Maximum Depositional Age Constraints from U-Pb Detrital Zircon Geo-Chronology of the Bokkeveld Group (south Africa) Suggests Extension of Early-Middle Devonian Biocrises to the South Pole

Early-Middle Devonian invertebrate faunas from South Africa may be described as belonging to at least one of three faunal complexes (Eo-Malvinoxhosan, Established Malvinoxhosan, Post-Malvinoxhosan) based on their endemic high-latitude Malvinoxhosan biotas. Pragian-earliest Emsian aged Eo-Malvinoxhosan faunas are typified by their low diversity showing a sharp increase in diversity during the Emsian where these endemics comingle with Eastern Americas faunas. Collectively this assemblage is referred to as the Established Malvinoxhosan faunas. Starting at least as early as the Eifelian, a gradual decline in diversity is observed with most endemic and Eastern Americas biotas being replaced by Post-Malvinoxhosan faunas. Post-Malvinoxhosan faunas comprise a mix of cosmopolitan and limited migrant taxa following their extinction during the early Givetian. Within these faunal complexes intervals marked by declining biodiversity can be explained in at least 8-9 interval biozones, the total stratigraphic ranges of which correspond with systematic changes in local 2nd and 3rd order base-level. The interval biozones identified in South Africa, furthermore, show close parallels with numerous stratigraphic intervals across West Gondwana suggesting a regional cause for the extinction of the Malvinoxhosan biotas. These biodiversity turnover events in West Gondwana have been equated with several Early-Middle Devonian biocrises, of which the Daleje, Choteč, Kačák and Taghanic biocrises are prime candidates. The Daleje and Choteč biocrises have been attributed to a phased decline of the Malvinoxhosan biotas that precede an assumed Kačák event marking their final extinction. The Taghanic event in West Gondwana marks a shift towards cosmopolitan faunas and is thought to mark the progressive (but limited) invasion of cosmopolitan eurythermal taxa into higher latitudes, termed the “Great Devonian Interchange” (GDI). These biocrises as well as the GDI are traditionally thought to be proximal causes for the extinction of Malvinoxhosan faunas but has remained elusive until now owing to a lack of geochronological constraints. Maximum relative ages from detrital zircon geochronology of the Lower-Middle Devonian Bokkeveld Group found a youngest Early-Middle Devonian zircon population of ~405-380 Ma. Given its proximity to South Africa, convergent arc magmatism events in the Famatinian of South America and Western Magmatic belt of Patagonia are likely source candidates for this youngest source of detritus, allowing for maximum age constraints to be determined for the succession. Inferred maximum depositional ages closely parallel assumed biostratigraphic ages for the succession and suggest that the Malvinoxhosan biotas persisted for at least 15-18 Ma extending from the Emsian to earliest Givetian (~405-387 Ma). Key turnover events have been dated at 406±17 Ma, 396.4±7.1 Ma, and are in range of the Zlichov, Daleje, and Choteč events whilst the final disappearance of the Malvinoxhosan biotas is associated with of 387.4±7.2 Ma and is in range of the Kačák event. The appearance of the eurythermal brachiopod Rhipidothyris and abundance of Tropidoleptus in younger strata proceeding the Kačák event and are attributed to the Taghanic event at c. 386 Ma. These findings imply a true global extent for biocrises of the Early-Middle Devonian and that their effect on south polar regions was particularly devastating.