptor_/_Saurornitholestes
>> langstoni_?
>
>Straight from the paper-
>
>Although nearly contemporaneous with Saurornitholestes langstoni (Sues,
>1978), the new taxon is distinguished by its relatively long, anteriorly
>tapering frontal. While this feature may be exaggerated due to the
>immaturity of the holotype, it is unlikely to be entirely an ontogenetic
>difference. The frontals in the holotypes of both species are
>nearly the same length, but the orbital rim of B. feinbergi is twice as long
>as that of S. langstoni. There is little basis for further comparison since
>the holotype of the latter is so incomplete and poorly preserved.
>Bambiraptor feinbergi is different from Velociraptor mongoliensis (Osborn,
>1924) since the holotype does not have the ventrally depressed nasals or the
>resulting bul-bous snout (Barsbold and OsmÛlska, 1999). V. mongoliensis has
>several additional maxillary teeth. The furcula is V-shaped with a rounded
>cross section as well as a small hypocleidium, different from the more
>Archaeopteryx-like condition of Bambiraptor (Norell, Mackovicky, and Clark,
>1997). The pectoral girdles in the described skeletons of V. mongoliensis
>(Norell and Makovicky, 1999) are imperfect,
>but they appear to have a longer scapulocoracoid suture (fused in the
>available specimens) and a smaller acromion. The postacetabular blade of the
>ilium is higher (as in Deinonychus) and less tapered (Norell and Makovicky,
>1997). Finally, the femur of Velociraptor mongoliensis is reported to have a
>fourth trochanter, a feature absent in Bambiraptor and other dromaeosaurids.

There is a large fourth trochanter in a specimen of Velociraptor or
whatever it is just published on by the AMNH team, but I think some of the
others lack it; and I think Novas has noted the presence of TR 4 in some of
the Deinonychus material at Harvard. The thing appears to be ontogenetic in
Eumaniraptora: it starts as a shallow pit like in Microvenator or
Sinornithoides (juveniles) and only in the most mature eumaniraptora (ones
that also have adult stuff like fused scapulocoracoids and tarsometatarsi)
it becomes a fourth trochanter. This contrasts with basal Maniraptora like
Coelurus in which the type is very immature but has a huge fourth
trochanter. The pattern is probably complex though; ontogeny varies across
species/families (e.g. the very well-developed hindlimbs of juvenile
troodontids). There is no tarsometatarsus or scapulocoracoid in the
Mononykus type but is has a fourth trochanter, what exactly this means (the
fourth trochanter develops first? Mononykus eliminated some of these
fusions?) I have no idea.

        Regarding troodontid relationships discussed recently, my guess at
this point is that Ostrom had it right all along, and it's one of the few
low-level relationships I really consistently get in my phylogenetic
analyses. For one thing, they have a hyperextensible toe and
oviraptorids/therizinosaurs don't (if you think they could've lost it, I'll
agree with you, but I don't think we can argue that unless we have some
evidence), while all basal birds and dromaeosaurs have it. The sickle of of
deinonychosaurs differs from all other sickle claws, however, in its sheer
development. The Archaeopteryx' claw is not a mass-killing implement, it's
just a toe with a bit of retractibility which maybe allowed it to hold the
digit off the ground and keep it from getting dull so it could climb
trees/spear small animals (in other words, it's convergent on the
retractible claws of cats), but in dromaeosaurs and troodontids, it's damn
big, slasher-movie style (Rahonavis of course has one but this does not
necessarily argue against a monophyletic Deinonychosauria). Convergent?
Sure, possibly, but why not assume that it is a shared derived feature?
It's hardly the only thing linking the two.
 For another thing, both have a crest running down the midline of the skull
formed by the parietals, where presumably the jaw muscles attached. The
posterior of each frontal also has a sigmoidal curved margin which
delineates the anterior extent of the supratemporal fenestrae; I don't know
of anything else that shares this distinctive look to the frontal bones. I
believe Currie noted this in dromaeosaurs but didn't point out that it is
also in troodontids as well. When the teeth are serrated they have smaller
serrations on the anterior of each tooth than the posterior, in both
families.
        The splenials have a large, triangular, lateral exposure in both
which isn't known in anything else so far as I know. Chevrons in both are
widely forked anteriorly and each embraces the preceeding one; I don't know
of anything else that has this although it wouldn't surprise me if
Rahonavis had it.
        The transverse processes of the proximal caudals are lower down on
the centra than in oviraptorosaurs, in which the transverse processes are
well above the body of the centrum (troodonts and dromaeosaurs are like
birds in this respect). The distal caudals completely lack neural spines,
which is also the case in dromaeosaurs and archaeopteryx. Distal caudals
are also extremely elongate, only matched in Archaeopteryx and Rahonavis.
The accessory trochanter (NOT the lesser trochanter) is this teeny thing
fused way high up on the trochanteric crest (I don't know what the
situation is in Archaeopteryx/Rahonavis). This isn't the full extent of the
similarities either.
        My guess is that this argues for putting them with Archaeopteryx
and dromaeosaurs, and in particular the dromaeosaurs; they share a lot of
features which don't pop up anywhere else, in the skull, feet, tail, etc.
It's this widespread agreement of anatomy and the uniqueness of many of
these features  which, in my opinion, makes the case so convincing. Sure,
they share lots of differences, which Russell and Dong's Sinornithoides'
paper list in detail. But they are irrelevent unless you can categorize
them as either synapomorphy or plesiomorphy in a phylogenetic context; many
of these differences are  autapomorphic and therefore don't shed light on
the problem one way or the other- troodontids and dromaeosaurs are
different. Okay, tell us something we don't know. A lot of the other
differences are pretty dubious- elongate legs and arctometatarsus pop up in
a lot of maniraptorans and probabably have far more to tell us about
ecology and function than phylogeny; the propubic nature of the pelvis is
probably of little significance given the wide variety of pro-, opistho-,
and vertipubic arrangements seen in Maniraptora, which follow no
discernable pattern, arm elongation or lack thereof is a similarly
homoplastic character which is probably of little or no use in
reconstructing phylogeny.
-N