The Great Devonian Interchange (gdi): A Viewpoint from Gondwana

The Great Devonian Interchange (GDI) was a biotic migration event that has been well documented in the fossil record of Euramerica and has only recently been investigated from several localities in Gondwana. This study focused on a Gondwana-first approach to assess changes in brachiopod bioregionalisation during the Early, Middle and Late Devonian. A timesliced, presence-absence database of 658 brachiopod genera from 29 depocentres across West and East Gondwana (in addition to peri-Gondwana) was amassed to evaluate how these taxa migrated throughout the Devonian and gauge the stability of these Gondwanan brachiopod bioregions against the backdrop of the Middle Devonian biocrises. The database was assessed using various multivariate methods, including non-metric multidimensional scaling, as well as cluster and network analyses. Our analyses recovered a consistent two-fold, latitudinal first-order division in Gondwana, comprising a “high-intermediate-latitude” (~30°–90° S) bioregion and a “low latitude” (~0°–30° S) bioregion. During the Early-Middle Devonian, the high-intermediate bioregion could be latitudinally subdivided into the Colombian-West African (~30°–50° S), Amazonian (~50°–70° S) and Malvinoxhosan (~70°–90° S) regions, whilst the low latitude bioregion could broadly be divided into a “northwestern Gondwana” and “eastern Gondwana” region. Mixing zones were additionally identified between these bioregions, as the Amazonian bioregion, the South Saharan and Sahel Region, and the New Zealand Region seemingly enabled faunal exchange between high-intermediate and low latitudes at this time. These findings do provide some support to the traditional Old World, Eastern Americas and Malvinoxhosan Realms thought to exist during the Devonian; however, the results indicate that the depocentres of the Old World cannot be discretely clustered into smaller-scale natural groupings, and the depocentres of the Eastern Americas region sustain an admixture of fauna from these traditional regions. The Malvinoxhosan bioregion remains the most well-supported natural grouping among the traditional bioregions, but it subsists as a second-order division within Gondwana during the Devonian. During the Early Devonian, brachiopods within these regions experienced a burst in diversification, with a peak in diversity being reached in the Emsian-Eifelian. Lower latitude regions appear to show recovery during the latest Givetian-Frasnian onwards; however, higher latitude areas continue to show a diversification decline, seemingly being depopulated by the end of the Frasnian. Closer analyses also point toward a trend of depressed proportional originations observed throughout the Devonian. These trends reflect the gradual collapse of the Gondwanan brachiopod bioregions by the Late Devonian, as the extinctions brought about by the GDI and the Devonian biocrises devastated endemic populations. Our results show that this shift towards cosmopolitanism among bioregions is most evident at lower latitudes in Gondwana; however, no new migrations or originations are registered in high-latitude bioregions during the GDI. Thus, the GDI was an exclusively low-latitude event that did not extend to the isolated brachiopod communities of high-latitude Gondwana. The dataset further suggests that the globally widespread Rhipdothyris is a more suitable marker for the onset of the GDI, whilst Tropidoleptus seems to have been an opportunistic brachiopod which persisted in western Gondwana from the Early Devonian onwards.