If some of the extremes were found as fossils and the intermediates were
not known, they would surely be given different species names, probably be
given different generic names (as the dinosaur varieties are), and possibly
even divided at the family level.
Even living canids which are known to be able to interbreed are given
different generic names.
Someone requested the article reference -- I don't remember who. It
quantifies some of the variation. The reference:
R.K. Wayne. 1986. Cranial morphology of domestic and wild canids: the
influence of development on morphological change. Evolution 40(2):243-261.