Romer's gap is an example of an apparent gap in the tetrapod fossil record used in the study of evolutionary biology. Such gaps represent periods from which excavators have not yet found relevant fossils. Romer's gap is named after paleontologist Alfred Romer, who first recognised it.
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 high diversity of fishes in the end Devonian and more modern aquatic and terrestrial assemblages of the early Carboniferous.
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 the Late Devonian extinction, a 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.
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. 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. 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.
For many years after Romer's gap was first recognized, only two sites yielding Tournaisian-age tetrapod fossils were known; one is in East Lothian, Scotland and another in Blue Beach, Nova Scotia, where in 1841, Sir William Logan, the first Director of the Geological Survey of Canada, found footprints from a tetrapod. Blue Beach maintains a fossil museum that displays hundreds of Tournaisian fossils, which continue to be found as the eroding cliff continues to reveal new fossils. In 2012, tetrapod remains from four new Tournaisian sites in Scotland were announced. 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). As with East Kirkton Quarry, tetrapods at these sites were discovered through the long-term efforts of Stan Wood and colleagues.
In April 2013 scientists associated with the British Geological Survey (BGS) and the National Museums of Scotland announced the TW:eed project (Tetrapod World: early evolution and diversification), which will drill a continuous 500-meter borehole at an undisclosed location near Berwick-upon-Tweed that is expected to produce a complete sampling of Tournaisian sediment, without discontinuities; the rock core will provide a timeline on which fossil discoveries can be accurately placed.
- Smithson, T.R.; Wood, S.P.; Marshall, J.E.A.; and Clack, J.A. (2012). "Earliest Carboniferous tetrapod and arthropod faunas from Scotland populate Romer's Gap". Proceedings of the National Academy of Sciences of the United States of America. in press. doi:10.1073/pnas.1117332109.
- Coates, Michael I.; Clack, Jennifer A. (1995). "Romer's gap: tetrapod origins and terrestriality". Bulletin du Muséum national d'Histoire naturelle 17: 373–388. ISSN 0181-0642.
- By 1955 (perhaps even earlier), Romer states that few good fossils of tetrapods have been recovered from early Carboniferous deposits. See: Romer, Alfred Sherwood (presented: November 11, 1955 ; published: June 28, 1956) "The early evolution of land vertebrates," Proceedings of the American Philosophical Society, 100 (3) : 151-167; see especially page 166. Available on-line at: JSTOR.
- Ward, Peter; Labandeira, Conrad; Laurin, Michel; Berner, Robert A. (7 November 2006). "Confirmation of Romer's Gap as a low oxygen interval constraining the timing of initial arthropod and vertebrate terrestrialization". PNAS 103 (45): 16818–16822. doi:10.1073/pnas.0607824103. JSTOR 30051753. PMC 1636538. PMID 17065318.
- Sallan, Lauren Cole; Coates, Michael I. (1 June 2010). "End-Devonian extinction and a bottleneck in the early evolution of modern jawed vertebrates". PNAS 107 (22): 10131–10135. doi:10.1073/pnas.0914000107. PMC 2890420. PMID 20479258.
- Coates, Michael I.; Ruta, Marcello; Friedman, Matt (2008). "Ever Since Owen: Changing Perspectives on the Early Evolution of Tetrapods" (PDF, 1.0 MB). Annual Review of Ecology, Evolution, and Systematics 39: 571–592. doi:10.1146/annurev.ecolsys.38.091206.095546.
- Sallan, Lauren Cole; Kammer, Thomas W.; Ausich, William I.; Cook, Lewis A. (17 May 2011). "Persistent predator-prey dynamics revealed by mass extinction". PNAS 108 (20): 8335–8338. doi:10.1073/pnas.1100631108. PMC 3100987. PMID 21536875.
- Clack, Jennifer A. (June 2002). Gaining Ground: The Origin and Evolution of Tetrapods (1st ed.). Bloomington, IN: Indiana University Press. ISBN 978-0-253-34054-2. LCCN 2001004783. OCLC 47767251.
- Kammer, Thomas W.; Ausich, William I. (June 2006). "The "Age of Crinoids": A Mississippian biodiversity spike coincident with widespread carbonate ramps" (PDF, 0.6 MB). Palaios 21 (3): 238–248. doi:10.2110/palo.2004.p04-47.
- "Paleos Proterozoic: Proterozoic sites". 9 April 2002. Archived from the original on 2009-02-24. Retrieved 2012-03-06.
- "Blue Beach Fossil Museum". 9 May 2012. Retrieved 2012-05-09.
- "Blue Beach Fossil Museum". 9 May 2012. Retrieved 2012-05-09.
- "Fossil hunters dig deep in Scottish Borders", news.Scotsman.com: accessed 6 April 3013