Temporal range: Ordovician–Devonian 
Gabbott, Aldridge & Theron 1995
A conodont, Promissum had a primitive mouth under its eyes with mineralized teeth, which are both typical for conodonts, some call it the Niagara Foundation fish. 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.
Promissum was probably capable of maintaining a cruising speed, but not of bursts of speed.
- Gabbott, S.E.; Aldridge, R.J.; Theron, J.N. (1998). "Chitinozoan chains and cocoons from the Upper Ordovician Soom Shale lagerstatte, South Africa; implications for affinity". Journal of the Geological Society 155 (3): 447–452. doi:10.1144/gsjgs.155.3.0447.
- Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 23. ISBN 1-84028-152-9.
- Gabbott, S.E.; R. J. Aldridge; J. N. Theron (1995). "A giant conodont with preserved muscle tissue from the Upper Ordovician of South Africa". Nature 374 (6525): 800–803. doi:10.1038/374800a0.
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