A review of the areoligeracean dinoflagellate cyst <i>Cyclonephelium</i> and morphologically similar genera
Creators: name: Fensome, Robert A.
name: Williams, Graham L.
name: Wood, Stephanie E. L.
name: Riding, James B.
Publisher: name: Taylor & Francis
Year: 2019
Subjects: subject: Evolutionary Biology
subject: FOS: Biological sciences
subject: FOS: Biological sciences
subject: 59999 Environmental Sciences not elsewhere classified
subject: FOS: Earth and related environmental sciences
subject: FOS: Earth and related environmental sciences
subject: Ecology
subject: Developmental Biology
subject: Inorganic Chemistry
subject: FOS: Chemical sciences
subject: FOS: Chemical sciences

description: A group of mainly Cretaceous areoligeracean dinoflagellate cyst genera, which we call the ‘<i>Cyclonephelium</i> group’, has proved difficult to classify. The group comprises <i>Aptea</i>, <i>Canningia</i>, <i>Canninginopsis</i>, <i>Cassidium</i>, <i>Cauveridinium</i>, <i>Cerbia</i>, <i>Circulodinium</i>, <i>Cyclonephelium</i>, <i>Doidyx</i>, <i>Senoniasphaera</i> and <i>Tenua</i>. As a group, they also converge morphologically with ceratiaceans. <i>Cyclonephelium-</i>group taxa show considerable morphological diversity and gradation. However, the most important criteria for discriminating dinoflagellate cyst-based genera – tabulation and archaeopyle type – are uniform among areoligeraceans and ceratiaceans and so are not useful in this case. Any subdivision of the <i>Cyclonephelium</i> group will break apparently natural continuities; nevertheless, any resolution must involve a hierarchy of morphological criteria. In developing a ‘best-fit’ hierarchy of morphological criteria for the <i>Cyclonephelium</i> group and its separation from ceratiaceans, we consider historical concepts, morphological variation, illustrations in the literature (especially of types) and taxonomic stability. We conclude that the most pragmatic distinguishing feature of ceratiaceans in contrast to the <i>Cyclonephelium</i> group (and areoligeraceans in general) is that the former possess a lateral horn or distinct prominence on the inner body or wall. The hierarchy we favour within the <i>Cyclonephelium</i> group (in decreasing importance) is: (i) wall structure; (ii) whether the ornament is linear or free standing; and (iii) the distribution of the ornament. As a consequence, we propose one new genus (<i>Trimuridinium</i>), one new species (<i>Aptea cassis</i>), two new names (<i>Canningia glomerata</i> for <i>Senoniasphaera rotundata</i> and <i>Circulodinium vectensis</i> for <i>Pseudoceratium distinctum</i>), 49 new combinations and one new status. We emend the descriptions/diagnoses of <i>Aptea</i>, <i>Circulodinium</i>, <i>Cyclonephelium</i>, <i>Pseudoceratium</i>, <i>Senoniasphaera</i>, <i>Tenua</i> and <i>Tenua hystrix</i>. <i>Cyclonephelium</i>-group taxa predominate in neritic marine palaeoenvironments, and their use in palaeoecological analyses should be improved by a more cohesive and consistent taxonomy. The group may have been the root stock for ceratiaceans in the Late Jurassic, the two families becoming more clearly separate from the Late Cretaceous onwards. <i>Cyclonephelium</i>-group areoligeraceans are sparse in the Paleogene and confirmed species are absent in the Neogene.