ROMER'S GAP AND THE TERRESTRIALIZATION OF PALEOZOIC ARTHROPODS AND LIMBED VERTEBRATES

Romer's Gap was proposed over 40 years ago as an interval of approximately 15 m.y. encompassing the Early Mississippian (Tournaisian), represented by few limbed vertebrate (stegocephalian) occurrences, minimal cladogenesis, and the persistence a few major lineages surviving into the Middle Mississippian (Visean). Romer's Gap is immediately preceded in the Late Devonian by several genera of aquatic, limbed vertebrates with features that later became exaptations for a terrestrial lifestyle, referred to as terrestrialization Phase 1B. A considerably earlier Early and Middle Devonian diversification of terrestrial arthropods (hexapods, arachnids and myriapods) consisting of 22 major lineages conventionally considered as orders, is termed Phase 1A. Romer's Gap is succeeded during the Middle Mississippian through Lower Permian by a diverse expansion of major arthropod and limbed vertebrate lineages (Phase 2), representing a threefold increase of 66 additional ordinal-level lineages.

Explanation of Romer's Gap is linked to ambient atmospheric oxygen levels. A recent recalculation of Devonian ambient oxygen levels based on the ¹³C composition of brachiopod calcite is congruent with the (smoothed) trend established by the earlier GEOCARBSULF model in 2006. The only significant departure is the raising of O₂ level by 1 to 3 % during the Middle (Eifelian) to Late (Famennian) Devonian, corresponding to a low of 16 % (mid Frasnian) but throughout remaining significantly depressed compared to the present atmospheric level of 21 %. This O₂ minimum is associated with arrested cladogenesis of arthropod lineages, indicated by a prolonged and zero ordinal origination rate. This minimum continues into the Visean, through Romer's Gap, but is followed by a dramatic monotonic elevation to 29 % at the mid Early Permian (Phase 2). This latter increase parallels physiological advances in arthropod and vertebrate respiratory efficiency, new modes of insect herbivory, arthropod gigantism, and increased complexity in terrestrial food webs. The four independent replicates of macroscopic animal clades (hexapods, arachnids, myriapods and stegocephalians) indicate that a minimal threshold of around 20 % atmospheric oxygen is necessary for diversification of land-dwelling life.