Late Ordovician (Katian) chitinozoans from northwest Saudi Arabia: Biostratigraphic and paleoenvironmental implications

Investigation of 37 samples from continuously cored intervals of the upper part of the Quwarah Member of the Qasim Formation yielded well-preserved and diverse chitinozoan assemblages. Two biozones previously defined from the North Gondwanan Domain were recognized in the examined cores: the Tanuchitina elongata and Ancyrochitina merga. One new genus, Alhajrichitina, and five new species, Alhajrichitina adamantea, Hyalochitina incomposita, Rhabdochitina curvata, Tanuchitina contracta and Tanuchitina densimura, are described. The vertical distribution of these new taxa form the basis of the Tanuchitina elongata Partial Range Subzone, Hyalochitina incomposita Total Range Subzone, Tanuchitina contracta Partial Range Subzone, and Tanuchitina densimura-Belonechitina aff. robusta Concurrent Range Subzone. Two other assemblages, T. ontariensis and Angochitina cf. curvata are also recognized from the merga Biozone. These new subzones and assemblages improve regional and local stratigraphic correlations, and the recognition of the degree of the Hirnantian glacial erosion at the top of the Quwarah Member. An evolutionary lineage is proposed for the species assigned to Alhajrichitina nov. gen., originating with the Sandbian Lagenochitina dalbyensis, through the Katian Alhajrichitina adamantea and culminating with Rhuddanian species, Alhajrichitina nuayyimensis. Three minor diversity and abundance increases are recognized within the merga–elongata biozone interval. These events could represent minor pulses of sea level rise within a progressive, overall sea level drawdown. For this upper part of the core, a general trend of decreasing diversity is recognized and may indicate an upward shallowing. This trend is compatible with diversity trends from all paleocontinents. It could be speculated that fluctuations in sea level were caused by ice advance and retreat during the initiation of Hirnantian continental glaciation south of the study area.