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The genus Loftusia

The genus Loftusia

Micropaleontology New York 47: 1-73 (Supplement 1)

Loftusia is a well-known genus among larger benthic foraminifera reported from various Maastrichtian strata in the Mediterranean and the Middle East. It is an important genus for palaeontologists, stratigraphers, petroleum geologists and field geologists because it is a good paleoenvironmental indicator and provides excellent resolution in age interpretations of sediments. This study is a synthesis of the Loftusia genus and its species. The focus, here, is on data obtained from Turkey and other Middle Eastern countries involving the general features of the genus using quantitative data, distribution and paleobiogeography of Loftusia, a biofabric approach and systematics. The species of Loftusia are categorized by size as small, medium and large. Size is a useful criterion for identification and is also important for paleoenvironmental interpretations. Other external (diameter-d, the ratio of diameter to length-d/l and polar-p) and internal parameters or features (number of whorls-nW, nucleoconch-nc, septa-s and endoskeleton content) are also distinguishing characteristics for description of species. Species comparisons show that much attention has been given to external measurements; however, detailed analyses of dimorphism, wall structure and the relationship of the nucleoconch with the chambers have been poorly studied. The identification of Loftusia species by taking into consideration dimorphism and statistical data in the same measured sections may provide clues for distinctions among species. The following observations can be evaluated with respect to species identification. Until recently, Loftusia ketini Meric, Loftusia minor Cox, and Loftusia turcica Meric and Avsar were described from microspheric (B) and macrospheric (A) individuals. The others are known from either microspheric (B) or megalospheric (A) forms. Loftusia elongata Cox (microspheric B) and Loftusia harrisoni Cox (megalospheric A) were found together, while Loftusia baykali Meric (megalospheric A), Loftusia oktayi Meric, and Loftusia kahtaensis Meric, Loftusia morgani Douville (microspheric B) also co-occur. The co-occurrence of Loftusia anatolica Meric and Loftusia morgani Douville is another interesting point to note. Quantitative data presented previously on the external and internal features of Loftusia species is re-evaluated herein. After comparing these parameters, new quantitative values for external (length-1; ratio of diameter to length-d/l) and internal features (number of whorls-nw) are suggested. Mean values for these features obtained from previous and present statistical studies are used. A few new parameters, such as, coiling parameter (cp) and growth rate (gr) may also be important features for statistical studies in the future. Furthermore, this study contains paleobiogeographical re-evaluations with a plate-tectonic approach of the distribution of various Loftusia species known to exist mainly in the Middle East (Late Cretaceous-Maastrichtian). We discuss the spatial and temporal similarities and differences of various species distribution on small plates between two giant continents. The abundance, distribution and size of Loftusia species is evaluated, herein, by taking into consideration species diversity, abundance on various continental platforms, salinity, temperature, and proximity to the equator. Smaller forms occurred at the end of the Late Maastrichtian and the stratigraphical range of Loftusia species in Turkey is from Middle to Late Maastrichtian. Morever, we think that tiny ophiolitic rock fragments within Loftusia tests are related to ophiolitic movements and substrate features during the Late Maastrichtian. A biofabric approach to Loftusia assemblages shows that fusiform-shape in larger foraminifera might have been hydrodynamically advantageous. Occurrences of microspheric (B) and macrospheric (A) forms within the same community are usually rare among Loftusia. Although Loftusia anatolica Meric and Loftusia morgani Douville have more or less the same size, they are differentiated from each other by their nucleoconchs. Thus, size for microspheric (B) and megalospheric (A) forms do not always allow us to understand their biofabric. Their packing and imbrication features seem to be more important. In the light of indicated features and with our biofabric approach, we think that individuals were mainly in-situ, and allochthonous Loftusia assemblages were rare in fore-reef areas. The following species of Loftusia are described herein: Loftusia anatolica Meric, Loftusia arabica El Asa'ad, Loftusia baykali Meric, Loftusia coxi Henson, Loftusia elongata Cox, Loftusia harrisoni Cox, Loftusia kahtaensis Meric, Loftusia ketini (B and A) Meric, Loftusia minor (B and A) Cox, Loftusia morgani Douville, Loftusia occidentalis Milovanovich, Loftusia oktayi Meric, Loftusia persica Brady, Loftusia turcica (B and A) Meric and Avsar. We also describe a new species, Loftusia matsumarui Meric and Gormus n. sp., from the Mid dle Maastrichtian of Turkey. This new species is easily distinguished from the other Loftusia species by its medium size, a large nucleoconch and distinctive wall structure. The Loftusia ancestral form is thought to be Loftusia arabica El-Asa'ad due to their age distribution and fossil assemblage data. Thus, Loftusia species are found spread across from the Arabian plate to other plates, where they are abundant in Middle Maastrichtian fore-reefal areas. Where they are particularly associated with rudists, they have very large tests. At the end of Maastrichtian, their size became smaller possibly due to differing paleoenvironmental conditions. Loftusia morgani Douville is a common species at the end of the Maastrichtian Stage. Consequently, loftusiid foraminifera allow us to better understand and interpret accumulation of Upper Cretaceous sediments which are important as hydrocarbon reservoirs.

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Accession: 011510369

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Related references

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