The evolution of Orbitoides media (Foraminiferida) in the late Campanian

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Abstract
Biometric analysis of several thousand specimens of Orbitoides media, from the upper Campanian of southwestern France, shows that the evolution of this species involves two periods of stasis, interrupted by a relatively short interval of transition in which a new morphotype replaced the previous one. Since O. media lived in a high-energy environment, its fossil record is fragmentary, and the exact amount of time involved in the transition is not known. The break in lineage permits the distinction of two chrono-subspecies: O. media media and O. media "megaloformis", which are redefined using the distribution of population variables. The boundary between the two subspecies is placed where the populations first contain less than 50% of the old morphotype. Comparison with an analysis of some 500 specimens from the Maestrichtian stratotype shows that the difference between the two subspecies of O. media is less distinctive than the difference between O. media and its successor, O. apiculata. However, in a purely typological sense, O. media still occurs in the upper Maestrichtian and O. apiculata may be found in the Campanian. Because of the high correlation between the variables measured, I suggest that for this portion of the Orbitoides lineage, one parameter (e.g., the easily measured maximum diameter of the embryon) is sufficient for biostratigraphical purposes. The large variation within each sample, however, requires measurements on some tens of specimens to determine their taxonomic position.