Sphenolithus heteromorphus

Sphenolithus heteromorphus Deflandre, 1953

Small to very large (3.5-32.4µm) species of Sphenolithus Deflandre in Grassé, 1952 having segments arranged in three cycles. It is having an almost quadrate base with well-developed and about equal size proximal and lateral cycles. The frustum shape proximal cycle shows a hollow conical cavity that is open proximally.

The tapering, three-blade apical spine is extinct (dim) at 0° but show maximum birefringence at 45°. In well preserved, almost in pristine condition, specimens show that three blades placed at 120° to each other. Under adverse preservational conditions, the blades fuse, altering their appearance.

The apical spine naturally settles with the keel of the middle blade facing outward, exposing the face of the two outer blades. The thickness and the width of the blades increase considerably towards the proximal end.

Sphenolithus heteromorphus differs from Sphenolithus belemnos Bramlette & Wilcoxon,1967 by having a quadrate base, whereas the latter has an elongated base. The proximal cycle is frustum shape in Sphenolithus heteromorphus but cylindrical with or without minor outward proximal flaring in Sphenolithus belemnos. Moreover, the segments of the proximal cycles are different in the axial section. Sphenolithus heteromorphus almost kite shape segments, but Sphenolithus belemnos has irregular quadrilateral shape segments.

Sphenolithus heteromorphus is distinguished from Sphenolithus preasii Bergen & de Kaenel in Bergen et al., 2017 and Sphenolithus milanetti Maiorano & Monechi, 1998 by having a frustum shape proximal cycle. In contrast, Sphenolithus preasii has an open umbrella shape lateral cycle, and Sphenolithus milanetti has a semi-open umbrella shape lateral cycle.

Bergen, J. A., de Kaenel, E., Blair, S. A., Boesiger, T. M., Browning, E., 2017. Oligocene-Pliocene taxonomy and stratigraphy of the genus Sphenolithus in the circum North Atlantic Basin: Gulf of Mexico and ODP Leg 154. Journal of Nannoplankton Research 37(2-3), 77-112.

Bramlette, M. N., Wilcoxon, J. A., 1967. Middle Tertiary calcareous nannoplankton of the Cipero section, Trinidad, W.I. Tulane Studies in Geology and Paleontology 5, 93-131.

Deflandre, G., 1952. Classe des Coccolithophoridés. (Coccolithophoridae. Lohmann, 1902). In: Grassé, P. P. (ed.), Traité de Zoologie. Masson, Paris 439-470.

Deflandre, G., 1953. Hétérogénéité intrinsèque et pluralité des éleménts dans les coccolithes actuels et fossiles. Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences, Paris 237, 1785-1787.

Maiorano, P., Monechi, S., 1997. New Early Miocene species of Sphenolithus Deflandre, 1952 from the North Atlantic Ocean. Journal of Nannoplankton Research 19(2), 103-107.