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

Reflections on Recent "Dromaeosaurs"



  It is becoming more and more clear to me that the
Dromaeosauridae, sensu Holtz (1995) and Padian et al.
(1999), is polyphyletic. Descriptions in the past
couple years of four new "dromaeosaurids"
(Sinornithosaurus: Xu et al., 1999; Variraptor:
Buffetaut and leLoeuff, 1999; Bambiraptor: Burnham et
al., 2000; Achillobator: Perle et al., 1999) have
expanded our knowledge of the Paravian/Eumaniraptoran
dinosaurian taxa (sensu stricto). The problem has been
that rather abrupt means by which some of these taxa
have been dealt with, especially *Achillobator*, that
pre-emtps qualification of its material taxonomically.
There are numerous plesiomorphies in some tax, most
especially *Achillobator*, that may be homoplasies at
best, and some complex advanced features in other taxa
arguing for a more dynamic taxonomy than previously
presented.

  Velociraptorine dromies tend to share more features
of the braincase, skull, and pelvis, with birds than
less "advanced" dromaeosaurs ("dromaeosaurines"). The
original grouping of these animals based on the
structure of the skull and lower jaw presented a
robust, deep skulled Dromaeosaurinae and a
shallow-skulled, narrow, birdy Velociraptorinae (Paul,
1988a,b); Kirkland et al. (1993) presented a likewise
deep skulled animal older than the velociraptorine
*Deinonychus*, *Utahraptor*, but lack of skeletal
details have made this animal less distinguishable
than is liked in a taxonomic analysis. Both
*Achillobator* and *Bambiraptor* suggest that skulls
of basal dromies were deep, rather than shallow, with
vertical, robust lachrymals; Bambi itself shares a
deeply curved dentary and elongate orbit with
velociraptorines, but a pelvis morphology similar to
*Unenlagia* and *Rahonavis* (Buchholz, comm. to the
list, 1998, 1999; pers. obs.) that suggest that, not
only are Unen and Rahona part of the commonly
considered "dromie" grouping, Bambi is plesiomorphic
of the Dromaeosauridae and possibly lies closer to
birds than to velociraptorines.

  Additionally, *Achillobator* has a pelvis that is so
unlike the maniraptoran trend that it may not even be
a maniraptoran (for instance, a propubic process and
proximal obturator process), but other features
(dorsoposterior process of ischium, strong
posteroventral extension of the ilium, and squared
pubic peduncle of the ilium) suggest some strong
dromie affinities, and the second (strong
posteroventral extension of the ilium) makes the ilium
so like *Rahonavis* and *Unenlagia*, its incredible.
The pectoral girdle looks like it could have come from
a ornithomimosaur (by the sub-ovate coracoid with deep
anteroventral/posterior sternal process, scapular
blade reduced from dorsal margin of the
coracoid/scapular contact, lack of a axial twist in
the scapular shaft, ventrally facing glenoid) that
even troodontids lack (see Russell and Dong, 1993);
these suggest three things: one, that *Achillobator*
is not a dromie and is convergent in anatomy to
dromies, two, that the taxon is a dromie, and that the
forelimb is convergent or homoplasious, and three,
that it is an assemblage of more than one taxa (as
suggested by Burnham et al., 2000) a dromie, and
another maniraptoriform, or more. The caudals, aside
from being pleuroceolous, have elongated
prezygapophyses that extend, as preserved, to the
middle the segment preceeding the one before, that is,
1.5 segments; the chevrons may be shorter, and these
are usually considered autapomorphic of dromaeosaurids
(Ostrom, 1990; Xu et al., 1999; Burnham et al., 2000).

  *Bambiraptor* and *Sinornithosaurus* have another
feature that is not mentioned in Burnham et al.:
lateral ornamentation of the maxillary antorbital
fossa by pnuematic fossae, seen also in
sinraptorine/id allosaurs (Currie and Zhao, 1993).
This might be construed as either a synapomorphy of
the two, as a plesiomorphy of all dromaeosaurs
reversed in more advanced forms, or a homoplasy.
*Velociraptor*, based on visual inspection of
photographs and illustrated figures (Osborn, 1924;
Webster, 1996; Barsbold and Osmolska, 1999), does not
exhibit the pneumatization; similarly, *Achillobator*
(photograph, illustration [Perle et al., 1999]) lacks
it, and *Deinonychus* (illustrations [Paul, 1988a,b])
lacks it.

  As previously commented on, *Ozraptor* Long and
Molnar, 1999, is a Western Australian Jurassic form
that has a tibial form and depression for the
astragalar ascending process (which conforms very
closely to the actual ascending process in other
theropods) that are remarkably similar to
*Deinonychus* and *Velociraptor*, as inspected using
Norell and Makovicky, 1997, and Ostrom, 1990.
Similarly, the shape is also similar to *Utahraptor*
Kirkland et al., 1993. I would then suggest that the
animal is a basal dromie, using a polyphyletic term
for Eumaniraptorans, and the term Deinonychosauria may
conform even closer.

  *Bambiraptor* is also well united with bird-trend
dromies on the basis of what Burnham et al. suggest is
a triosseal canal, with separation of the coracoid
dorsal margin from the acromion by a concavity of the
edge, and extreme retroversion of the coracoid from
the long axis of the scapula, which is lacking in
velociraptorines, and relatively more "avian" than
*Sinornithosaurus* (Burnham et al., 2000; Xu et al.,
1999). This is a unique animal, and supposedly adult
specimens of the taxon (FIP 002 and on through FIP
036; the holotype skeleton is FIP 001, and all other
specimens refer to distinct bones except for two
caudal series identical [only larger] to FIP 001) are
only between 1.46 and 1.34 times larger, so Bambi was
more than half-grown, and the skull, pectoral, and
pelvic architecture may not have differed too much, as
suggested by nearly identical morphology of the limb
bones between the type and refered material; so Bambi
may be a distinct taxon from *Saurornitholestes*. This
is the bulk of Burnham et al.'s differentiation, so
I'm not too willing to trust it, but it seems a
likelyhood. At worst, Bambi is a unique "species," if
not a genus.

-----------

  The following is my suggestion of the phylogeny of
paravians / eumaniraptorans / deinonychosaurs, based
on the above, and extensive reading of the literature.
Troodontids and oviraptorosaurs are excluded for
simplicity.

|
`--+--*Achillobator giganticus*
   `--+--*Ozraptor subotaii*
      |--*Variraptor mechinorum*
      `--+--*Dromaeosaurus albertensis*
         `--+--*Utahraptor ostrommaysi /
ostrommaysorum*
            `--+--+--*Bambiraptor feinbergi /
feinbergorum*
               |  |--*Adasaurus mongoliensis*
               |  `--+--*Unenlagia comahuensis*
               |     `--+--*Rahonavis ostromi*
               |        `?-+--*Archaeopteryx
lithographica*
               |           `--+--*Archaeopteryx
bavarica?*
               |              `--Aves
               |?-*Sinornithosaurus millenii*
               `--+--*Deinonychus antirrhopus*
                  `--+--*Velociraptor mongoliensis*
                     `--*Saurornitholestes langstoni*

  I have not had the benefit of reading Kirkland et
al., 1993 or Ostrom, 1969b (monograph), or Sues, 1979,
so this is hesitant. Illustration and description of
the material elsewhere (Glut, 1997; Ostrom, 1990) have
been the basis of my interpretation of these animals.
I also place the Aves and Archie in the cladogram,
contra previous statements to reserve judgement on
these, but do this to show trends in different
lineages. The clade places *Achillobator* (= Achilles'
Hero) as the most basal due to plesiomorphy.
*Ozraptor* has tibial morphology similar the group as
a whole, and *Variraptor* has a sacral and
cervicodorsal morphology similar to the avian trend,
with the most highly developed hypopophysis of any
dinosaur (sensu stricto) save *Avimimus* (Buffetaut
and LeLeouff, 1999; pers. obs.), so I place them
closer to velociraptorines and "unenlagiines" [to use
a conventional and possibly paraphyletic term] than
plesiomorpic. *Dromaeosaurus* lacks features that
*Velociraptor* and *Bambiraptor* share, such as the
upturned frontal process of the squamosal, slender
ascending quadratojugal and descending squamosal
processess, upturned dentary, and strongly recurved
teeth.

  A short form cladistic analysis on a few diagnostic
cranial remains (7 taxa, 6 characters) done by hand,
and used to find those taxa that share the most
features, suggests the above cladogram is not entirely
acurate, and produces the following results:

Velociraptor--|
Deinonychus------|
Dromaeosaurus-------|
Bambiraptor------------|
Sinornithosaurus..../  |
Archaeopteryx-------------
Saurornitholestes....../

        | 1 2 3 4 5 6
----------------------
Bambi   | 2 0 1 1 0 2
Veloci  | 1 0 1 1 0 0
Deino   | 0 0 0 1 0 0
Dromaeo | 0 1 0 0 1 0
Sinorn  | ? ? 1 ? 0 1
Archae  | 2 0 0 ? 1 2
Saurorn | / / 1 / / 0

  I haven't checked all the lit for
*Saurornitholestes* and *Archaeopteryx,* and I have
lost my *Sinornithosaurus* paper :), so / = missing, ?
= indetirminate;
1 = jugal suborbital portion [0, medium; 1, medium
long; 2, long],
2 = quadratojugal/squamosal processes juncture [0,
slender; 1, robust],
3 = dentary [0, strait; 1, upturned],
4 = squamosal frontal process [0, strait; 1, turned
up]
5 = form of lachrymal shaft [0, long and strait; 1,
short and robust]
6 = dorsal surface of frontals [0, strait across; 1,
partially convex; 2, strongly convex]

  Actually running this through PAUP or MacClade may
change the results.

---------------

  Just for fun, the misspelling *Sinornithosaurus
milleni* has been officially published in a
peer-reviewed article, etc., so qualifies as an
official misspelling :) [pg. 10, for those interested].

=====
Jaime "James" A. Headden

"Come the path that leads us to our fortune."

Qilong---is temporarily out of service.
Check back soon.

__________________________________________________
Do You Yahoo!?
Send online invitations with Yahoo! Invites.
http://invites.yahoo.com