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Ireneusz Walaszczyk, University of Warsaw (Poland)
W.M. John Jagt, Natuurhistorisch Museum Maastricht (SCZ) (Netherlands)
Norbert Keutgen, Natuurhistorisch Museum Maastricht (SCZ) (Netherlands)
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The inoceramid bivalve "Inoceramus" ianjonaensis Sornay, 1973 was a widely distributed species during mid-Maastrichtian time. It is best documented from all around the African continent, having been first described from Madagascar and later recorded from Libya, Egypt, Tunisia, Nigeria and South Africa. It is also well known from Europe, where it is represented by "Inoceramus" morgani Sornay, a form interpreted to be its geographical subspecies. In Europe, "I." ianjonaensis morgani occurs in the Maastrichtian type area, the Netherlands-NE Belgium, the Calcaire à Baculites, northwest France, and at the Bay of Biscay sections, northern Spain. Although poorly documented, the species appears to be present in the US Western Interior as well. Thus, "I." ianjonaensis extended throughout the entire Euramerican region of the North Temperate Realm, in the Tethyan Realm as well as the East African Province of the South Temperate Realm. The species usually occurs in mass abundance and in a narrow stratigraphical interval. The evolutionary interpretation of the species is not yet entirely clear, but it seems to have evolved separately from the "basic" Cataceramus lineage during the mid-Maastrichtian, and is not linked directly with other radially ribbed Maastrichtian inoceramids of the genus Trochoceramus. In ammonite terms, the species was documented to appear first at the approximate level of the first occurrence of Anapachydiscus fresvillensis, which has occasionally been proposed as a good proxy for the base of the Upper Maastrichtian. In the stratotypic Maastrichtian it is shown to appear at, or close to, the base of the Belemnitella junior Zone, the basal Upper Maastrichtian zone in the classic European belemnite subdivision of the stage. The top of the range zone of "I." ianjonaensis marks the highest occurrence of "true" inoceramids. The first appearance of "I." ianjonaensis is thus a very convenient marker for the base of the Upper Maastrichtian and potentially may be considered to be used in its formal definition.
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