Abstract
The standard reconstruction of species of Orbitoides d’Orbigny into a single lineage during the late Santonian to the end of the Maastrichtian is based upon morphometric data from Western Europe. An irreversible increase in the size of the embryonic apparatus, and the formation of a greater number of epi-embryonic chamberlets (EPC) with time, is regarded as the main evolutionary trends used in species discrimination. However, data from Maastrichtian Orbitoides assemblages from Central Turkey and the Arabian Platform margin (Southeastern Turkey and Oman) are not consistent with this record. The Maastrichtian Besni Formation of the Arabian Platform margin in Southeastern Turkey yields invariably biconvex specimens, with small, tri- to quadrilocular embryons and a small number of EPC, comparable to late Campanian Orbitoides medius (d’Archiac). The upper Maastrichtian Taraklı Formation from the Sakarya Basin of Central Turkey contains two distinct, yet closely associated forms of Orbitoides, easily differentiated by both external and internal features. Flat to biconcave specimens possess a small, tri- to quadrilocular embryonic apparatus of Orbitoides medius-type and a small number of EPC, whereas biconvex specimens possess a large, predominantly bilocular embryonic apparatus, and were assigned to Orbitoides ex. interc. gruenbachensis Papp–apiculatus Schlumberger based on morphometry. The flat to biconcave specimens belong to a long overlooked species Orbitoides pamiri Meriç, originally described from the late Maastrichtian of the Tauride Mountains in SW Turkey. This species is herein interpreted to be an offshoot from the main Orbitoides lineage during the Maastrichtian, as are forms that we term Orbitoides ‘medius’, since they recall this species, yet are younger than normal occurrence with the accepted morphometrically defined lineage. The consistent correlation between the external and internal test features in O. pamiri implies that the shape of the test is not an ecophenotypic variation, but appears to be biologically controlled. We, therefore, postulate that more than one lineage of Orbitoides exists during the Maastrichtian, with a lineage that includes O. ‘medius’ and O. pamiri displaying retrograde evolutionary features.
Highlights
Orbitoides d’Orbigny is an orbitoidal larger foraminifer that thrived in the tropical and sub-tropical shallow marine carbonate platforms and ramps from Central America to Asia during the late Santonian to the end of the Maastrichtian (Goldbeck & Langer, 2009; Loeblich & Tappan, 1987; van Gorsel, 1978)
In addition to the well-known species O. ex. interc. gruenbachensis–apiculatus, we show that these late Maastrichtian populations include predominantly flat- to biconcave specimens, possessing small, tri- to quadrilocular embryonic apparatus of Orbitoides medius-type and a small number of EPC, previously recorded from southern Turkey as O. pamiri, but not known from Europe
Lithraphidites occurs with the presence of the marker species Lithraphidites quadratus Bramlette and Martini and Lithraphidites praequadratus Roth
Summary
Orbitoides d’Orbigny is an orbitoidal larger foraminifer that thrived in the tropical and sub-tropical shallow marine carbonate platforms and ramps from Central America to Asia during the late Santonian to the end of the Maastrichtian (Goldbeck & Langer, 2009; Loeblich & Tappan, 1987; van Gorsel, 1978). The exact stratigraphic distribution of the species within this lineage is not yet precisely calibrated against the standard geologic time scale. In spite of these uncertainties, three species, O. gruenbachensis, O. apiculatus and O. gensacicus, have commonly been reported from Maastrichtian sediments. These species correspond to an advanced developmental stage of the presumed lineage and are characterised by a relatively large embryonic apparatus (the size of which is expressed by Li + li) and many epi-embryonic chamberlets (expressed by E, the total number of primary and accessory epi-embryonic chamberlets (EPC))
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