ABSTRACT

The evolutionary line which leads to the living Sphaeroidinella dehiscens (Parker and Jones) has been divided into two genera, at least six species, and several subspecies. The sequence of forms is discussed here as a temporal cline.

Irrespective of nomenclature, the evolutionary forms of Sphaeroidinella s. l. have proven stratigraphically useful in the Upper Miocene and younger sediments of Louisiana. Development progressed from a trochospiral form with discrete, inflated chambers and a single aperture, to the living form in which the coiling pattern is obscure, the chambers are merged, and multiple apertures are present. Single aperture forms (Sphaeroidinellopsis of Banner and Blow) disappeared at the end of the Pliocene. Late Miocene and Earliest Pliocene specimens have a thickened apertural lip at maturity. Pleistocene and Recent specimens resemble irregularly ruptured spheres. The lips of the several apertures tend to be recurved and are never thickened. The coiling spiral becomes lower with time. Whereas there is a trend in the direction of fewer chambers in the final whorl of stratigraphically younger forms, this feature must be used with great caution since juveniles always have fewer chambers than adults. The safest single index to stratigraphic age is the degree of chamber separation. All stages of gradation between "species" can be observed.

If one uses the Globoratalia fohsi forms to mark time datums, then the stratigraphic ranges of Sphaeroidinella forms in Louisiana are not contemporaneous with their ranges in the Caribbean region. The evolution of Sphaeroidinella is apparently similar in both areas.

There is some evidence that Sphaeroidinella dehiscens is a deep-water variant of Globigerinoides sacculifer. Benthonic associates of fossil specimens suggest that Sphaeroidinella did not require great water depths in the past.