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ABSTRACT

A diversified assemblage of cryptospores and trilete miospores are reported from the Nuayyim-2 borehole, Saudi Arabia. Four new genera and six new species are described: Chelinohilates maculatus sp. nov., Complectitetras alhajrii gen. and sp. nov., Imperfectotriletes patinatus gen. and sp. nov., Laevolancis chibrikovae sp. nov., Sphaerasaccus glabellus gen. and sp. nov., and Vestitusdyadus qalibahinus gen. and sp. nov. Independent biostratigraphical control using chitinozoans and graptolites indicates that all the studied samples are Llandovery (Rhuddanian) in age. A new biostratigraphic scheme for strata near the Ashgill-Llandovery boundary is proposed: one assemblage biozone is divided into two interval biozones that are both further subdivided into two sub-biozones. The criteria used to create these biozones are the first occurrences of taxa (including the earliest trilete spores) and the relative abundance of envelope-enclosed cryptospores. The palaeogeographically widespread distribution of similar spore assemblages during the Late Ordovician-Early Silurian suggested that the parent plants were cosmopolitan and thrived in a variety of climatic conditions. Therefore, they were little affected by climatic changes associated with the Hirnantian glaciation. This might provide an explanation for the apparent lack of significant impoverishment in spore biodiversity during this glaciation.

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