THE NEARSHORE CRADLE OF EARLY VERTEBRATE DIVERSIFICATION

The ancestral habitats of vertebrates have been the subject of controversy for decades, obscured by limited early sampling (“A Sea Without Fishes”) and conflicting data. However, the environmental origin of vertebrates has bearing on everything from the quality of their Paleozoic record to the ecological context of major innovations such as jaws. To resolve this mystery, we combined Bayesian phylogenetic comparative methods with new databases and supertrees for skeletonized, jawed vertebrates and their jawless relatives (gnathostomes), covering the interval from their first appearance in the mid-Ordovician to the end-Devonian extinction. Occurrences were assigned to Boucot’s benthic assemblage zones, which represent habitats along the depth gradient (e.g. BA 1=intertidal, BA 6=open water). A phylogenetic threshold model was used to determine both ancestral habitats and the cost (liability) of movement between adjacent zones (threshold parameters). We found that all major vertebrate divisions, from the earliest jawless fishes to sharks, originated in highly-restricted intertidal-subtidal waters (~BA 1-2) during the first 100 million years of vertebrate diversification. This is in stark contrast to the coincident distribution of fossiliferous strata, invertebrate occurrences, and benthic communities, all concentrated on deeper settings including reefs (BA 3-4). It also contrasts with the more even spread of taphonomically-similar conodonts across the shelf (BA 1-6). We found that divergence into new body forms nearshore greatly influenced the subsequent dispersal abilities of vertebrate clades. More robust, benthic forms tended to shift shoreward following origination, while more gracile, pelagic forms shifted into deeper waters, although excursions onto the reef and deep shelf were short-lived until the later Devonian. In contrast, all fish divisions invaded freshwater (BA 0) rapidly and repeatedly. In sum, we find that nearshore origins highly influenced the early fossil record of vertebrates and the course of their initial diversification.