The Central Andean Basin as a dispersal centre: Biogeographic patterns of olenid trilobites during the late Cambrian – Early Ordovician

Abstract

The biogeographic histories of two cosmopolitan groups of Olenida: Hypermecaspididae and Parabolinella (Olenidae) are studied herein. Different biogeographical models were compared, and the results were discussed in a palaeogeographic and palaeoceanographic context. Dispersal pathways were proposed for both groups, identifying potential environmental factors that affected their distribution. Calibrated phylogenetic hypotheses for each taxon were used to compare different models of geographical evolution with R package BioGeoBEARS. Time-stratified analyses were carried out, using different distance matrices, for three intervals (Furongian, Early Ordovician and Middle Ordovician). Numbers and types of biogeographical events were estimated under the best-fitting models by using Biogeographic Stochastic Mapping (BSM). For both groups, the best-fit model incorporated jump dispersal, indicating that this was an important speciation strategy. Vicariance was unimportant for these trilobites, which would have been favoured by the stable conditions of their habitat. Ocean currents were key in the dispersal patterns of Parabolinella, but not in those of Hypermecaspididae. The main strategy used for both groups would have been “island hopping”, taking advantage of the microterranes and volcanic island arc chains surrounding Laurentia, Baltica, Avalonia, and Gondwana. Creeping also operated as a dispersal mechanism in Parabolinella. This genus originated in the late Miaolingian, when the more uniform sea conditions would have favoured its dispersal capabilities. On the other hand, Hypermecaspididae diversified mainly in the Early Ordovician, when cooling seas and changes in ocean circulation patterns generated more segmented habitats, affecting chances of dispersal, and favouring jump-founder events. Despite these differences, in both clades founder and dispersal events were more abundant from the western margin of Gondwana, which constituted an important dispersal centre during the Furongian and Early Ordovician.