Romer's Gap - Mechanism Behind The Gap

Mechanism Behind The Gap

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. However, the existence of a true low point in vertebrate diversity has been supported by independent lines of evidence.

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 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.

Aquatic vertebrates, which include most tetrapods during the Carboniferous, were recovering from a Late Devonian extinction major extinction event that preceded Romer's gap, one on par with that which killed the dinosaurs. 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. 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.