Isocrystallithus compactus Verbeek, 1976
Isocrystallithus is a holococcolith characterized by a prominent hollow distal process, which either tapers or remains parallel-sided and terminates with a plug. The complete specimens naturally settle in a side-view position. From the side view, Isocrystallithus exhibits parallel extinction with length-slow (+) elongation.
In the plan view, the holococcolith’s base is noted for its narrow rim and central opening surrounded by struts. In the plan view, Isocrystallithus displays parallel extinction with length-fast (-) elongation.
Isocrystallithus is distinguished from Kahvecia, Owenia, and Swapansahooia by its parallel extinction pattern observed at both the base and distal processes. In contrast, Kahvecia, Owenia, and Swapansahooia exhibit inclined extinction patterns.
Both Isocrystallithus and Sarmania exhibit parallel extinction , but differ in the morphology of their distal processes and the base. In Isocrystallithus, the distal process is hollow, whereas in Sarmania, it is solid and comprises four ridges. The base of Sarmania, as viewed in the plan, features a rim and two pores separated by the base of the solid distal process, which appears as a diagonal cross in the plan view. In comparison, the base of Isocrystallithus in the plan view has a narrow rim and a central opening surrounded by struts. The side view of the base in Sarmania reveals two levels and the ridges of the distal process.
Verbeek, J. W. 1976. Upper Cretaceous calcareous nannoplankton from Ballon and Théligny in the type area of the Cenomanian stage (Sarthe, France). Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen. B79: 69-82.