THE HALF-MADE WORLD: PERSISTENT ‘AQUATIC’ COMMUNITY STRUCTURES DURING THE ORIGIN AND TERRESTRIALIZATION OF TETRAPODS

In the mid-20^th century, AS Romer placed the anatomical origin of tetrapod terrestriality in the late Pennsylvanian; concurrently, EC Olson proposed that tetrapod communities at this time, despite containing semiaquatic and terrestrial taxa, were still ‘aquatic’ in the sense that they were reliant on aquatic primary productivity. While terrestrial tetrapods are now known from the mid-Mississippian, the ecosystem changes accompanying tetrapod terrestrialization remain unclear. Here we present the results of two studies examining Devonian-Carboniferous vertebrate food webs to understand the origin and early development of terrestrial vertebrate communities.

Analysis of 14 modeled food webs from the Late Devonian-end Mississippian reveals that the end-Devonian mass extinction (EDME) caused a substantial change in taxonomic composition, but guild richness remained similar; the emergence of tetrapods onto land in the Mississippian did not precipitate a restructuring of existing terrestrial communities. Similarly, analysis of 11 additional North American terrestrial communities across the Carboniferous finds limited change, even during Pennsylvanian diversification of crown amniotes. Expansion of lower trophic levels is driven by the proliferation of arthropod primary consumers, with tetrapod herbivores being marginal components.

These surprising results suggest that at a mass extinction and substantial faunal turnover were unable to upset an ecological paradigm that apparently persisted for 80 million years. They support Olson’s hypothesis that terrestrial vertebrate communities were, in their first phase, still structurally and energetically tied to the water. Moreover, this phase lasted for tens of millions of years after the origin and diversification of firmly terrestrial crown tetrapod lineages. Further work will expand the food web dataset and integrate paleoenvironmental and paleofloral information to better understand the water-land transition of vertebrate ecosystems.