Ordovician Biota from the Arabian Plate: A Fresh View in Understanding the Austral Gondwanan High-Latitude Environments

Recent palaeontological and field research in the Ordovician successions of Jordan and Turkey (northern Arabian Plate, Levant region) has significantly increased the regional knowledge on the associated biota and depositional environments. From southern Jordan, a relatively large number of body fossils (bivalves, brachiopods, arthropods, graptolites: e.g., Modiolopsis cf. modiolaris, Kerfornella besnevillensis, Orbiculothyris costellata, Trematis tenuiornata, Palaeoglossa cf. attenuata, Lingulobolus cf. gnaltaensis, Hanadirella cf. bramkampi, Didymograptus murchisoni) and trace fossils (cruzianids and others) were documented. Additionally, the critical re-evaluation of the previously published complete Ordovician skeletal fossil fauna from Jordan led to a significantly reduced number of taxa: of nearly thirty assumed body-fossil taxa, only eleven are validated as occurring in the Ordovician strata. First detailed systematic palaeoichnological research of the entire Ordovician succession provided excellently preserved material, partly of Lagerstätte quality. The ichnotaxa include various ichnospecies of the ichnogenera Cruziana, Rusophycus, Diplichnites, Monomorphichnus, Bergaueria, as well as Arenicolites, Treptichnus, Skolithos, Gyrochorte, Palaeophycus, Planolites, Psammichnites, and Zoophycosof various stratigraphic levels that significantly support the biostratigraphic determination of the sequence as late Early to early Late Ordovician. The fieldwork, the palaeontological review and the new fossil findings indicate the existence of a trans-regional, quite large stratigraphic gap (encompassing about the lower half of the Darriwilian) in the Jordanian succession, and have led to a revision of the stratigraphic position of the post-gap formations higher up in the Middle and Late Ordovician. Especially the occurrence of surprisingly large-sized molluscs (polar gigantism) and the reduced bio- and ichnodiversity we interpret as adaptation to high-latitude ecological conditions. We propose the extensive and excellent exposures of the sedimentary succession in southern Jordan as the regional reference for the Ordovician system in the southern Levant. The Ordovician succession in south-eastern Turkey (northernmost rim of the Arabian Plate) is very similar in depositional facies expression (fluvial-deltaic and shallow fully marine environments) and general succession, indicating close palaeogeographic proximity to southern Jordan. The observed trace fossil content is largely identical. Especially the excellently preserved cruzianids, belonging to the C. rugosa group as well, but also typical assemblage characteristics, are useful for ecostratigraphic correlation. Recently published trace fossils and additional, yet unpublished, own ichnological data from the central and south-eastern Arabian Plate enable a good correlation with these more distant parts of the Arabian Plate. Due to conspicuous similarities in the sedimentary and biotic characteristics – indicating high-latitude conditions – close ecostratigraphic relations of the northern Arabian Plate not only with the central and southern Arabian Plate, but also with northern Africa (Libya, Egypt, Algeria), and, to a minor extent, south-western Europe are evident. These characteristics point to the close palaeogeographic vicinity of all these regions within a well-connected shallow shelf with active faunal exchange along this austral coastal margin of the Gondwana palaeocontinent. The new data from the Ordovician of the Arabian Plate open new opportunities for more comprehensive models of the Ordovician evolution of this palaeogeographic Gondwanan sector. They point to a large transregional scientific potential of the region, and they strongly encourage intensified regional research.