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.

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.

There has been long debate as to why there are so few fossils from this time period. Some have suggested the problem was of fossilization itself, suggesting that there may have been differences in the geochemistry of the time that did not favour fossil formation. Also, excavators simply may not have dug in the right places. The existence of a true low point in vertebrate diversity has been supported by independent lines of evidence. 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.

While initial arthropod terrestriality was well under way before the gap, and some digited tetrapods might have come on land, there are remarkably few terrestrial or aquatic fossils that date from the gap itself. 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. The new sedimentary logging in the Ballagan Formation in Scotland challenges this, suggesting oxygen was stable throughout Romer's Gap.

Aquatic vertebrates, which include most tetrapods during the Carboniferous, were recovering from the Hangenberg event, a major extinction event that preceded Romer's gap, one on par with that which killed the dinosaurs. In this end-Devonian extinction, most marine and freshwater groups became extinct or were reduced to a few lineages, although the precise mechanism of the extinction is unclear. Before the event, oceans and lakes were dominated by lobe-finned fishes and armored fishes called placoderms. After the gap, modern ray finned fish, as well as sharks and their relatives were the dominant forms. The period also saw the demise of the Ichthyostegalia, the early fish-like amphibians with more than five digits.

The low diversity of marine fishes, particularly shell-crushing predators (durophages), at the beginning of Romer's gap is supported by the sudden abundance of hard-shelled crinoid echinoderms during the same period. The Tournaisian has even been called the "Age of Crinoids". 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. There is increasing evidence that lungfish and stem tetrapods and amphibians recovered quickly and diversified in the rapidly changing environment of the end-Devonian and Romer's Gap.

The gap in the tetrapod record has been progressively closed with the discoveries of such early Carboniferous tetrapods as Pederpes and Crassigyrinus. There are a few sites where vertebrate fossils have been found to help fill in the gap, such as the East Kirkton Quarry, in Bathgate, Scotland, a long-known fossil site that was revisited by Stanley P. Wood in 1984 and has since been revealing a number of early tetrapods in the mid Carboniferous; "literally dozens of tetrapods came rolling out: Balanerpeton (a temnospondyl), Silvanerpeton and Eldeceeon (basal anthracosaurs), all in multiple copies, and one spectacular proto-amniote, Westlothiana", Paleos Project reports. In 2016, five new species were found across the Ballagan Formation: Perittodus apsconditus, Koilops herma, Ossirarus kierani, Diploradus austiumensis, Aytonerpeton microps. These stem tetrapods and amphibians provide evidence for an early split between the two groups, and rapid diversification in the Early Carboniferous.

However, tetrapod material in the earliest stage of the Carboniferous, the Tournaisian, remains scarce relative to fishes in the same habitats, which can appear in large death assemblages, and is unknown until late in the stage. Fish faunas from Tournaisian sites around the world are very alike in composition, containing common and ecologically similar species of ray-finned fishes, rhizodont lobe-finned fishes, acanthodians, sharks, and holocephalans.