In a message dated 3/16/2005 6:28:15 A.M. Central Standard Time, [log in to unmask] writes:
The problem of the fusion together of lumbar vertebrae is one I am dealing
with in my current research project, comparing riding, traction and
free-living horses. Such phenomena are described in the veterinary
literature. For example:

Townsend, H. G. G. and D. H. Leach (1984). "Relationship between
intervertebral joint morphology and mobility in the equine thoracolumbar
spine." Equine Veterinary Journal 16(5): 461-65.

Stecher, R. M. (1961). "Ankylosing lesions of the spine of the horse."
J.A.V.M.A. 138(5): 248-55.

There doesn't seem to be any concensus about why the vertebrae fuse. And
there probably is no simple answer. Such fusions are not uncommon, as you
can see from the above articles. I have seen fusion between the 5th and 6th
lumbar vertebrae in a 4 year old Chinese chariot horse. I think that the
horse was too young for the fusion to be either age related or probably
even life-style related, since at 4 years it couldn't have been used long
for traction.
Interesting. I don't have enough of a sample of Romano-Dutch horses to make any firm statements about the vertebral fusion as a breed or population trait (as Melanie Wilson suggests), but the frequency of the phenomenon in general certainly raises that possibility. I also have an email out to a colleague who is a lifelong rider and has worked as a veterinarian's assistant. If she offers any additional insight, especially from the medical end, I'll be sure to post it here.
 
Erik Filean, M.A.
Department of Anthropology
114 Macbride Hall
The University of Iowa
Iowa City, Iowa, USA 52242