Nannotax3 - ntax_cenozoic

Citation: Zygodiscales Young & Bown, 1997

Taxonomic discussion: Taxa included: The extant families Helicosphaeraceae and Pontosphaeraceae and extinct Family Zygodiscaceae. These show highly variable shape, but similar structure, and there is strong palaeontological evidence for their evolutionary connections (Romein 1979; Aubry 1989).

Distinguishing features:
Parent taxon (ntax_cenozoic): Extant coccolithophores and Cenozoic calcareous nannofossils - Mesozoic nannofossils are in a separate module
This taxon: 2N: Heterococcoliths with V-units forming upper/outer cycle of imbricated elements and R-units forming basal plate and central mass of irregular elements. Usually motile
N: Form holococcoliths formed of single block

Original description: Description: Muroliths, and modified descendants, with an outer rim-cycle ofV-units showing anticlockwise imbri- cation and an inner rim-cycle showing clockwise imbrication - the opposite imbrication sense to Eiffellithales. This is a diverse group but with clear evolutionary relationships (e.g. Romein, 1979; Aubry, 1989). Central-area structures include disjunct transverse bars, diagonal crosses and perforate plates but no spines.

Coccolith structure: V-units form outer rim; in Pontosphaeraceae and Zygodiscaceae, this is a narrow cycle of anticlockwise-imbricating elements, in the Helicosphaeraceae, a helical flange. The R-units form a proximal plate of rather regular, inward-growing elements and a distal blanket, which typically appears as a mass of minute, tangentially-elongated crystallites. Growth does not occur downwards from the proto-coccolith ring and so the alternating belt of V-R nuclei remains clearly visible on the proximal surface.

Helicosphaera carteri has been cultured repeatedly (Inouye pers. comm., Probert unpubl. data) and Scyphosphaera apsteinii once (Probert unpubl. data). Life-cycle transitions have been observed in Helicosphaera cultures, and combination coccospheres have been observed for Helicosphaera (Cros et al. 2000, Geisen et al. 2002), Pontosphaera and Scyphosphaera (Frada et al. 2009). These indicate that the haploid phase forms holococcoliths with distinctive rhomboid-array ultrastructure - these were previously placed in the genus Syracolithus.

Geological Range:
Last occurrence (top): Extant. Data source: Total of ranges of the species in this database
First occurrence (base): within NP1 zone (65.47-66.04Ma, base in Danian stage). Data source: Total of ranges of species in this database

References:

Aubry, M. -P. (1989a). Handbook of Cenozoic calcareous nannoplankton. Book 3: Ortholithae (Pentaliths, and others), Heliotithae (Fasciculiths, Sphenoliths and others). Micropaleontology Press, American Museum of Natural History, New York. 1-279. gs

Cros, L., Kleijne, A., Zeltner, A., Billard, C. & Young, J. R. (2000b). New examples of holococcolith-heterococcolith combination coccospheres and their implications for coccolithophorid biology. In, Young, J. R., Thierstein, H. R. & Winter, A. (eds) Nannoplankton ecology and palaeocology, Proceedings of the INA7 conference, Puerto Rico, February 1998. Marine Micropaleontology . 39(1-4): 1-34. gs

Frada, M., Percopo, I., Young, J. R., Zingone, A., de Vargas, C. & Probert, I. (2009). First observations of heterococcolithophore-holococcolithophore life cycle combinations in the family Pontosphaeraceae (Calcihaptophycideae, Haptophyta). Marine Micropaleontology. 71(1): 20-27. gs

Geisen, M., Billard, C., Broerse, A. T. C., Cros, L., Probert, I. & Young, J. R. (2002a). Life-cycle associations involving pairs of holococcolithophorid species: intraspecific variation or cryptic speciation? European Journal of Phycology. 37: 531-550. gs

Perch-Nielsen, K. (1985). Cenozoic calcareous nannofossils. In, Bolli, H. M., Saunders, J. B. & Perch-Nielsen, K. (eds) Plankton Stratigraphy. Cambridge University Press, Cambridge (1): 427-555. gs

Romein, A. J. T. (1979). Lineages in Early Paleogene calcareous nannoplankton. Utrecht Micropaleontological Bulletin. 22: 1-231. gs O

Young, J. R. & Bown, P. R. (1997). Cenozoic calcareous nannoplankton classification. Journal of Nannoplankton Research. 19(1): 36-47. gs

Young, J. R., Geisen, M., Cros, L., Kleijne, A., Probert, I. & Ostergaard, J. B. (2003). A guide to extant coccolithophore taxonomy. Journal of Nannoplankton Research. S1: 1-132. gs

Daughter taxa (time control age-window is: 0-800Ma)		Granddaughter taxa
	Helicosphaeraceae Varimorphic; liths placolith-like but with helical shield structure; arranged in spiral on coccosphere	Helicosphaera
	Pontosphaeraceae Monomorphic or dimorphic; liths muroliths with perforate central area	Pontosphaera Scyphosphaera
	Zygodiscaceae Muroliths with rim formed from well-developed V- and variably developed R-units, with opposite imbrication directions, and central area structures that include disjunct transverse bars and diagonal crosses. R-units vestigial or not observed in several genera (Chiphragmalithus, Isthmolithus, Neococcolithes).	Lophodolithus Neochiastozygus Zygodiscus Chiphragmalithus Isthmolithus Neococcolithes Nannotetrina

Zygodiscales

References:

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