A few years ago at a rock quarry north of Abilene TX I was told about a great way to cut limestone when it is quite a bit harder than the usual chalky stuff. Whenever your fossil is in a huge rock and you need to go in after it most of us don't carry a portable saw along so a hammer and chisel is the next best thing. Adam A. showed me a chisel with a blade like this:
They cut rock quicker than a flat blade.The notches in the blade seem to make a big difference, maybe because they are taking off smaller chunks on each strike. This chisel is 9” X 5/8” or about 228mm X 15mm.
I found it at a company who makes them for carving marble so they are good quality. They seem a bit pricey but you would probably spend the same if you bought a cold chisel and had a machine shop cut the notches. They would also have to re-temper it for you. If you have the equipment to do this to an existing chisel yourself all you spend is some time so let me know if you do. See if it cuts better than a flat blade.
I found mine at Trow & Holden. On their website click on: Shop Online, Steel Hand Tools, then Steel Marble Hand Chisels. They have several tooth patterns. This is “H” and it's what I was shown at the quarry but I haven't tried the others. Some are clearly meant for sculpting.