ROMER's gap: tetrapod origins and terrestriality

New data derived from recent discoveries of Devonian tetrapods necessitate a revision of frequently expressed palaeoecological and morphogenetic scenarios concerning the fish-tetrapod transition. Many current hypotheses link tetrapod origin closely to terrestrialisation in a teleological explanation of tetrapod evolution. In contrast, Romer emphasised the separation of these two events. ROMER's view is corroborated by new data that indicates that tetrapods were originally aquatic and preceded the earliest known terrestrial fauna by ~25 mya. Three varied Upper Devonian tetrapods are now known in detail: Acanthostega, Ichthyostega, and Tulerpeton. New reconstructions of Acanthostega and Ichthyostega are presented, including preliminary details of the acanthostegid vertebral column, pelvic girdle, and hind limb. Acanthostega/i>, anatomically closest to osteolepiform fish, is preserved in active fluvial channels; Ichthyostega is reinterpreted as having a seal-like postcranial skeleton; Tulerpeton, resembling more recent Carboniferous tetrapods, is found in shallow marine sediments. There is little support for an exclusively freshwater tetrapod origin. Comparison of acanthostegid anatomy with Panderichthys, and quantified changes between small and large specimens of Eusthenopteron, suggest dissociated heterochronic changes in early tetrapod evolution. Early tetrapod remains coincide with the Frasnian-Famennian extinction, which included massive depletion of the oxygen-content of marine surface-waters. High faunal turnover, the adaptive advantage conferred upon air-breathers, and the possibility that early tetrapods occupied marine environments, suggest that this extinction event facilitated the evolutionary radiation of early or near-tetrapods.