Romer's gap is an apparent gap in the Paleozoic tetrapod fossil record noted in the studies of paleontology and evolutionary biology, which represent periods in the Early Carboniferous from which excavators have not yet found relevant transitional fossils. It is named after American paleontologist Alfred Romer, who first recognised it in 1956. Studies published in 2016 and 2025 describing discoveries in Scotland and Australia began to close this gap in palaeontological knowledge.
Age
Romer's gap runs from approximately 360 to 345 million years ago, corresponding to the first 15 million years of the Carboniferous, the early Mississippian (starting with the Tournaisian and moving into the Visean). The gap forms a discontinuity between the primitive forests and high diversity of fishes at the end Devonian and more modern aquatic and terrestrial assemblages of the early Carboniferous.
thumb|right|[[Crassigyrinus, a secondarily aquatic non-amniote tetrapod from Romer's gap]]
Mechanism behind the gap
There has been long debate as to why there are so few fossils from this time period. However, recent finds in five new locations in Scotland have yielded multiple fossils of early tetrapods and amphibians. They have also allowed the most accurate logging of the geology of this period. This new evidence suggests that - at least locally - there was no gap in diversity or changes in oxygen geochemistry. Recent work on Paleozoic geochemistry has provided evidence for the biological reality of Romer's gap in both terrestrial vertebrates and arthropods, and has correlated it with a period of unusually low atmospheric oxygen concentration, which was determined from the idiosyncratic geochemistry of rocks formed during Romer's gap. Once the number of shell-crushing ray-finned fishes and sharks increased later in the Carboniferous, coincident with the end of Romer's gap, the diversity of crinoids with Devonian-type armor plummeted, following the pattern of a classic predator-prey (Lotka-Volterra) cycle. In 2016, five new species were found across the Ballagan Formation: Perittodus apsconditus, Koilops herma, Ossirarus kierani, Diploradus austiumensis, Aytonerpeton microps.
In 2025, the first evidence of crown group amniotes during Romer's gap was identified in clawed footprints from the Snowy Plains Formation of Victoria, Australia. These footprints, which closely resemble those of sauropsids, suggest that the radiation of crown group amniotes occurred very early in the Carboniferous or even during the Devonian, contrasting with all previous theories that this only occurred during the mid-late Carboniferous. Blue Beach maintains a fossil museum that displays hundreds of Tournaisian fossils, which continue to be found as the cliff erodes to reveal new fossils.
In 2012, 350-million-year-old tetrapod remains from four new Tournaisian sites in Scotland were announced, including those from a primitive amphibian nicknamed "Ribbo". In 2016, five more species were unearthed from these localities,
These localities are the coast of Burnmouth, the banks of the Whiteadder Water near Chirnside, the River Tweed near Coldstream, and the rocks near Tantallon Castle alongside the Firth of Forth. Fossils of both aquatic and terrestrial tetrapods are known from these localities, providing an important record of the transition between life in water and life on land and filling some of the lacunae in Romer's gap. These new localities may represent a larger fauna, as all lie within a short distance of each other and share many fishes with the nearby and contemporary Foulden fish bed locality (which has not produced tetrapods thus far). In the most recent paper to be produced by the TW:eed team, they announced some initial results from the core, including the apparent lack of oxygen excursion across Romer's Gap.