# Cat gap
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{{short description|Gap in the fossil record}}
The '''cat gap''' is a period in the [fossil record](/source/fossil_record) of approximately [25 million to 18.5 million years ago](/source/Neogene) in which there are few fossils of [cats or cat-like species](/source/Feliformia) found in [North America](/source/North_America). The cause of the "cat gap" is disputed, but it may have been caused by changes in the climate ([global cooling](/source/global_cooling)), changes in the habitat and [environmental](/source/Environment_(biophysical)) [ecosystem](/source/ecosystem), the increasingly [hypercarnivorous](/source/hypercarnivore) trend of the cats (especially the [nimravids](/source/nimravids)), [volcanic activity](/source/volcanic_activity), [evolution](/source/evolution)ary changes in [dental](/source/Tooth) [morphology](/source/Morphology_(biology)) of the [Canidae](/source/Canidae) species present in North America, or a periodicity of extinctions called [van der Hammen cycles](/source/Extinction_event).[{{cite journal |last1=Meehan |first1=T. J. |last2=Martin |first2=L. D. |year=2003 |title=Extinction and re-evolution of similar adaptive types (ecomorphs) in Cenozoic North American ungulates and carnivores reflect van der Hammens cycles |journal=Naturwissenschaften |volume=90 |issue=3 |pages=131–135 |url=http://mypage.iu.edu/~pdpolly/Papers/Polly%20et%20al,%202006,%20Viverravus.pdf |archive-url=https://web.archive.org/web/20110524193602/http://mypage.iu.edu/~pdpolly/Papers/Polly%20et%20al,%202006,%20Viverravus.pdf |archive-date=May 24, 2011 |url-status=dead |doi=10.1007/s00114-002-0392-1 |pmid=12649755 |bibcode=2003NW.....90..131M |s2cid=21117744 |access-date=November 28, 2008}}]
==Cat evolution==
{{main|Felidae#Evolutionary history}}
right|upright=1.36|thumbnail|Feliform evolutionary timeline
All modern [carnivora](/source/carnivora)ns, including [felids](/source/Felidae), evolved from [miacoids](/source/Miacoidea), which existed from approximately 66 to 33 million years ago. There were other earlier cat-like species but ''[Proailurus](/source/Proailurus)'' (meaning "before the cat"; also called "Leman's Dawn Cat"), which appeared about 30 million years ago, is generally considered the first "true cat".[{{cite book|last1=Hunter|first1=Luke|last2=Hinde|first2=Gerald|title=Cats of Africa: Behavior, Ecology, and Conservation|url=https://books.google.com/books?id=qQhuqVC4bnEC|year=2006|publisher=Johns Hopkins University Press|location=Cape Town|isbn=978-0-8018-8482-5|pages=40–42}}]
{{quote|Following the appearance of the dawn cat, there is little in the fossil record for 10 million years to suggest that cats would prosper. In fact, although ''Proailurus'' persisted for at least 14 million years, there are so few felid fossils towards the end of the dawn cat's reign that paleontologists refer to this as the "cat gap". The turning point for cats came about with the appearance of a new genus of felids, ''[Pseudaelurus](/source/Pseudaelurus)''.[{{Cite journal |last1=Poust |first1=Ashley |last2=Barrett |first2=Paul Z. |last3=Tomiya |first3=Susumu |date=2022 |title=An early nimravid from California and the rise of hypercarnivorous mammals after the middle Eocene climatic optimum |journal=Biology Letters |volume=18 |issue=10 |pages=333–347 |doi=10.1098/rsbl.2022.0291 |hdl=2433/276689 |pmc=9554728 |s2cid=252818430 |hdl-access=free}}] The extinction of nimravids in North America marked the beginning of the "cat gap".[{{cite journal |last1=Joeckel |first1=R. M. |last2=Peigneb |first2=Stephane |last3=Hunt |first3=Robert M. |last4=Skolnick |first4=Robert I. |year=2002 |title=The Auditory Region and Nasal Cavity of Oligocene Nimravidae |journal=Journal of Vertebrate Paleontology |volume=22 |issue=4 |pages=830–841 |url=http://www.bioone.org/perlserv/?request=get-abstract&issn=0272-4634&volume=022&issue=04&page=0830 |doi=10.1671/0272-4634(2002)022[0830:TARANC]2.0.CO;2 |s2cid=85608067 |url-access=subscription |access-date=November 28, 2008}}][{{cite book |last=Antón |first=Mauricio |author-link=Mauricio Antón |title=Sabertooth |date=2013 |publisher=University of Indiana Press |isbn=978-0-253-01042-1 |location=Bloomington, Indiana |page=90}}]
==Possible causes==
===Hypercarnivorous tendency===
The history of carnivorous mammals is characterized by a series of rise-and-fall patterns of diversification, in which declining [clades](/source/clades) are replaced by [phylogenetic](/source/phylogenetic)ally distinct but functionally similar clades. Over the past 50 million years, successive clades of small and large carnivorous mammals diversified and then declined to extinction. In most instances, the cause of the decline was energetic constraints and pervasive selection for larger size ([Cope's rule](/source/Cope's_rule)) that lead to [hypercarnivory](/source/hypercarnivory) dietary specialization. Hypercarnivory leads to increased vulnerability to extinction.
The [nimravid](/source/nimravid)s were large cat-like animals that occupied this ecomorphic niche in the [ecosystem](/source/ecosystem) until 26 Mya. It is highly likely that their hypercarnivory led to their extinction in North America. After the extinction of the nimravids, no other feliform or cat-like species were present in North America. This gap ended when felids arrived from Eurasia after crossing the [Bering land bridge](/source/Bering_land_bridge) 18.5 million years ago. During this time there was great diversity among the other carnivorous mammals in North America – both hypocarnivorous and hypercarnivorous species – and other hypercarnivorous species existed before, during, and after the cat gap.
===Changes in climate and habitat===
[[File:Leopard on the tree.jpg|right|thumbnail|Many cats tend to be [arboreal](/source/arboreal) hunters. The disappearance of forests in North America may have caused the mass extinction.]]
Another possible explanation for the extinction of feliforms in North America is [changes in the ecology](/source/Ecological_succession) of the continent. Evidence from the [geologic temperature record](/source/geologic_temperature_record) shows that the earth was experiencing a period of [global cooling](/source/global_cooling), causing [forest](/source/forest)s to give way to [savanna](/source/savanna)s.[{{cite journal |last1=Averianov |first1=Alexander |last2=Obraztsova |first2=Ekaterina |last3=Danilov |first3=Igor |last4=Skutschas |first4=Pavel |last5=Jin |first5=Jianhua |date=2016 |title=First nimravid skull from Asia |journal=Scientific Reports |volume=6 |bibcode=2016NatSR...625812A |doi=10.1038/srep25812 |pmc=4861911 |pmid=27161785 |article-number=25812}}][{{cite book |last=Antón |first=Mauricio |author-link=Mauricio Antón |title=Sabertooth |date=2013 |publisher=University of Indiana Press |isbn=978-0-253-01042-1 |location=Bloomington, Indiana |page=47}}] with a few exceptions such as ''[Eusmilus](/source/Eusmilus)'', which showed more adaptations for open environments.[Castellanos, Miguel (2024). [https://huskiecommons.lib.niu.edu/cgi/viewcontent.cgi?article=8874&context=allgraduate-thesesdissertations ''Hunting Types in North American Eocene and Oligocene Carnivores and Implications for Nimravid Extinction''] (Graduate Research Thesis & Disserations)]
Climatic changes to arid conditions that muted variation at about 25.8 Ma coincides with the first appearance of [hoglike](/source/Pig) [creodont](/source/creodont)s and of [pocket gophers](/source/pocket_gophers), and this also is the beginning of the "cat gap", as well as the "[entelodont](/source/entelodont) gap". [Fauna](/source/Fauna)l overturn at 25.8 Ma is the basis for division of the [Arikareean](/source/Miocene) time period (30.5–19 Ma), and the Arikareen NALMA (North American Land-Mammal Ages), into the [Monroecreekian](/source/Monroecreekian) period (29.5–25.8 Ma), and then the [Harrisonian](/source/Harrisonian) period (25.8–23.5 Ma).[{{cite journal |last=Retallack |first=Gregory J. |year=2004 |title=Late Oligocene bunch grassland and early Miocene sod grassland paleosols from central Oregon, USA |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=207 |issue=3–4 |pages=203–237 |url=http://www.uoregon.edu/~gregr/Papers/Oligograss.pdf |archive-url=https://web.archive.org/web/20080830080512/http://www.uoregon.edu/~gregr/Papers/Oligograss.pdf |archive-date=August 30, 2008 |url-status=dead |doi=10.1016/j.palaeo.2003.09.027 |bibcode=2004PPP...207..203R |access-date=November 28, 2008}}]
{{quote|Why did these cat-like creatures die out in North America (while surviving in Eurasia) with no replacement by the true cats? Their fate may be owed to the same factors that created the diversity of herbivorous mammals, for most cats need forest or cover from which to hunt. In an increasingly open America the nimravids may have found themselves without an ecological perch to hunt from, particularly if the competition with dogs prevented them from colonising the savannas.[{{cite book |title=The Eternal Frontier: An Ecological History of North America and Its Peoples |last=Flannery |first=Tim |year=2002 |publisher=Grove Press |location=New York |isbn=0-8021-3888-8 |pages=113–114}}]}}
===Other===
Volcanic activity has also been promoted as a possible cause of the cat gap as well as other extinctions during this time period. The [La Garita Caldera](/source/La_Garita_Caldera) is a large volcanic [caldera](/source/caldera) located in the [San Juan Mountains](/source/San_Juan_Mountains) in southwestern [Colorado](/source/Colorado), United States, and is one of a number of calderas that formed during a massive [ignimbrite](/source/ignimbrite) flare-up in Colorado, Utah, and Nevada during the [Oligocene](/source/Oligocene) [Epoch](/source/Series_(stratigraphy)). The La Garita Caldera was the site of the [Fish Canyon eruption](/source/Fish_Canyon_eruption), an enormous eruption about 27 million years ago. The scale of the Fish Canyon eruption was far beyond anything known in human history (erupting more than {{convert|10000|km3|cumi|abbr=on|disp=or}} for a [VEI](/source/Volcanic_Explosivity_Index) 8+ magnitude), and was possibly the most energetic event on Earth since the [Chicxulub impact](/source/Chicxulub_crater), which is thought by many paleontologists to have caused the [extinction](/source/extinction) of the [dinosaurs](/source/dinosaurs) in the [Cretaceous–Paleogene extinction event](/source/Cretaceous%E2%80%93Paleogene_extinction_event). The resulting explosive volcanism probably ejected large amounts of dust and debris into the [stratosphere](/source/stratosphere) causing major cooling (see [volcanic winter](/source/volcanic_winter)). Climatic effects could also have been caused by [sulphur](/source/sulphur) ejected into the [stratosphere](/source/stratosphere), which rapidly converts to [sulphuric acid](/source/sulphuric_acid), an [aerosol](/source/aerosol) which cools the [troposphere](/source/troposphere) by blocking incoming [solar radiation](/source/solar_radiation).
Another possible cause of the cat gap could have been the [Late Cenozoic Ice Age](/source/Late_Cenozoic_Ice_Age) that began 33.9 million years ago. This ice age caused [glaciation](/source/glaciation) in [Antarctica](/source/Antarctica) that eventually spread to [Arctic](/source/Arctic) regions of southern [Alaska](/source/Alaska), [Greenland](/source/Greenland), and [Iceland](/source/Iceland). Glaciers on the North American continent, as well as the cooling trend, could have made the ecosystem uninhabitable for feliformia cat-like species, although habitable for cold-weather [caniformia](/source/caniformia) species such as [canids](/source/canids) (dog-like species), [mustelids](/source/mustelids) (weasel-like species), and [ursids](/source/ursids) (bear-like species).
{{quote|There is also evidence that during the Miocene a sill surrounding the Arctic Ocean, known as the Greenland–Scotland Ridge, subsided, allowing more cold polar water to escape into the North Atlantic. As the salinity of the North Atlantic grew and as outflow of cold polar water increased, so the [thermohaline circulation](/source/thermohaline_circulation) increased in vigour, providing the mild winter temperatures and large amounts of moisture to the North Atlantic, which are prerequisites to the build-up of the large continental ice caps on the adjacent cold continents.[{{cite book |chapter=Ice-age Theories |title=The Oxford Companion to the Earth |last=Haggart |first=B. A. |year=2000 |publisher=Oxford University Press |location=New York |chapter-url=http://www.encyclopedia.com/doc/1O112-iceagetheories.html}}]}}
==Evolution of caniforms during the gap==
[[File:Ysengrinia.jpg|right|thumbnail|Some paleontologists argue that caniforms like [Amphicyonidae](/source/Amphicyonidae) – "bear dogs" - responded to the cat gap by evolving to become more cat-like, to fill the [hypercarnivore](/source/hypercarnivore) [ecological niche](/source/ecological_niche)[{{cite journal |last=Wesley-Hunt |first=Gina D. |year=2005 |title=The morphological diversification of carnivores in North America |journal=Paleobiology |volume=31 |issue=1 |pages=35–55 |url=http://www.redorbit.com/news/science/120516/the_morphological_diversification_of_carnivores_in_north_america/index.html |doi=10.1666/0094-8373(2005)031<0035:TMDOCI>2.0.CO;2 |bibcode=2005Pbio...31...35W |s2cid=10989917 |url-access=subscription |access-date=November 28, 2008}}]
{{quote|During or just prior to this "cat gap", numerous caniform species evolve catlike features indicative of hypercarnivory, such as reduced snouts, somewhat enlarged canines, and fairly extreme reduction of their crushing [molar](/source/Molar_(tooth))s. In North America the first caniform group of moderate body size to move in the direction of hypercarnivory were the [endemic](/source/endemic) [hesperocyonine](/source/Hesperocyoninae) canids, with three genera (''[Parenhydrocyon](/source/Parenhydrocyon)'', ''[Enhydrocyon](/source/Enhydrocyon)'', and ''[Mesocyon](/source/Mesocyon)''), ranging in size from jackals to small coyotes, appearing in the early [Arikareean](/source/Arikareean) (circa 28 MYA). Notably, these three evolved alongside the last [hyaenodont](/source/hyaenodont) and the remaining three nimravids, two of which were [puma](/source/Cougar)-sized. The small hypercarnivorous canids were soon joined by and ultimately replaced by numerous species from other families which also had evolved more specialized meat-eating teeth and skulls. These included at least three larger genera of similarly adapted [amphicyonid](/source/amphicyonid)s, one endemic (''[Daphoenodon](/source/Daphoenodon)'') and two from the Old World (''[Temnocyon](/source/Temnocyon)'' and ''[Mammocyon](/source/Mammocyon)''), a leopard-sized [mustelid](/source/Mustelidae) (''[Megalictis](/source/Megalictis)'') as well as two hypercarnivorous bears, the [hemicyonine](/source/hemicyonine)s ''[Cephalogale](/source/Cephalogale)'' and ''[Phoberocyon](/source/Phoberocyon)''.[{{cite journal |last=Van Valkenburgh |first=Blaire |year=1999 |title=Major Patterns in the History of Carnivorous Mammals |journal=Annual Review of Earth and Planetary Sciences |volume=27 |issue=1 |pages=463–493 |doi=10.1146/annurev.earth.27.1.463 |bibcode=1999AREPS..27..463V |url=https://zenodo.org/record/890156}}]}}
However, other paleontologists take issue with this conclusion:
{{quote|It has been suggested that canids evolved hypercarnivorous morphologies because feliforms were absent during this period (the "cat-gap", 26–16 Ma). The data presented here do not support this hypothesis. In the calculated [morphospace](/source/morphospace) ... Canids never occupy the area of morphospace in which felids, nimravids, and hypercarnivorous [creodont](/source/creodont)s are found. More pertinent to the issue at hand, however, is that most of these hypercarnivorous canids were present before the disappearance of the nimravids, and all became extinct before the appearance of felids ... There was a progressive and marked decrease in hypercarnivorous forms during the "cat-gap". 28–20 Ma are characterized by above average extinction intensities and below average origination intensities. 20 Ma was marked by an increase in origination intensity, and 18 Ma showed a decrease in extinction intensity and a large increase in origination intensity. Nonetheless, despite increased origination intensities and decreased extinction intensities near the end of the "cat-gap" (20–16 Ma), there was still no substantial invasion of hypercarnivorous morphospace until the immigration of felids into North America.