Does Furcatolithus akropodus have two morphotypes useful for biostratigraphic purposes?

The genus Furcatolithus is derived from Sphenolithus ; its rapid biological evolution and high diversity make its species excellent markers. This study aims to investigate the distribution of two morphotypes of Furcatolithus akropodus in three ocean basins: the equatorial Pacific Ocean (IODP Hole U1333A), the equatorial Indian Ocean (ODP Hole 711A), and the South Atlantic Ocean (ODP Hole 1265A). The two morphotypes differ by the presence or absence of a midline or groove on the bifid spine, visible under a light microscope, which determines different extinction patterns. Following the approach outlined in previous work, we initially named the form without a line or groove (morphotype A) and the form with a line or groove (morphotype B). To classify the specimens of the species under study, the identification criteria proposed by Howe (2021) were followed. Ultrastructural analysis by scanning electron microscopy (SEM) suggests that morphotype A presents a bifurcation of the bifid spine, probably lateral to morphotype B, in which a central groove or line of the bifid spine is clearly visible, starting from its base and extending to its apex. At the biostratigraphic level, in all three analysed holes, a simultaneous lowest occurrence (LO) of the morphotypes is observed (located at the base of the CNO1 zone), while the highest occurrence (HO) of F. akropodus B precedes that of F. akropodus A, both located in the CNO3 zone. The highest occurrence of F. akropodus A appears to be contemporaneous with the lowest occurrence of F. distentus (base of CNO4 zone). The biostratigraphic order of events resulting from the three holes suggests the biostratigraphic potential of F. akropodus at mid- and low latitudes. The original description, different morphology, and different biostratigraphic distribution allow us to designate F. akropodus A as F. akropodus sinsu strictu and F. akropodus B as F. xiphopodus sp. nov.