I'm a geology grad student at NMSU and have seen many of these cool fossils here in NM. That's a partial fossil of a gastropod shell (a big one too!) made up of limestone. PBK was on the right track by suggesting this is an ammonite. Gastropods can look similar to ammonites, but gastropods lack the "complex sutures" (those lines that go all around the shell would be more "squiggly" on an ammonite) that are characteristic of ammonites.