:
: http://www.livescience.com/7488-world-strangest-creature-part-mammal-part-reptile.html

: http://en.wikipedia.org/wiki/Synapsid

Technically they can't be any of those things because the categories aren't arbitrary - at least they haven't been since we understood why animals fall into categories and what they mean.

TL;DR they are the result of descent with modification.

Jackals didn't descend from any earth species. They don't have a common ancestor with anything on earth. Unless this common ancestor was some spore, something pre-bacteria/archea/viral. If this was the case they still wouldn't be members of any of those categories (clades) because those clades are derived from the, supposed, common ancestor-spore-thingy.

A simple tree (that assumes common ancestry) would be:

spore-thingy
|
|- lots of things that we don't know about -> jackals
|
|- eukaryotes -> metazoa -> amniotes -> sauropsids

jackals are not sauropsids