Bridge species
Correlation across palaeo-provinces: a comparison of Nuskoisporites and Densipollenites
Michael H. Stephenson,
Stephenson Geoscience Consulting, Nottingham UK, NG21 5HU; mikepalyno@me.com
Evelyn Kustatscher
Museum of Nature South Tyrol Via Bottai 1 39100 Bolzano Italy; evelyn.kustatscher@naturmuseum.it
Ivan Rodriguez Barreiro
Museum of Nature South Tyrol Via Bottai 1 39100 Bolzano Italy Ivan.RodriguezBarreiro@naturmuseum.it
The Permian is one of the most palaeophytographically differentiated periods in the Phanerozoic (Stephenson 2016) with the result that it is sometimes difficult to correlate across and between Permian continents. This is a particular problem between Gondwana and Euramerica in the Cisuralian because the GSSPs of the Cisuralian are not Gondwanan, and because the two regions were very different in the Permian phytogeographically and faunally. This means that ‘bridge taxa’ or ‘bridge species’ that occur between or throughout distinct Permian palynological provinces become very important because they can allow cross-province stratigraphic correlation.
Recent illustrations and descriptions of the monosaccate prepollen Nuskoisporites dulhuntyi Potonié et Klaus 1954 from the Southern Alps (Kustatscher et al., 2024) crucially from a cone of Ortiseia (an early conifer) have highlighted its similarity with the genus Densipollenites Bharadwaj 1962. Densipollenites is a distinctive element of the Tethyan Lopingian (e.g. Stephenson and Korngreen, 2021, Stephenson 2024) and is present in the similar-aged Indian Gondwana succession from where it was first described by Bharadwaj (1962). Some early comparisons using the light microscope have been carried out and are reported here. Although there are close similarities between the two taxa, for example in the relationship between the intexinal body and the exoexine, and the composition of the exoexine, more careful examination is needed, particularly of the type material of Densipollenites, to determine if species of Densipollenites and Nuskoisporites could be considered conspecific.
Nuskoisporites dulhuntyi from the Lopingian of Europe
Nuskoisporites dulhuntyi is considered a biostratigraphic marker species for the Lopingian of Europe. It is a monosaccate grain characterized by a circular outline and a prominent trilete mark. The biostratigraphic importance of this species is based, among others, on its characteristic morphology and because it is one of only few pollen taxa found in situ in Permian conifer cones. It is produced by male cones of the genus Ortiseia, the so far youngest representative of the Walchiaceae, well-known from the Wuchiapingian of the Dolomites (Clement-Westerhof, 1984) and Lopingian of Germany (e.g., Kerp et al., 2022).
Klaus (1963) supplied detailed description and interpretative illustration of Nuskoisporites dulhuntyi concluding that it had a large monosaccus, mostly hollow in nature, with a dense, columellate infrastructure and a density at the margin that suggested the appearance of a limbus. Its internal intexinal body as illustrated, is attached substantially at the proximal pole (i.e. around the trilete mark), but only weakly at the distal pole, thus producing a series of characteristic lateral compression-preservation states (e.g. Klaus, 1963, figs 13, 14). Kustatscher et al. (2024) described a wide range of sizes for Nuskoisporites dulhuntyi, but most specimens fall within the range of 120-140 µm in diameter.
The provision of slides of Nuskoisporites dulhuntyi from Ortiseia to one of us (MHS) allowed the detailed examination of specimens, alongside specimens assigned to Densipollenites in the last two decades in several publications (e.g. Stephenson and Korngreen 2021, Stephenson et al. 2024, Stephenson 2008), all from Middle Eastern locations far outside the known range of Nuskoisporites dulhuntyi in the Lopingian (e.g. Kustatscher et al. 2024, fig. 7). Densipollenites (mainly D. indicus Bharadwaj 1962) has been described by other authors from the Middle East (e.g. Eshet and Cousminer 1986, Eshet 1990, Horowitz 1973, Nader et al., 1993), and Pakistan (e.g. Balme 1970). The large size and relatively dark appearance of Densipollenites makes it a distinctive and easily spotted element in Middle Eastern material and a useful marker for formations such as the Arqov and Saad formations in Israel, the Umm Irna Formation in Jordan, and the upper Gharif Formation in Oman (e.g. Stephenson and Korngreen 2020, Stephenson 2008). This project therefore seeks to establish if Nuskoisporites dulhuntyi is conspecific with any Densipollenites species, and therefore could be used as a bridge taxon between Euramerica, Tethyan regions, and parts of Gondwana.
Densipollenites
At least five species of Densipollenites have been described in the Indian subcontinent by Bharadwaj (1962), Bharadwaj and Salujah (1964), Bharadwaj and Srivastava (1969) and Tiwari and Rana (1981). The species are listed and compared by Vijaya and Tiwari (1986). Like Nuskoisporites, Densipollenites is large (holotype with diameter 122 µm), with a limbus-like structure, and interpreted to have a broadly hollow saccus with a smaller internal intexinal body inside, only attached at the distal pole (see Bharadwaj 1962, text-fig. 7). This, like Nuskoisporites, presumably provides the wide range of compression-preservation states in Densipollenites, often with the inner body in a non-centred position. This at least tends to separate other monosaccate pollen such as Plicatipollenites and Cannanoropollis from Densipollenites and Nuskoisporites, because these Gondwanan taxa have much more substantial saccus attachment, and therefore almost never have non-centred inner bodies. Densipollenites was described as rarely having a haptotypic mark of any kind (Bharadwaj 1962).
Specimens of Densipollenites from the Middle East tend to be smaller that Indian specimens but otherwise have the same range of characters including a limbus-like structure, and a dark inner body (see for example Balme 1970, pl. 7, 8; Stephenson 2008, pl. 5, 1-3).
Comparison
Specimens of Densipollenites from the Dyke Plateau locality of the Umm Irna Formation (Changhsingian, Jordan; DG1), are shown alongside specimens of Nuskoisporites dulhuntyi from the Ortiseia cone Bletterbach, Italy(BLE19) in Plate 1.
Plate 1. Densipollenites from the Dyke Plateau locality of the Umm Irna Formation (Jordan; DG1); and Nuskoisporites dulhuntyi from Bletterbach (Italy; BLE19).
The specimens in the two assemblages are similar in size, though the Bletterbach Nuskoisporites dulhuntyi tend to be slightly larger. It was also noted that cone specimens of Nuskoisporites dulhuntyi are larger than other Alpine dispersed specimens (see Kustatscher et al. 2024). A common feature of both sets of specimens is the commonly non-centred position of the intexinal body, and the presence of a ‘limbus’. A similar characteristic in the Jordan specimens and Bletterbach specimens is the presence of folding in the exoexine independent of the inner body, despite the two being in proximity and close to one of the grain’s poles. This suggests detachment between the inner body and the exoexine at least at one pole, or over part of one pole. The converse is also true where folds in the exoexine are continued into the intexine suggesting adpression or attachment of the intexinal body at another pole (see Plate 1). Trilete marks are indeed rare in the Densipollenites specimens though they can occasionally be seen (see Plate 1).
Conclusions
Some early comparisons of Nuskoisporites dulhuntyi and Densipollenites species have been carried out using the light microscope. Although there are close similarities between the two taxa, for example in the relationship between the intexinal body and the exoexine, and the composition of the exoexine, there are some differences for example in size, and the frequency of occurrence of the trilete mark. In the future, examination is needed of the type material of Densipollenites, which is held at the Birbal Sahni Institute of Palaeosciences, with which contact has been made.
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