Orbitolina, AS A BIOSTRATIGRAPHIC TOOL

ORBITOLINA AS A BIOSTRATIGRAPHIC TOOL View in CoL

Orbitolina View in CoL and its allies are widely cited as important tools for biostratigraphy in shallow-marine carbonate and marl facies from mid-Cretaceous Neotethys ( Schroeder & Neumann, 1985, Moullade et al., 1985; Simmons et al., 2000). The inception of O. concava View in CoL has been regarded as a proxy for the base of the Cenomanian, and to range no younger than a level within the Cenomanian ( Schroeder & Neumann, 1985; Tröger & Kennedy, 1996; Velić, 2007; Schlagintweit et al., 2015; Simmons & Bidgood, 2023). Our review suggests that this is essentially correct – O. concava View in CoL sensu stricto is not known from late Albian strata and appears to have definitely been recorded from both early and middle Cenomanian strata. On the other hand, other species of Orbitolina View in CoL are definitely found or might be found within the late Albian (although not excluding a range into the Cenomanian). These include O. duranddelgai View in CoL and possibly O. sefini View in CoL , O. qatarica View in CoL , and O. hensoni View in CoL . O. pauletensis View in CoL is too poorly known to be confident of its range.

There is thus an important caveat on the range of O. concava View in CoL : confidently identified specimens can indicate an early – middle Cenomanian age range; but for those specimens that cannot be confidently identified as O. concava View in CoL or another species of Orbitolina View in CoL and are thus Orbitolina spp. as used herein, then an age range of late Albian – middle Cenomanian is indicated. This likely includes forms described in the literature as “ Orbitolina concava View in CoL ” with a late Albian – intra-Cenomanian age range, but not illustrated (e.g. Saint-Marc, 1981). It is evident from the remarks above that there are some definite occurrences of Orbitolina as View in CoL old as the pricei ammonite zone in the late Albian (recorded in Figure 3 View Fig as Orbitolina spp. ), but it cannot yet be confirmed what species they are.

Age range summaries for the constituent species of Orbitolina including Orbitolina spp. are given in Figure 3 View Fig .

In this work we have used three broad categories to depict stratigraphic range confidence:

Confident and Common - a wide, solid, green line on Figure 3 View Fig

A relatively large number of (correctly) illustrated records (or if unillustrated, from a generally reliable source) with at least plausible age-control.

Confident but Scarce – a narrow, solid, green line on Figure 3 View Fig

At least one, but relatively fewer records but which fit the same criteria as above.

Uncertain – a series of orange “?” on Figure 3 View Fig Occurrences that lie outside of the “confident” ranges that are neither confirmed in terms of identity nor age-control but cannot be completely dismissed (e.g., an illustrated record with poor age-control or an unillustrated record from a generally reliable source with good age control).

As can be readily understood, subjectivity plays a role here, and the boundaries between one category and another are inevitably gradational.

Records which exist of named species occurring in rocks outside of these ranges, but which are based on very uncertain (i.e. dubious) identity and/or age control are not shown on Figure 3 View Fig .

Species of Orbitolina sensu stricto can be demonstrated to occur across an area extending from north-west Europe, the circum-Mediterranean, and to Somalia and the Arabian Plate. There may be some endemic patterns in this distribution as suggested by Luger (2018). For example, O. concava and O. duranddelgai are not convincingly found outside of the Northwest Europe - Mediterranean region. It is unclear if different lineages of Orbitolina independently arose from Mesorbitolina (?) ancestral stock in the Mediterranean and Arabian Plate, or if taxa arose in Arabia and rapidly migrated westwards (following prevailing palaeocurrents – Simmons & Bidgood, 2023, fig. 84 therein), with some morphological changes to create new species. Firm conclusions await the necessary taxonomic revisions derived from as yet unavailable data as highlighted in this paper.

O. concava View in CoL and O. qatarica View in CoL have been reported from Tibet ( Ho et al., 1976; Yang et al., 1982; Zhang, 1982; BouDagher-Fadel et al., 2017), but these records cannot be validated or are clearly other taxa ( Rao et al., 2017). Other species of Orbitolina (Orbitolina) View in CoL reported by Zhang (1982, 1986) ( O. (O.) birmanica View in CoL , O. (O.) aliensis View in CoL , O. (O.) deltoides View in CoL , O. (O.) bangoinica View in CoL , O. (O.) toibaica View in CoL ) are all species of Mesorbitolina View in CoL , either valid (as in the case of O. (O.) birmanica View in CoL = M. birmanica View in CoL (see Schlagintweit & Wilmsen, 2014)) or synonyms of other Mesorbitolina species). Indeed, Zhang (1994) concluded that no Orbitolina View in CoL sensu stricto or Cenomanian orbitolinids occurred in Tibet. A similar situation appears to be the case in Borneo ( Schlagintweit & Simmons, 2023) and Japan ( Iba et al., 2011).