Reconstructing a radiation: the chiton genus Mopalia in the north Pacific

The chiton genus Mopalia Gray, 1847 is highly speciose despite showing little morphological differentiation. Many of the 24 extant species are conspicuous, large-bodied and ecologically important today, but pre-Pleistocene fossils for the genus are rare. Here, we use a combined analysis of four gene regions (16S and COI mtDNA, 18S and 28S rDNA) to estimate the phylogenetic relationships for Mopalia species and use the inferred phylogeny to analyse the group's biogeography and patterns of speciation. We then use these molecular data to distinguish between two alternative interpretations of the fossil record, as there is a large temporal gap between the oldest fossils tentatively identified as Mopalia and the next oldest fossils (Miocene versus Plio-Pleistocene). Based on the estimated substitution rates from a wide variety of other marine animals, we conclude that the observed rates in Mopalia are consistent with a Miocene origin for the genus. Given this age for the group and assuming a molecular clock, most speciation events in Mopalia are inferred to have occurred on average similar to 5 Mya. The phylogenetic results indicate that most of the speciation events leading to extant species must have occurred along the western North American coast, though there appear to have been multiple spreading events across the Pacific. When considered along with results for the many other near-shore taxa that have similar distributions to Mopalia, our findings suggest the emergence of a coherent historical biogeography of the northern Pacific.