Order ISOCHRYSIDALES Pascher 1910
Families included: Noelaerhabdaceae, the Palaeogene Family Prinsiaceae (see Young & Bown 1997) and the extant family of non-calcifying haptophytes Isochrysidaceae (Edvardsen et al. 2000). The order Isochrysidales is used instead of Prinsiales Young & Bown, 1997, since it has priority.
Grouping of the Isochrysidaceae and Noelaerhabdaceae is based on flagellar characters (haptonema vestigial) and is supported by biochemical characters (production of alkenones) and molecular genetics (Edvardsen et al. 2000; Fujiwara et al. 2001; Sáez et al. 2004). Grouping of Prinsiaceae and Noelaerhabdaceae is based on coccolith structure and stratophenetic data (Young et al. 1992; Young & Bown 1997).
Description: Coccoliths are placoliths, but unlike the Coccolithaceae, growth does not occur downward from the proto-coccolith ring. The R-unit is always well developed, forming a proximal shield element, two tube-elements with opposite senses of imbrication, and usually a central-area element. The locus of the proto-coccolith ring is usually marked by a ring of slits on the proximal surface. Central-area structures are always conjunct, being formed from either the central-area element or the inner tube-element of the proximal shield.
In the Prinsiaceae, the V-unit is well developed (Toweius-type structure); forming an upper layer to the proximal shield, an outermost tube and the distal shield.
In the Noelaerhabdaceae, the V-unit is virtually absent (Reticulofenestra-type structure) and the outer of the two R-unit tube-cycles is extended to form the distal shield.