Alfred R. Loeblich

Peridinialean cyst affinity rather than gymnodinialean motile stage of the late cretaceous dinoflagellate dinogymnium

TAPPAN, H. & LOEBLICH, A. R. JR. 1977. Peridinialean cyst affinity, rather than Gymnodinialean motile stage, of the Late Cretaceous dinoflagellate Dinogymnium. Trans. Amer. Micros. Soc., 96: 497-505. Morphological comparison of fossil Dinogymnium to both motile cells and cysts of living representatives of the orders Gymnodiniales and Peridiniales suggests an affinity with the modern family Lophodiniaceae (Peridiniales). The thick, acid-resistant, highly punctate to perforated wall resembles the exospore layer of the resting zygote cyst of Woloszynskia and probably of Lophodinium. The meridional ridges of some Dinogymnium, interrupted only by the median cingulum, are similar to features of Lophodinium. The archeopyle of Dinogymnium develops at the time of excystment. The rarely present flagellar pores result from the late retention of the flagella by the planozygote, prior to completion of the thick cellulosic endospore layer of the wall of the later hypnozygote stage. Within the perforated exospore, this thicker endospore layer protects the pre-meiotic zygote, although the cellulosic composition precludes its preservation. Other than the endoskeletal Actiniscaceae, all fossil dinoflagellates thus appear to represent cysts, and from analogy with modern species, probably all are hypnozygotes.