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The evolution of the earliest representatives of the genus Orbitoides: implications for Late Cretaceous biostratigraphy 
S. Albrich, G. Frijia, M. Parente, E. Caus,  (2014)
Cretaceous Research, 51, 22–34.
1-s2.0-S0195667114000913-gr4The biostratigraphy of Campanian-Maastrichtian carbonate platforms is largely based on the larger foraminiferal genus Orbitoides. However, while the taxonomy and the chronostratigraphic age of the younger species of this genus are well established, there are still many controversies on the earliest species. We have restudied their morphological characters using a large collection of samples from the type-localities and from continuous sections in the southern Pyrenees. Based on these new observations, the long forgotten species O. sanctae-pelagiae is reinstated, while O. dordoniensis is considered a junior synonym. Successive populations of O. hottingeri, O. sanctae-pelagiae and O. douvillei show gradual morphological changes in time marked by an increase in the size and complexity of the macrospheric embryonal apparatus, an increase of the size of the adult specimens of both generations and the pro- gressive appearance and development of true lateral chamberlets. The Font de les Bagasses Unit in the southern Pyrenees preserves a high-resolution archive of the evolution of the earliest Orbitoides. Strontium isotope stratigraphy indicates that the oldest species, O. hottingeri, made its first appearance in the earliest Campanian, close to the Santonian-Campanian boundary, and was replaced by O. sanctae- pelagiae at a level closely corresponding to the boundary between the Placenticeras bidorsatum and Menabites delawarensis ammonite zones.
Larger foraminifera; Biostratigraphy; Strontium isotope stratigraphy; Late cretaceous; Orbitoides
Fig. 4. 

A: panorama of the Font de les Bagasses Unit on the road C-13 (section of the Terradets gorge). Note the position of the type-level of O. hottingeri and the stratigraphical distribution of O. hottingeri and O. sanctae-pelagiae populations. The samples FB/FBO/P86 (Sr in the figure) were taken from this section. B: skeletal-rich packstone, comprising common benthic foraminifera (Alexina payracea, L. cantabrica, rotaliids and minor very small Orbitoides hottingeri); rare, very fine-grained, detrital quartz grains are observed; sample 131. C: peloidal wackestone-packstone with O. hottingeri andL. elongata Schlumberger; fine-grained detrital quartz is also present; sample 142. D: peloidal packstone-wackestone with common Orbitoides sanctae-pelagiae and minor Lacazina elongata. Bivalve and echinoid fragments are also present; sample 155. Scale bar is 1 mm for figures B–D.