Orbitolina

Bidgood, Michael, Schlagintweit, Felix & Simmons, Michael, 2024, The Genus Orbitolina D’Orbigny, 1850 (Larger Benthic Foraminifera) And Its Constituent Species: Notes On Identity And Stratigraphic Ranges, Acta Palaeontologica Romaniae 20 (2), pp. 33-59 : 45-51

Orbitolina is an important larger benthic foraminifera genus, often found in abundance in mid-Cretaceous shallow-marine facies within the former Neotethys realm. Following the work of Schroeder (1962) the genus has a clear and precise definition based on the characteristics of a tripartite subdivision of an apically-situated macrospheric embryonic apparatus. Many taxa once regarded as Orbitolina belong in allied genera.

There are six potential species of Orbitolina ( O. concava , O. sefini , O. qatarica , O. hensoni , O. duranddelgai , and O. pauletensis ). Their identity depends partly on details of the size and shape of the embryonic apparatus and its constituent parts, and on the shape of the chamber passages that may be triangular, oval, or rectangular, or grade between different shapes (and also vary between macrospheric and microspheric forms). Other than O. concava and O. duranddelgai , the other species of Orbitolina need better description requiring access to topotype material. Because confident identification requires observation of (ideally multiple) sections through the embryonic apparatus and of chamber passage shape and density, many specimens encountered cannot be identified to species level. It is proposed that these be referred to as Orbitolina spp ..

O. concava is confirmed as a useful larger benthic foraminifera species for the recognition of Cenomanian (specifically early and middle Cenomanian) strata, but care needs to be taken to separate it from other similar species of Orbitolina that may range into the late Albian, whilst recognition of the more general Orbitolina spp. concept implies an age within the range late Albian - middle Cenomanian.

The limited confident data available suggests that the distinct species of Orbitolina may have at least partly endemic palaeogeographic distribution.

Michael D. Bidgood, Felix Schlagintweit & Michael D. Simmons

Appendix – An Inventory of “ Orbitolina ”

The authors of this paper are aware of more than 150 taxa that have been originally assigned to the genus Orbitolina , following its introduction by d’Orbigny (1850), or have commonly been assigned to the genus. This includes species, subspecies, and varieties. Table 1 below lists these in alphabetical order, noting the broad age and location of the types (with comments as appropriate), and the modern synonymy of each taxon if it can be determined.

Tabel 1. List of taxa assigned to Orbitolina since its introduction by d’Orbigny (1850), or commonly assigned to that genus. Where possible, a modern taxon equivalent is shown, either based on literature or as reviewed herein.

Many taxa were described purely from external morphology and without further study of type material (if it can be located) are effectively indeterminate in terms of modern taxonomy ( Figure 4 ). This includes taxa introduced by Parker & Jones (1860) and Prever (e.g., 1909) who also used a very loose sense of the meaning of the genus Orbitolina . Many others where the internal structure was described can be placed in the allied genera to Orbitolina sensu stricto (sensu Schroeder, 1962) such as Palorbitolina , Mesorbitolina or Conicorbitolina . This includes the plethora of seemingly unnecessary taxa introduced from Tibetan stratigraphy by Zhang (1982, 1986). However, illustration of internal structure is not a guarantee of the ability to assign a taxon to a modern species concept. Figure 4 demonstrates our approach to determination:

This listing is almost certainly not exhaustive although we have reviewed a large amount of historical and international literature, assisted by tools such as the Ellis and Messina Catalogue (2002) and WORMS database (WoRMS Ed. Board, 2024). Our hope is that it will provide a useful tool for research into the Orbitolininae .

Figures

Fig. 4. Approach to determination of Orbitolina taxa listed in this appendix. All images taken from type specimens of various sorts as illustrated in the Catalogue of Foraminifera (Ellis & Messina, 1940-2015) except Orbitolina concava illustrated by Schroeder & Neumann (1985), Lamarck never providing an image.