Calcidiscaceae

Thu, 11/10/2007 - 14:20 — Jeremy Young

FAMILY CALCIDISCACEAE YOUNG AND BOWN, 1997

Description: Coccoliths are placoliths with rim structure similar to Calcidiscus, i.e. the V-unit forms distal shield and central-area/tube; R-unit forms proximal shield only. As in the Coccolithaceae, growth occurs downward from the proto-coccolith ring which becomes embedded within the structure so that alternating V- and R-units are only visible on specimens where the proximal shield has broken off.

Distal shield sutures usually show laevogyral curvature. The proximal shield is usually formed of a single layer of elements with sub-radial sutures; sometimes a lower layer is developed, with elements showing strong dextral obliquity (in proximal view). The connection between the proximal and distal shields is weak and they frequently separate.

Genera included:

Calcidiscus	Circular to broadly elliptical with closed central area or narrow opening	Paleocene-Recent
Umbilicosphaera	Circular to broadly elliptical with wide central opening	Paleocene-Recent
Oolithotus	Sub-circular with proximal shield off-centre	Pliocene to Recent
Hayaster	Nearly flat, with few elements and small proximal shield	Eocene-Recent
Cryptococcolithus	Ellptical with grill in central area	Miocene
Ceratolithus HET (old name Neosphaera)	Only one shield, circular. These do NOT belong in the Calcidiscaceae, but they do look rather similar	Pliocene-Recent

NB All these genera were included within the Coccolithaceae in older classifications but were separated by Young & Bown (1987) into the family Calcidscaceae on the grounds of them sharing a simpler coccolith structure than that of the Coccolithaceae. The grouping has been supported by molecular genetic data (Sáez et al., 2003, 2004).

Remarks: Dominant phase of life-cycle non-motile with placolith heterococcoliths. Holococcolith bearing motile phase has been documented in the extant species C. leptoporus and C. quadriperforatus (refs. Gesen et al. 2002, Houdan et al. 2004).

Selected references: Inouye & Pienaar (1984) - cytology of Umbilicosphaera; Kleijne (1993) - detailed description of extant species; Knappertsbusch (2000) Calcidiscus evolution; Young et al. (2004) - coccolith structure.

Syracolithus schilleri
Thu, 03/01/2008 - 10:03 — Jeremy Young

Syracolithus schilleri (Kamptner 1927) Loeblich & Tappan 1963

Description: Coccolith structure - coccoliths are open tubes with internal septae (walls). The outer tube wall shows hexagonal fabric and the crystallites have
radial c-axes (birefringent in plan view in LM). The fabric of the
septae is not obviously different but they are non-birefringent in plan
view in LM and so the calcite crystallites must have vertical c-axes.

Coccospheres monomorphic, nearly spherical.

Coccolith form - the internal septae define 8-20 pores, tube 10-15 crystallites high, upper surface of coccolith convex; higher in centre than edges. Essentially similar to C. quadriperforatus HOL but larger and with more pores.

Remarks:

Original description:

Size: Liths 3-4 µm long.

Possible life-cycle association: S. schilleri is so similar to the Calcidicus quadriperforatus holococcolith phase that it seems likely that it is a closely related species and so a member of the Calcidiscaceae. This might, however, be a species of Oolithotus or Umbilicosphaera rather than Calcidiscus, so in the absence of direct evidence the species is left in its traditional holococcolith genus Syracolithus.

Synonym:

Holodiscolithus macroporus (Deflandre in Deflandre and Fert, 1954) Roth, 1970 - S. schilleri is sporadically present in Neogene sediments but it is usually identified as H. macroporus, which is probably a junior synonym.

Comment and discussion:
Calcidiscaceae Young & Bown 1997
Fri, 06/02/2009 - 12:13 — Jeremy Young

Comment and discussion: