Woodland Hypothesis for Devonian Tetrapod Evolution

The rarity of Devonian tetrapods and the absence of tetrapods during the first 14 million years of the Mississippian (Romer's Gap) have inspired hypotheses of fish-tetrapod evolutionary transition as an escape from difficult habitats such as deserts or stagnant waters. These hypotheses and Romer's Gap are tested here using depth to calcic horizon in paleosols of the northern Appalachians as a proxy for precipitation and vegetation changes during the Devonian and Mississippian. All tetrapod bones and trackways in this region, as well as fossil tree remains, were found at times of high precipitation (indicated by deep calcic paleosols), when subhumid woodlands expanded at the expense of semiarid shrublands in alluvial lowlands. No tetrapods or tree fossils were found in abundant paleosols with the shallow calcic horizons of aridland soils. The mean annual precipitation requirement of Devonian-Mississippian tetrapods and trees was at least 571 +/- 72 mm. Global distribution of prototetrapods and aquatic tetrapods also shows that they lived earliest (Eifelian) in coastal lagoon or estuary margin soils of humid regions rather than in aridlands or intertidal flats. Tetrapods later spread to a variety of habitats, but continued preference for noncalcareous soils of woodlands and forests may explain Romer's Gap as a preservational artifact. A woodland hypothesis of tetrapod evolution is presented here: limbs and necks were selected for by scavenging and hunting in shallow-flooded woodlands and oxbow lakes during a unique period in Earth history, after evolution of flood-ponding trees and before effective terrestrial predator resistance.