BIOGEOGRAPHY AND RATES OF EVOLUTION DURING A TAXIC RADIATION: CALMONIID TRILOBITES (DEVONIAN) OF THE MALVINOKAFFRIC REALM

The integration of biogeography with phylogenetics can enhance our understanding of macroevolutionary patterns. Patterns of phylogenetic biogeography can be used to determine if geological change had an overarching influence on evolution; the biogeographic patterns can also be used to study tectonic events. A phylogenetic approach was used to examine biogeographic patterns and speciation rates in the “Metacryphaeus + Malvinella group” trilobites of the family Calmoniidae (Devonian, Malvinokaffric Realm). This is an extremely diverse group of trilobites that have sometimes been offered as a model of adaptive or taxic radiation. The known phylogeny of 40 taxa of these calmoniids was used in a modified Brooks Parsimony Analysis that sampled from several regions in the Malvinokaffric Realm, including South Africa, the Falklands, Brazil, Bolivia, and Argentina. The results showed repeated patterns of vicariance and geo-dispersal that likely reflect cycles of sea-level rise and fall. Area cladograms reveal a strong relationship between South African and Bolivian provinces, as well as Falkland and Brazilian (Parnaíba) provinces. Biogeographic history and geologic events often have an important influence on evolutionary rates. Calmoniid trilobites have reputedly undergone a period of rapid diversification during the Devonian. Speciation and extinction rates in the group were determined using stratigraphic information calibrated to several timescales, and compared to other exceptional rates of evolution. Typically, trilobite groups that display repeated patterns of vicariance and geo-dispersal do not have high rates of speciation characteristic of taxic radiations. This study presents evidence that much of the radiation of these calmoniid trilobites is over before the component taxa appear in the fossil record. This may indicate a more general macroevolutionary theme regarding other evolutionary radiations preserved in the fossil record. For example, in the case of the Cambrian radiation of animals or the Cenozoic radiation of mammals, repeated lines of evidence indicate short to moderate fuses preceding what were perceived as explosive radiation events. Here we will focus on to what extent the fuse in the calmoniids may be related to an episode of biogeographic invasion.