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

Re: ornithomimosaur

Regarding the oft commented on uniqueness of Ornithomimus velox's
Russell (1972) writes, "there are no contacts between the distal and
proximal fragments of any metatarsal in the pes.  The metatarsus of O. velox
is therefore not known to differ significantly from the usual morphology of
this structure in ornithomimids."  And furthermore.... "It is not possible
to separate Ornithomimus edmontonicus from O. velox on the basis of existing
materials."  He goes on to say the stratigraphic separation may indicate
that further material will show them to be separate species.  Note that the
metacarpus of O. velox shares the condition of metacarpal I being longest
with O. edmontonicus, but not other ornithomimids.  A specimen that may
complicate matters is MNA Pl.1762A, described by DeCourten and Russell
(1985) from the Kaiparowitz Formation of Utah.  This is referred to O. velox
based on- straight pedal unguals with straight margins; pedal ungual II >75%
of phalanx II-1 length.  The ungual structure is only compared to
Struthiomimus and an undescribed Hell Creek taxon, but Archaeornithomimus at
least also has straight unguals.  The straight edges are caused by the lack
of proximoventral spurs found on the unguals of Archaeornithomimus,
Struthiomimus and Gallimimus.   This is a plesiomorphy of O. velox and may
indicate MNA Pl.1762A belongs to that taxon.  The ratio between pedal ungual
II and phalanx II-1 not only varies considerably within species
(Ornithomimus edmontonicus 31-61%, Gallimimus 49-65%), but is also
plesiomorphic, being high in Harpymimus (78%), Alectrosaurus (78%) and
Gorgosaurus (73%) for instance.  The pubic shaft is curved posteriorly as in
Archaeornithomimus, the Hell Creek taxon and Garudimimus, but unlike other
ornithomimids.  Because of this, the authors suggest O. velox may not be
congeneric with O. edmontonensis.  I'm unsure what to think of this.  The
lack of proximoventral ungual spurs, elongate pedal ungual II and curved
pubic shaft are all plesiomorphic characters.  The second is too highly
variable for me to trust though.  This leaves one character that refers MNA
Pl.1762A to O. velox, the plesiomorphic lack of ungual spurs.  Of course,
only the elongate first metacarpal groups O. velox and O. edmontonensis
together.  Further study of actual specimens is needed to determine the
relationship of Ornithomimus velox, O. edmontonensis and MNA Pl.1762A.

As for "Coelosaurus" antiquus, I think a more in depth comparison of
ornithomimosaur tibiae would have to be made before a synonymy could be
accepted.  Examining Sullivan's two characters-
1. cnemial crest forming prominent anterior ridge that descends along the
shaft for almost 1/4 its length.
The cnemial crest is prominent and anteriorly placed in all ornithomimids
and is no longer in O? antiquus (21%) than Struthiomimus (23%) or Gallimimus
(24%) for instance.
2. anteriorly directed upper cnemial crest almost parallel to shaft of tibia
for 23 mm, descending sharply towards shaft, forming distinctive somewhat
angular ridge in medial view.
This is just saying the outline of the cnemial crest in medial view is
angled sharply, which I agree with.  It looks that way in both O? antiquus
and O. edmontonicus, but Struthiomimus' is angled slightly while Gallimimus
and Archaeornithomimus aren't angled.  Dromiceiomimus also has this
character though (Struthiomimus ingens holotype), so it is not unique to
Ornithomimus.  Then again, there are suggestions Dromiceiomimus and
Ornithomimus are synonymous, or at least sister taxa.  They are only
separated by Russell because Dromiceiomimus has- humerus shorter than
scapula; ulna ~70% of femoral length; preacetabular process, tibia,
metatarsus and third pedal digit shorter relative to femoral length.
Perhaps Dromiceiomimus should be a separate species of Ornithomimus.  In any
case, it has not been demonstrated that "Coelosaurus" antiquus is closer to
O. edmontonicus than Dromiceiomimus brevitertius, so I reject Sullivan's
synonymy.  I do not think referral of most of the material that's been
assigned to O? antiquus can be justified either, most of it being
unassociated and not comparable.

Mickey Mortimer