ORIGIN AND EARLY EVOLUTIONARY HISTORY OF THE ORDER LAGENIDA (CLASS FORAMINIFERA)

The order Lagenida is a monophyletic group of calcareous foraminifers that originated in Middle Pennsylvanian time via acquisition of hyaline-radial wall structure and loss of microgranular wall structure, the latter being characteristic of the close sister group and likely ancestor, the Fusulinida. Early lagenides are delineated hierarchically into subgroups on the basis of presence or absence of partitioning within their tests, and among partitioned forms, on continuous versus discontinuous growth styles. Partitioned, discontinuously growing forms may be further delineated on the basis of radial versus bilateral symmetry (as seen in transverse section) and on modifications to chamber shape and apertural complexity. Early lagenides underwent a phase of rapid taxonomic differentiation during late Moscovian and early Kasimovian time. Taxonomic differentiation was accompanied, and possibly facilitated, by rapid dispersal from their presumed center of origin in the midcontinent-Andean area to tropical and subtropical shelves worldwide. By Early Permian time certain lagenides were adapted to cool water paleoenvironments, as evidenced by their occurrences in high paleolatitudes (~60° N and S) and even in glaciomarine basins. Early Permian lagenide occurrences can be grouped into eight paleogeographic areas. The midcontinent-Andean and present day Arctic areas contain diverse and similar lagenide faunas (low- to mid paleolatitudes), as do the Europe-Urals, Siberian and Australian areas (mid- to high paleolatitudes). Although not as well studied as other areas, the Cimmerian blocks and the eastern Paleotethys, both of which consist of island terranes, seemingly contain less diverse faunas.