Palorbitolinoides orbiculata Zhang, 1986

SCHLAGINTWEIT, FELIX, 2021, Orbitolinids And Other Larger Benthic Foraminifera From The Aptian-Albian Of Tibet: Critical Discussion Of Some Recently Published Data, Acta Palaeontologica Romaniae 18 (1), pp. 17-23 : 18

Record of Palorbitolinoides orbiculata Zhang, 1986 and Palorbitolinoides hedini Cherchi & Schroeder, 1980

Referred illustration: Palorbitolinoides orbiculata Zhang , figure 8.8, Palorbitolinoides hedini Cherchi & Schroeder, 1980 , figure 8.9 in BouDagher-Fadel et al. (2017).

Both species have been described from Aptian-Albian limestones of Tibet ( Cherchi & Schroeder, 1980; Zhang, 1986). A third species, Palorbitolinoides tenuis Zhang, 1986 , is considered a junior synonym of the type-species P. hedini by Schroeder et al. (2010). The differences of both species refer to the size and complexity of the embryo. The diameter of the central embryonic chamber is 0.21-0.4 mm in P. orbiculata (see Zhang, 1986, table page 207) against 0.45-0.7 mm in P. hedini (see Schroeder et al., 2010, p. 60). The embryo of P. hedini is much larger as in P. orbiculata , flattened, and with a well developed and inflated peri-embryonic zone. In addition, the partitions at the top of the embryonic chamber produce a system of small alveoli of different lengths. Both specimens illustrated by BouDagher-Fadel et al. (2017) display a hemi-spherical embryonic chamber (diameter ~0.31-~ 0.39 mm) with rather short partitions of seemingly equal length. They are interpreted as belonging to P. orbiculata . The stratigraphy of the two species P. orbiculata and P. hedini appears not as well fixed as other Orbitolininae ( Cherchi & Schroeder, 1980, 2013; Zhang, 1986; Marcoux et al., 1987; BouDagher-Fadel et al., 2017). Transitional types between P. orbiculata and P. hedini have been illustrated by Marcoux et al. (1987, pl. 2, figs. 2-4) from the late Aptian of Tibet. It is worth noting that representatives of Palorbitolinoides co-occuring with Mesorbitolina aperta (Erman) in the upper Albian of the Kazhdumi Formation in SW Iran are currently under study. Also with respect to the palaeobiogeographic distribution there seems to be some kind of dissens as Cherchi & Schroeder (2013) considered Palorbitolinoides cf. orbiculata as a typical taxon of the southern Neotethysian margin.