Resumen de The Sphaeroidinella gens from Miocene till Recent
In their wall-structure specimens from the Miocene (Sphaeroidinellopis seminulina (Schwager) = S. subdehiscens Blow) do not differ from later forms of this gens. The walls consist of an inner calcareous lamella, covered by the primary pseudochitinous lamella, succeeded by a second calcareous lamella in which eventually spines can be detected, and then a third thicker calcareous lamella is added, as in Globigerina rubra (Hofker, 1971, Pl. VI). The difference with Sphaeroidinella is that dorsal secondary apertures are lacking. But young specimens of S. dehiscens from Basal Pleistocene in a JOIDES sample lack spece openings also, whereas the chambers following the embryonic ones all of the spines are found in the form of fine open canals which do not pierce show the third thickening lamella. In these pleistocene individuals remmants the chitinous lamella nor the primary calcitic one. Last formed embryonic chambers may show traces of these spines at their outsides. Young specimens of the Recent form, S. excavata Blow, are built as the Miocene ancestor. This recent form shows bullae in many specimens.
During this evolution from Miocene to Recent the gens gradually increases its size in the adult specimens, as is found in many other planktonic gentes. Ontogenetically as well as phylogenetically this gens shows no proof of the theory proposed by Bé, Hemleben and Bandy, that Sphaeroidinella is the deep water form of Globigerina sacculifera Brady. The forming of secondary apertures in the later larger forms of this gens is also found in many other planktonic gentes increasing their size during time, as is the forming of bullae. There is no reason to split off the genus Sphaeroidinellopsis from Sphaeroidinella as the young specimens of the latter genus should in that case be put in Sphaerodinellopsis. In the Recent form the hollows left by the spines are filled up with calcite when the third thickening lamella is formed.
