A conodont, Promissum had a primitive mouth under its eyes with mineralized teeth, which are both typical for conodonts, and also a primitive backbone. It probably looked like a small eel or large worm, lacking any kind of fins except for perhaps a small one on the tail. It was relatively large for a conodont, reaching about 40 centimetres (16 in) in length.
Well-preserved specimens were discovered in the Soom shale of South Africa in 1994.
Promissum was probably capable of maintaining a cruising speed, but not of bursts of speed