HOLLOW BONES IN ANKYLOSAURS

[darren.naish@port.ac.uk]

Betty asked....

> Has anyone resolved whether ankylosaurids had air sacs similar to 
> birds? (ie: their own natural flotation devices?)

Like all ornithischians (so far as we know), ankylosaurs were not 
pneumatic - within Dinosauria this is either exclusive to Saurischia 
or, alternatively, was present in the dinosaurian common ancestor and 
then lost in Ornithischia (pterosaurs represent a pneumatic outgroup 
within the Ornithodira.. though yes I am aware that there is some 
controversy about their position as members of this group). Thus no 
air sacs in ankylosaurs.

HOWEVER, it's interesting to note that an indeterminate North 
American nodosaur described by Walter Coombs and someone else in 
_Journal of Paleontology_ a few years back did actually have hollow 
ribs. Seeing as this ankylosaur was found preserved in marine 
sediments, might the hollow ribs represent some sort of bizarre 
lightening device to help it float? If memory serves, Coombs and 
colleague discussed this, but discounted it, plumping for the 
alternative - that the hollowed ribs were the result of scavenging 
from (unknown) marine animals. 

No amphibious/aquatic tetrapods (other than birds!) have hollow ribs 
- instead, the ribs are usually filled with dense trabecular bone or 
maybe even pachyosteosclerotic bone to help control buoyancy - 
EXCEPTING the recently described (1995) _Omphalosaurus wolfi_, a 
European omphalosaur recently given a redescription. Omphalosaurs are 
strange Triassic marine beasts that may be ichthyosaurs (certainly 
the newest stuff looks ichthyosaurian, but Ryosuke Motani has an MS 
in press where (like Merriam) I think he questions their 
ichthyosaurian status). The fact that _O. wolfi_, alone out of the 
four omphalosaur species, has hollow bones argues that this feature 
is the result of scavenging, as Coombs and colleague suggested for 
the ankylosaur.

"He's a credit to your race"

DARREN NAISH 
PALAEOBIOLOGY RESEARCH GROUP
School of Earth, Environmental & Physical Sciences
UNIVERSITY OF PORTSMOUTH
Burnaby Building
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Portsmouth UK                          tel: 01703 446718
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