CALAMOSAURUS

[darren.naish@port.ac.uk]

I'm responding to a fairly old message here. WRT biogeography of 
Australian dinosaurs, Jaime wrote on _Kakuru_...

> very unusual tibia in that the distal end is very much
> expanded medial, as in *Acrocanthosaurus*,
> *Calamosaurus*, *Microvenator*, and *Ingenia* (see
> Molnar and Pledge, 1980, original paper). 

The _Calamosaurus_ tibia Jaime refers to (BMNH R186) is, in fact, 
not _Calamosaurus_. This genus, erected by Lydekker (1891) when he 
realised that his _Calamospondylus foxi_ was preoccupied by 
_Calamospondylus oweni_ Fox in Anon. 1866, was given to the two 
cervical vertebrae BMNH R901. BMNH R186 came from the same 
horizon as BMNH R901 and was roughly the same colour: these facts 
led Lydekker to provisionally refer the tibia to the same species. Size 
disparity indicates that they cannot come from the same individual, but 
there is also no good reason to refer them to the same taxon. 

BMNH R186 is about the strangest theropod tibia I have seen. While 
Lydekker and most subsequent authors regarded a distal depression on 
one surface as the facet for the ascending process of the astragalus, and 
a proximal projection on the same surface as the cnemial crest, it is in 
fact a proximal structure on the OTHER side that has the distinctive 
lateral curve of a cnemial crest: the structure identified by Lydekker 
does not, and is only a projecting bump. Furthermore, there is a 
triangle-shaped impression on the distal end of the same size as this 
'new' cnemial crest.. Lydekker's 'cnemial crest' is the caudal surface of 
this structure and he thus (apparently) got the bone the wrong way 
round. The diagrams in Lydekker (1891) are not accurate. All in all, 
BMNH R186 is a perplexing specimen in that it has triangular 
impressions on both sides of the distal end - it also has foramina on 
both sides of its shaft (the foramen for the medullary artery, on the 
caudal surface, is often a good indication of which way round a tibia 
goes (and it's position, relative to the fibular crest, may sometimes be 
phylogenetically informative (Naish 1999)). If this interpretation is 
correct (it needs further checking), BMNH R186 is a left tibia, not a 
right one, and its unusual flaring malleolus is on its lateral side, not its 
medial one. Oh yeah, the long axes of the proximal and distal ends are 
nearly parallel in BMNH R186: more normally they are about 
perpendicular.

There is a lot more that could be said about the specimen (it took up 
18 pages in my thesis) - one day I might publish. As to what it's from 
(other than a tetanuran), I'm not sure, though various groups can be 
potentially eliminated (including ornithomimosaurs, troodontids, 
tyrannosaurids, oviraptorosaurs, dromaeosaurids, and avians).

Finally, tibiae from the Wealden Group suggest that small theropod 
diversity in England at the time was quite high. If only they came with 
the rest of the skeleton attached.

"We should not have made this bargain"


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