Phylogenetic and Taxonomic Problems of Some Tertiary Planktonic Foraminiferal Lineages

Abstract

Some basic principles of taxonomy and classification are considered in relationship to planktonic foraminifers, along with various problems which face the specialist in his attempts to arrive at a suitable and "natural" classification of this group. The phylogenetic development of five major lineages of Tertiary planktonic foraminifers is discussed and the more important species illustrated. The definition of the genus Globorotalia includes keeled and non-keeled forms and its range is Danian to Recent. Globorotalia pseudobulloides (Plummer) is a highly polytypic species and the ancestral form of all later Tertiary members of the Globigerinacea with the exception of the Heterohelicidae, which had their own, separate evolutionary development. Globigerina daubjergensis Bronnimann (= Globoconusa daubjergensis Bronnimann) is classified in the Guembilitriinae; derivation from Guembelitria near the top of the Maestrichtian is suggested. Four lineages within the genus Globorotalia include: 1) leading towards low-conical, keeled and non-keeled forms (G. spiralispusilla- convexa-broedermanni), extinct within the middle Eocene; 2) leading towards compressed, keeled and non-keeled forms (G. compress a- pseudomenardii- chapmaniplanoconica- pseudoscitula), extinct at the top of the middle Eocene; 3) leading to low to high conical forms ("conical globorotaliids") , extinct at the top of the middle Eocene; and, 4) leading to the Neogene and Recent globorotaliids. The acarininids, lineage 5, are characterized by species which are distinguished from Subbotina and Globorotalia by strongly spinose wall texture. Two, and possibly three, branches within this lineage lead independently, through parallel trends, to the development of the polytypic, but monophyletic, genus Truncorotaloides. In terms of normal criteria of divergence (gaps) and monophyly, the acarininids are recognized as a genus, emended to include primarily spinose forms. The genus Truncorotaloides is also accepted, although it seems unlikely that this genus warrants a separate subfamily. On the other hand, there is insufficient reason for the erection of new genera or subgenera for the various branches of globorotaliids in the Paleogene and Neogene. A conservative, relatively unspecialized stock, or "mainstream," leading through Globorotalia wilsoni (Cole) and G. opima nana Bolli in the Eocene survived the planktonic diminution at the close of the Eocene and was the source of further progression and splitting of various branches leading to the wide divergence of forms in the Neogene and, ultimately, to the globorotaliid faunas in the present day seas.