[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Another response to an ancient Jaime post. Jaime wrote..
> *Ornithodesmus* has been hypothesized as a troodont, and it too,
> may lie anywhere, as a comparison of sacra has not been done so far
> on troodonts, and I am unaware of the full condition in Sinorn.
Pete Makovicky did lots of description of troodontid vertebrae,
including sacrals, in his unpublished MSc thesis (Makovicky 1995). It
turns out that troodontid sacrals do not have either pneumatic centra or
a neural platform, two features seen in _Ornithodesmus_. This makes
a troodontid identity for _Ornithodesmus_ unlikely, and Howse and
Milner's (1993) suggestion of a troodontid identity for the
_Ornithodesmus_ sacrum (BMNH R187) was based in part on their
comparison with the sacrum BMNH R4463. This specimen is more
probably from a dromaeosaurid (Norell and Makovicky 1997).
_Ornithodesmus_ does share some features with dromaeosaurids
(including fused lateral lamina formed from the zygapophyses, shallow
ventral sulcus, pneumatisation of first two centra), and I previously
thought it was therefore from a dromaeosaurid (hence published
abstracts which announce the discovery in the Wealden of
dromaeosaurid material). However, some of the features have a wider
distribution (plus some, like pneumatisation of centra, are
polymorphic in dromaeosaurids). Also, in _Ornithodesmus_ the
transverse processes are not dorsoventrally flattened, whereas I believe
they are in dromaeosaurids. I provisionally conclude that
_Ornithodesmus_ is not a dromaeosaurid (which prompts the
question: where the hell _are_ the Wealden Group dromaeosaurids??).
The final spin on this is that some of the features in _Ornithodesmus_
are seen in non-tetanurans (e.g., ankylosis of at least four sacrals,
sacrum with six sacrals, neural spine lamina) and, superficially at least,
there is one vague similarity with coelophysoid sacrals. As
controversial and silly as it sounds, I wonder if _Ornithodesmus_ is
therefore from a non-tetanuran.. this would be nice as there is a femur
from the Wessex Formation (MIWG 6214 - Naish in press) that also
"I beg your pardon but, what do you mean, 'naked'?"
PALAEOBIOLOGY RESEARCH GROUP
School of Earth, Environmental & Physical Sciences
UNIVERSITY OF PORTSMOUTH
Burnaby Road email: firstname.lastname@example.org
Portsmouth UK tel: 01703 446718
P01 3QL [COMING SOON: