It looks like elm to me as well. However, someone else mentioned locust and I have no experience cutting or splitting that wood. So, I can't say one way or the other on that.
The final confirmation for me is the smell. Elm has a very unique smell, and I can tell a mile away
Different varieties of elm split different. All are more difficult than your average hardwood. It ranges from "not too bad" to "darn near impossible." Some varieties of elm are so twisted and stringy that even when two pieces are split a part, "strings" can still be attached. That type is a real pain.
The other bad thing about elm is that the greener it is, the easier it is to split. When that twisted, stringy grain starts drying, it tightens everything up making it really tough.
Sounds like you had two things working against you there....twisted grain and had been dried awhile.
B