Re-examination of Archaeomonas mirabilis from the Late Cretaceous reveals its true identity as the oldest known fossil Parmales (Bolidophyceae)

Abstract

ABSTRACT SEM observations on siliceous microfossil assemblages of Late Cretaceous age from the southwest Pacific revealed the presence of bona fide Parmales. The subspherical tests are similar in morphology (i.e. fused plates) and diameter (4–6 µm) to those of extant Tetraparma silverae from the Sulu Sea and South China Sea, and of an undescribed form in Quaternary sediments from the Mexican Pacific. Previously published light micrographs of the archaeomonad Archaeomonas mirabilis are seemingly identical to those seen in other samples from the same core, and SEM micrographs from the type sample confirm that A. mirabilis is a fossil parmalean, which we classify as Tetraparma mirabilis comb. nov. Two specimens from another sample revealed the presence of hat-shaped shield and ventral plates, still attached to or disarticulated from the test. Since only a poorly preserved plate was seen in the type sample, specimens from other samples are collectively referred to as Tetraparma aff. mirabilis. The shape of these detachable plates provides species-specific differences; a flattened dome with no ornaments (Tetraparma aff. mirabilis), a flattened dome with central knob (Mexican specimens), a high dome with leaf-like ornament (T. silverae). The fused test plates of these three taxa probably increase their preservation potential in the sediments, whereas more delicate taxa have abutting or interlocking plates that are easily disarticulated and dissolved. The identification of T. mirabilis from the Late Cretaceous significantly extends the stratigraphic range of the Parmales and supports the general consensus that they may be ancestral to the diatoms.