Romer's Gap is an example of a gap in the tetrapod fossil record used in the study of evolution. Such gaps represent a period from which excavators have found no or very few fossils. Romer's gap is named after paleontologist Dr. Alfred Romer, who first recognised it.[1]
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Romer's Gap ran from approximately 360 to 345 million years ago, corresponding to the first 15 million years of the Carboniferous, the early Mississippian (Tournaisian). The gap forms a discontinuity between the primitive forests and the high diversity of fishes in the "Age of Fishes" - the Devonian and a more modern aquatic and terrestrial assemblages of the Carboniferous.[2][3]
There has been long debate as to why there are so few fossils from this time period.[2] 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.[2][3][4] Also, excavators may not have dug in the right places. However, the existence of a true low point in vertebrate diversity has been supported by independent lines of evidence. [2][3][5]
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 [2][6][3][4]Recent work on Paleozoic geochemistry has confirmed 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 independently determined from the idiosyncratic geochemistry of rocks formed during Romer's Gap.[2]
Aquatic vertebrates, which includes most tetrapods during the Carboniferous[6][4], were recovering from a major extinction event during the Romer's Gap period, one on par with the that which killed the dinosaurs [3]. In this Hangenberg event, most marine and freshwater groups went extinct or were reduced to a few lineages, although the precise mechanism of the extinction is unclear [3]. Before the event, oceans and lakes were dominated by lobe-finned fishes and armored fish called placoderms [3]. After the gap, modern ray finned fish, as well as shark and their relatives were the dominant forms.[3] The period also saw the demise of the Ichthyostegalia, the early fish-like amphibians with more than five digits.[3][4]
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. [5] The Tournaisian has even been referred to as the "Age of Crinoids" [7]. 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. [5]
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.[8] However, tetrapod material in the earliest stage of the Carboniferous, the Tournaisian, is typically scarce relative to fishes in the same habitats, which can appear in large death assemblages, and is unknown until late in the stage [4][3]. 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.[3]