Re: _Achillobator_

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

        Re: Achillobator

        Tom's comments are interesting- I think we do need to look at what
does and doesn't fall in Dromaeosauridae. The exclusion of Dromaeosaurus
from the classic "dromaeosaurs" in Norell's Yale Symposium talk was a bit
of a shocker in this regard, but we don't have any definitely assigned
postcrania if I recall correctly, and in the skull, a fair number of
velociraptorine features (e.g. lateral, triangular exposure of splenial,
that funky sigmoid posterior border for the supratemporal fenestra on the
frontals) are also troodontid characters as well. Of course we don't know
what the skulls in Rahonavis and Unenlagia looked like; and Achillobator is
unfortunately only represented by the maxilla I think. So this seems
entirely possible.
        Of the weird features- a proximally placed obturator process is
clearly an apomorphy assuming that it fits inside the
oviraptorid-velociraptorine clade. The pubic boot could argue for some kind
of primitive status, but according to gut feeling and the latest runs of my
highly preliminary matrix, troodontidae form a monophyletic
Deinonychosauria with the velociraptorines. So either that could be
synapomorphic somewhere in there, or just be one of those things that is
fairly homoplastic. Pelves- well, those are definitely homplastic.
Opisthopubic in Mononykus, Velociraptor, and therizinosaurs; slightly
opisthopubic/vertical in Archaeopteryx, Rahonavis; propubic in
oviraptoridae, Caenagnathids if I recall.
        What makes me think this critter is "dromaeosaur":
        -Posterior wing of acetabulum comes to a point, as in some
velociraptorines, Archaeopteryx, Rahonavis, Unenlagia modern birds,
        -Big antitrochanter to articulate with a trochanteric crest, as in
all maniraptorans excluding basal forms like Coelurus, ?Ornitholestes,
        -Pubic peduncle extends ventrally below ischiadic peduncle, as in
velociraptorines, Rahonavis, Unenlagia, Archaeopteryx
        -Ventral margin of pubic peduncle concave to recieve pubis, as in
Rahonavis, velociraptorines, Unenlagia, Archaeopteryx
        -hyperextensible digit II. Also in velociraptorines, Archaeopteryx,
Confuciusornis, Rahonavis.

         So I think it is pretty convincingly in the same region as
dromaeosaurs, archaeopteryx and their kin on these characters. This is not
to say it isn't also related to troodontids; we don't have a complete ilium
published on for them.


Velociraptorine-like characters
        -Gynglymous end to MT III. not present in troodontids,
oviraptorosaurs and kin. Present in Rahonavis, I don't recall for
Archaeopteryx.
        -enlarged sickle-claw. Also in Troodontidae (no I don't think it's
convergent), Rahonavis, velociraptorines, Megaraptor. Not found in
Archaeopteryx.
        -short anterior iliac blade. Typical for velociraptorines; unknown
for troodonts. Not the case in Rahonavis, Unenlagia, Archaeopteryx.
        -strong anterior notching of the iliac blade. Among derived
Maniraptora, unique to velociraptorines (where known), AFAIK.
        -Relatively short posterior caudals.  This is a plesiomorphic
character, but the primitive condition for the Archaeopteryx-sickle-claw
group seems to be to have those hyperelongate caudals seen in
Archaeopteryx, Rahonavis, and Troodontidae. So this is a reversal that
links Achillobator with velociraptorines, which also show short caudals.
        -Anteriorly hyperelongate prezygapophyses, with twin extensions.
Uniquely known for Velociraptorines (and Utahraptor, which has been called
a dromaeosaurine in the traditional scheme if I recall. It also has short
caudals).
        -chevrons widely forked anteriorly, to embrace preceeding chevron.
Found in Velociraptorinae, Troodontidae. I don't see this in Archaeopteryx
but the preservation makes it hard to tell. Published descriptions of
Rahonavis don't show enough here.
        -Short dorsals. Velociraptorinae have extremely short dorsals
relative to other theropoda; I haven't done the morphometrics but
Achillobator looks comparable.
        -serrations larger on posterior carinae than anterior carinae.
Troodontidae, velociraptorinae. (I can't recall dromaeosaurus off the top
of my head and don't have the paper handy).
        -stalklike parapophyses of dorsals. Hard to tell, there may be a
broken-off one of these in Achillobator.

        As Tom has argued in the past, it's a heck of a lot easier to
derive birds from Archaeopteryx than something like Dromaeosauridae. I'd
argue that the same applies to Achillobator+Dromaeosauridae;  they just
have too many weird apomorphies that traditional birds lack. Despite the
various weirdnesses displayed by Achillobator, it has even more weird
features that successively pair it up with Archaeopteryx, Unenlagia,
Troodontidae, Rahonavis, and Velociraptorines. I haven't talked much about
Sinoavisaurus and Dromaeosaurus, mostly 'cos I don't have the papers handy.
But the other interrelationships? That's pretty hard, especially since we
don't have a lot of overlap- hard to compare Dromaeosaurus to Rahonavis,
considering one is mostly the skull and the other is headless. So I think
we do need to be vigilant to other possibilities.
        Some people e-mailed me on Achillobator; I just got done working on
prelim exams so that's why I haven't had time to reply.


        -N