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TAXONOMIC REVISION OF THE LATE CENOMANIAN PLANKTONIC FORAMINIFERA ROTALIPORA GREENHORNENSIS (MORROW, 1934)

¹ Department of Paleobiology, MRC NHB 121, Smithsonian National Museum of Natural History, Washington, D.C. 20013-7912, USA.
² Department of Earth Sciences, Faculty of Science, Chiba University, Chiba 263-8522, Japan.
³ Now at Research Institute for Humanity and Nature, Motoyama 457-4, Kamigamo, Kita-ku, Kyoto 603-8047, Japan.

⁴ Correspondence author. E-mail: ando{at}chikyu.ac.jp

Rotalipora greenhornensis has long been considered a representative planktonic foraminiferal species of the single-keeled Rotaliporinae of late Cenomanian age. Its taxonomic identity, however, is still ambiguous because this taxon has been distinguished by morphological criteria (numerous, strongly curved chambers, etc.), whereas the other species of Rotalipora are distinguished primarily by their apertural and ornamental characters (raised sutures, periumbilical ridges, etc.). The presence of strongly raised sutures throughout the umbilical and spiral sides has also been used to identify R. greenhornensis, but this criterion is troublesome as it also applies to its direct ancestor Rotalipora globotruncanoides. Reexamination of the holotype and observation of many topotypes and some North Atlantic and Pacific hypotypes reveals that, despite its remarkable morphological variability, R. greenhornensis is distinguishable by having umbilical sutures that are not fully raised, with the last several being depressed, and supplementary apertures that are umbilical in position and surrounded by a continuous series of closely-spaced periumbilical ridges. Moreover, reduction of the raised sutures tends to occur simultaneously on both the umbilical and spiral sides. Such apertural and ornamental characters of R. greenhornensis serve as its primary distinguishing features, allowing a clear-cut differentiation of this taxon from other species of Rotalipora, particularly the closely associated species R. globotruncanoides and plano-convex Rotalipora deeckei.

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