Hollow structures in radiolarian skeletons

Spines are the major element of radiolarian skeletons; they are usually solid or, rarely, hollow. Subcylindrical hollow spines and other hollow structures of radiolarian skeletons appeared in the Middle Cambrian. Later, hollow skeletal structures repeatedly developed in the evolution of all radiolarian classes of the subphylum Polycystina throughout the Phanerozoic to the Recent. The analysis of appearance and distribuion in the Phanerozoic of morphotypes with hollow spines has shown that the formation of hollow skeletal structures allowed better adaptation for the planktonic mode of life and could become a key adaptation in struggle for life under stressful conditions.