Harker et al., 1990
From dinoflaj3
Harker, S.D., Sarjeant, W.A.S. and Caldwell, W.G.E., 1990: Late Cretaceous (Campanian) organic-walled microplankton from the interior plains of Canada, Wyoming and Texas: biostratigraphy, palaeontology and palaeoenvironmental interpretation. Palaeontographica, Abteilung B, v.219, 243 p., pl.1-13.