Fossil ceratiaceae: A restudy and new TAXA from the mid‐cretaceous of the western interior, U.S.A.

Abstract

Abstract Dinoflagellate cyst assemblages from the upper Albian Kiowa Formation in Kansas contain 13 species assignable to the Ceratiaceae. Three genera, Nyktericysta, Vesperopsis, and Balmula, are described as new, as are eight species, Odontochitina ancala, O. rhakodes, Pseudoceratium interiorense, Nyktericysta davisii, N. arachnion, Vesperopsis mayi, V. nebulosa, and Balmula tripenta. Several of these species occur elsewhere in the Western Interior in mid‐Cretaceous sediments. Morphologically, ceratioid cysts fall into three groups. The first contains Odontochitina Deflandre 1935 and Xenascus Cookson & Eisenack 1969 (which differs only in bearing spines) and is characterized by uncompressed cysts with one lateral horn. The second group contains Pseudoceratium Gocht 1957 (herein emended to include Aptea Eisenack 1958), Endoceratium Voz‐zhennikova 1965, and possibly some species of Cyclonephelium Deflandre & Cookson 1955 in which cysts are dorsoventrally compressed and have one (or no) lateral horn. The third group encompasses compressed cysts with paired lateral horns and contains Muderongia Cookson & Eisenack 1958, Phoberocysta Millioud 1969, Australisphaera Davey 1978, and the three new genera described herein. Some specimens of Nyktericysta davisii are fully paratabulate, combining plate relationships variously observed on other fossil and modern Ceratiaceae. The contrast between ceratioid paratabulation and that exhibited by dorso‐ventrally compressed gonyaulacoid cysts is stressed. For nontabulate cysts, a difference in plate relationships revealed by the archeopyle suture appears useful in distinguishing the two groups. In the ceratioids the first precingular paraplate is planate, whereas in the gonyaulacoids it is typically camerate.