ORNITHODESMUS AGAIN

[darren.naish@port.ac.uk]

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including sacrals, in his unpublished MSc thesis (Makovicky 1995).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 
looks non-tetanuran.

"I beg your pardon but, what do you mean, 'naked'?"


DARREN NAISH 
PALAEOBIOLOGY RESEARCH GROUP
School of Earth, Environmental & Physical Sciences
UNIVERSITY OF PORTSMOUTH
Burnaby Building
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