Marthasteritaceae

Marthasterites Deflandre, 1959

Three-armed, monocrystalline calcareous nannofossils exhibit distinct optical properties similar to discoasters. In a specific profile, its optic axis is oriented vertically relative to the microscope stage and appears in constant extinction (dim) under XPL. This extinction occurs because they do not double-refract light in this orientation, resulting in a dark appearance (Pl. 22, Figs. 9-16 in Varol, 2025a).

However, when observed from a different profile under XPL, these nannofossils display birefringence (leading to interference colours) caused by splitting incident light into two rays travelling at different velocities. During this observation, they exhibit parallel extinction, meaning they darken when their vibration directions align with the polariser’s and analyser’s vibration directions (Pl. 22, Figs. 1-8 and 17-24 in Varol, 2025a). Additionally, they demonstrate length-slow (+) elongation (Pl. 22 Figs. 6 and 24 in Varol, 2025a).

These optical characteristics are crucial for identifying and studying three-armed Marthasteritaceae under XPL, providing insights into their structural and compositional features.

Deflandre, G. 1959. Sur les nannofossiles calcaires et leur systématique. Revue de Micropaléontologie 2: 127-152.

Hay, W. W. 1977. Calcareous nannofossils. In: Ramsay, A. T. S. (Ed.) - Oceanic Micropalaentology. Academic Press, London 1055-1200.