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    A new study that explores the expansion of the orbitolinid group during the mid-Cretaceous
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        Rated 5 of 5.
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        Rated 5 of 5.
    Level of validity:
        Rated 5 of 5.
    Level of completeness:
        Rated 5 of 5.
    Level of comprehensibility:
        Rated 5 of 5.
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    Members of the Larger Benthic Foraminiferal (LBF) family Orbitolinidae occured from the Cretaceous to the Paleogene, however, they were most diverse during the mid-Cretaceous, and dominated the agglutinated LBF assemblages described from limestones of that period. Various orbitolinid species have been used to zone and date lithologies formed in the shallow, warm waters of the Aptian to the Early Cenomanian, and many, sometimes inaccurate, generic and sub-generic nomenclatures have been proposed to differentiate the often subtle morphological changes that orbitolinids exhibit over time. Until now, it has not been possible to develop an effective global overview of their evolution and environmental development because descriptions of specimens from Asia have been relatively rare. Following our recent study of over 1800 orbitolinid-rich thin sections of material from 13 outcrops of the Langshan Limestone, from the Southern Tibetan Plateau, and from the Barito Basin, South Kalimantan, Indonesia, it has been possible to compare the stratigraphic ranges of these orbitolinids with previously described Tethyan and American forms, based on the use of a planktonic zonal (PZ) scheme, itself tied to the most recent chronostratigraphic scale. This has allowed the reconstruction of the phylogenetic and paleogeographic evolution of the orbitolinids from their Valanginian origin in the Tethys. Although Tethys remained the paleogeographic focus for the orbitolinids, it is inferred here for the first time that a bi-directional paleogeographic migration of some orbitolinid genera occurred from Tethys to the Americas and also to the Western Pacific region. Our observations and dating confirm that global marine regressions in the Aptian were coincident with, and may well have facilitated, these orbitolinid transoceanic migrations. However, migration stopped after rising sea-level in the Early Albian appears to have again isolated these provinces from each other. Tectonic forces associated with the subduction of the Farallon Plate and further sea-level raises led to the opening of the Western Interior Seaway in the North America, which correlates with, and may have been the cause of, the Middle Albian (top of PZ Albian 2) extinction of the American orbitolinids. The extinction of the orbitolinids revealed that the Western Pacific province was split into two sub-provinces, with extinction occurring at the end of the Early Albian (top of PZ Albian 1) in the Northwest Pacific sub-province, and at the end of the Albian (top of PZ Albian 4) in the sub-province that is today South East Asia (on the margins and west of the Wallace Line). The final virtual extinction of the orbitolinids occurred at the end of the Cenomanian in the Tethyan province, which coincides with, and may have been caused by, global anoxic oceanic events that correlate with a near-peak Mesozoic eustatic sea-level high-stand that led to the overall global collapse of the paleotropical reef ecosystem at that time.

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      This work has been published open access under Creative Commons Attribution License CC BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Conditions, terms of use and publishing policy can be found at www.scienceopen.com.

      Earth & Environmental sciences
      Ecology,Foraminifera,palaeogeographic distribution,,Global anoxic events,biostratigraphy,,Sea-level changes,The Environment,Climate,phylogeny,,Extinctions,mid-Cretaceous,,Orbitolinids,

      Review text

      The paper presents a useful taxonomic, palaeoenvironmental and palaeogeographic summary of the Aptian to Cenomanian Orbitolinidae. The most recent significant works on this family include Schroeder et al. (2010), Boudagher-Fadel et al. (2017) and Boudagher-Fadel (2018). The present paper presents supplementary evidence to that presented in Boudagher-Fadel et al. (2017) to extend existing knowledge of the Tethyan and American forms to the flanks of the Western Pacific region. Studies of the orbitolinids from the Tibetan Plateau and South Kalimantan, stratigraphically calibrated with planktonic foraminifera, provide new evidence for the migration paths and their individual evolutionary lineage that will significantly assist the stratigraphic application of orbitolinids. The Tethys is maintained as the centre for orbitolinid development, from which one migration path extended west to the Americas and the other east to the Western Pacific, with subsequent evolution of local provincial forms.

      The photomicrographs easily display the features of taxonomic importance. Figures 4 and 5 clearly illustrate the morphological changes of orbitolinid embryonic apparatus and test morphology, with reference to the evolutionary groups described in the text. Episodes of migration are explained as responses to eustatic sea-level changes, as well illustrated in Figure 6, with figures 7 and 11 succinctly explaining the geographic and stratigraphic distribution of the various genera in the three provinces.

      The paper is important as it permits application of orbitolinid evolutionary stages to be better understood across the wide extent spanning the American to the Western Pacific. Calibration with the planktonic foraminiferal zones is an important tool for correlation at both regional and local levels. Links with eustatic controls on dispersal are highlighted as well as the tendency for provincial evolution in the three provinces respectively.


      Thank you for these positive comments.. 



      2019-04-08 18:53 UTC
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