Phylogenetics and the Cenozoic radiation of lampreys.

The development of a movable jaw is one of the most important transitions in the evolutionary history of animals. Jawed vertebrates rapidly diversified after appearing approximately 470 million years ago. Today, only lampreys and hagfishes represent the once dominant jawless grade and comprise less than 1% of living vertebrate species. Their relationship to other vertebrates ranks among the more contentious problems in animal phylogenetics. Further, the phylogenetic relationships within lampreys and hagfishes remain unclear, and the ages of their living lineages are largely unexplored. Because of their importance for the genomic and developmental changes that prefigured jawed vertebrate diversity, the evolutionary history of lampreys and hagfishes is a major frontier of organismal biology. Of these two clades, lampreys are more ecologically diverse, exhibiting freshwater, anadromous, and fully marine forms, as well as parasitic and nonparasitic species. Here, we present a new phylogeny and historical biogeographic reconstruction of all living lampreys. Whereas the early diversification of this clade tracks Pangaean fragmentation, lampreys also rapidly radiated in the northern hemisphere during the mid-Cretaceous and directly after the Cretaceous-Paleogene extinction. These radiations mirrored concurrent ones in other animals and plants and coincided with changes to lamprey ecology and feeding behavior. Our results suggest that 80% of living lamprey clades appeared in the last 20 million years of Earth history. Rather than gradually accumulating since the oldest stem-group forms appeared in the early Paleozoic, living lamprey biodiversity results from diversifications extending from the Cretaceous to present.