The genus Paracostellagerina nov. gen. is proposed to accommodate hedbergellid taxa bearing asymmetrical test ornamentation, with meridional costellae on the umbilical side and costellae oriented parallel to the periphery on the spiral side. Test ornamentation is a peculiar feature of the genus, with transitional forms being recently placed in a new species Hedbergella praelibyca by Petrizzo and Huber (2006). The asymmetric test ornamentation and extraumbilicalumbilical primary aperture separate Paracostellagerina nov. gen. from the early Santonian-early Campanian genus Costellagerina. The latter generally has a meridional ornamentation pattern on both sides of the test, although variations in test ornamentation were observed by Huber (1994).

Differences in test ornamentation, position of the primary aperture, and nature of the periapertural structures provide the impetus to distinguish Hedbergella libyca and Rugoglobigerina pilula as the type species of the two monospecific genera Paracostellagerina nov. gen. and Costellagerina. The two species were considered congeneric in the past by Loeblich and Tappan (1988, p. 462) and Premoli Silva and Sliter (1995, pl. 1, fig. 8) under Costellagerina, and by El-Nakhal (2002) under his genus Meridionalla. Meridionalla was based on the figures of the holotype of Hedbergella murphyi, which was named by Douglas (1969). Re-examination of the SEM photographs of the Hedbergella murphyi holotype by Petrizzo and Premoli Silva (2000) showed that there is no meridional ornamentation as was illustrated in the original figure. Therefore Meridionalla lacks taxonomic significance. Separation of Costellagerina and Paracostellagerina nov. gen. is supported by absence of forms with similar test morphology between the extinction of Paracostellagerina libyca and the first occurrence of Costellagerina pilula. This stratigraphic interval ranges from the earliest Cenomanian to the earliest Santonian and spans approximately 13 myr.
Phylogeny. Paracostellagerina nov.gen.  evolved from  hedbergellids that  bear  dense, randomly  scattered  pustules.  Hedbergella  praelibyca was  recently  proposed   to  accommodate  a  distinct  taxon   with  intermediate  morphological features between Hedbergella delrioensis and Paracostellagerina libyca (Petrizzo and Huber,  2006). Examination of the  type  material   of  Hedbergella   praelibyca  from   Ocean   Drilling Program   (ODP)  Sites  1050C  and  1052E  (Blake  Nose,  subtropical North  Atlantic)  showed  that  the  surface  texture  of  this  species has incipient,  randomly  arranged,  short  costellae  and  scattered  pustules. The ornamentation pattern  sharply  differs from  that  of the ancestor and descendant  species, Hedbergella delrioensis and Paracostellagerina libyca, respectively. The asymmetrical ornamentation pattern  in Paracostellagerina libyca is distinct  from  that  of the ancestral  species in presenting  preferential  arrangement of the ornamentation elements. Such complex ornamentation, meridional on the umbilical side and parallel to the periphery  on the spiral side, is unique  among  the early and middle Cretaceous  planktonic  foraminifera.

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