The Great Devonian Interchange, a (near) global event? Insights from Gondwanan brachiopod palaeobiogeography

The Great Devonian Interchange (GDI) was a biotic migration event that has been well documented in Euramerica and has only recently been investigated from several localities in Gondwana. This study evaluated the GDI through a Gondwana-first approach, assessing changes in brachiopod bioregionalization during the Early, Middle, and Late Devonian. A presence-absence database of 658 brachiopod genera from 29 depocentres across West and East Gondwana (in addition to peri-Gondwana) was compiled to investigate the migration of these taxa throughout the Devonian and assess the stability of Gondwanan brachiopod bioregions in relation to Devonian biocrises. The time-sliced database was assessed using a variety of 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) and a “low-latitude” (∼0°-30° S) bioregion. During the Early-Middle Devonian, the high-intermediate bioregion is latitudinally subdivided into the Colombian-West African (∼30°-50° S), Amazonian (∼50°-70° S) and Malvinoxhosan (∼70°-90° S) regions and the low-latitude bioregion may broadly be divided into a “northwestern Gondwana” and “eastern Gondwana” area. The results further identified the presence of mixing zones between these bioregions, as the Amazonian bioregion, the South Saharan and Sahel Region, and the New Zealand Region appear to have facilitated faunal exchange between high-intermediate and low latitudes during the period. The divisions proposed here do provide some support to the Old World, Eastern Americas, and Malvinoxhosan realms traditionally thought to exist during the Devonian; however, the results indicate that the depocentres of the Old World cannot clearly be clustered into smaller-scale natural groupings, and the depocentres of the Eastern Americas region sustain an admixture of fauna from these traditional bioregions. Furthermore, the findings suggest that 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. These bioregions gradually collapsed by the Late Devonian, as the extinctions brought about by the GDI and the Devonian biocrises devastated endemic populations. These events are correlated with global warming and pulsed sea-level rise, which brought about anoxic conditions on continental shelves. These events enabled cosmopolitan taxa to proliferate across areas affected by environmental stress, as well as breach previous barriers to migration, allowing them to supersede the niches of endemic and incumbent taxa. There is a trend towards cosmopolitanism among bioregions, clearly observable at lower latitudes in Gondwana; however, faunal exchange to high-latitude bioregions was significantly reduced 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. Additionally, the dataset suggests that the globally widespread Rhipidothyris 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.