Thecat gap is a period in thefossil record of approximately25 million to 18.5 million years ago in which there are few fossils ofcats or cat-like species found inNorth America. The cause of the "cat gap" is disputed, but it may have been caused by changes in the climate (global cooling), changes in the habitat andenvironmentalecosystem, the increasinglyhypercarnivorous trend of the cats (especially thenimravids),volcanic activity,evolutionary changes indentalmorphology of theCanidae species present in North America, or a periodicity of extinctions calledvan der Hammen cycles.[1]
All moderncarnivorans, includingfelids, evolved frommiacoids, which existed from approximately 66 to 33 million years ago. There were other earlier cat-like species butProailurus (meaning "before the cat"; also called "Leman's Dawn Cat"), which appeared about 30 million years ago, is generally considered the first "true cat".[2]
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, althoughProailurus 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.[2]
The increase in disparity through the early Miocene occurs during a time when few feliform fossils have been found in North America. The hypercarnivorous nimravid feliforms were extinct in North America after 26Ma and felids did not arrive in North America until theMiddle Miocene with the appearance ofPseudaelurus.Pseudaelurus crossed over to North America by way of theBering land bridge from surviving populations inAsia 18.5 million years ago. All modern-dayfelids are descended fromPseudaelurus.
Nimravids, also known as false-sabertooths cats, were a group of saber-toothed predators. Despite their cat-like appearance, they aren’t actually true cats, but instead a distinct family. Physically, some nimravids resembled the saber-toothed catSmilodon, which would not appear until many millions of years later. Nimravids first appeared in North America around 40 Ma, with the appearance ofPangurban.[3] The extinction of nimravids in North America marked the beginning of the "cat gap".[4][5]
The history of carnivorous mammals is characterized by a series of rise-and-fall patterns of diversification, in which decliningclades are replaced byphylogenetically 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) that lead tohypercarnivory dietary specialization. Hypercarnivory leads to increased vulnerability to extinction.
Thenimravids were large cat-like animals that occupied this ecomorphic niche in theecosystem 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 theBering 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.
Another possible explanation for the extinction of feliforms in North America ischanges in the ecology of the continent. Evidence from thegeologic temperature record shows that the earth was experiencing a period ofglobal cooling, causingforests to give way tosavannas.[2][5] Nimravids, the cat-like predators present in North America at the time, favored forested environments,[6][7] with a few exceptions such asEusmilus, which showed more adaptations for open environments.[8]
Climatic changes to arid conditions that muted variation at about 25.8 Ma coincides with the first appearance ofhoglikecreodonts and ofpocket gophers, and this also is the beginning of the "cat gap", as well as the "entelodont gap".Faunal overturn at 25.8 Ma is the basis for division of theArikareean time period (30.5–19 Ma), and the Arikareen NALMA (North American Land-Mammal Ages), into theMonroecreekian period (29.5–25.8 Ma), and then theHarrisonian period (25.8–23.5 Ma).[9]
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.[10]
Volcanic activity has also been promoted as a possible cause of the cat gap as well as other extinctions during this time period. TheLa Garita Caldera is a large volcaniccaldera located in theSan Juan Mountains in southwesternColorado, United States, and is one of a number of calderas that formed during a massiveignimbrite flare-up in Colorado, Utah, and Nevada during theOligoceneEpoch. The La Garita Caldera was the site of theFish 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 10,000 km3 or 2,400 cu mi for aVEI 8+ magnitude), and was possibly the most energetic event on Earth since theChicxulub impact, which is thought by many paleontologists to have caused theextinction of thedinosaurs in theCretaceous–Paleogene extinction event. The resulting explosive volcanism probably ejected large amounts of dust and debris into thestratosphere causing major cooling (seevolcanic winter). Climatic effects could also have been caused bysulphur ejected into thestratosphere, which rapidly converts tosulphuric acid, anaerosol which cools thetroposphere by blocking incomingsolar radiation.
Another possible cause of the cat gap could have been theLate Cenozoic Ice Age that began 33.9 million years ago. This ice age causedglaciation inAntarctica that eventually spread toArctic regions of southernAlaska,Greenland, andIceland. 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-weathercaniformia species such ascanids (dog-like species),mustelids (weasel-like species), andursids (bear-like species).
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 thethermohaline 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.[11]
It has been suggested by some that as a result of the cat gapcaniforms (dog-like species including canids, bears, weasels, and other related taxa) evolved to fill more carnivorous andhypercarnivorousecological niches that would otherwise have been filled by cats.[12] This conclusion, however, is disputed.[13]
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 crushingmolars. In North America the first caniform group of moderate body size to move in the direction of hypercarnivory were theendemichesperocyonine canids, with three genera (Parenhydrocyon,Enhydrocyon, andMesocyon), ranging in size from jackals to small coyotes, appearing in the earlyArikareean (circa 28 MYA). Notably, these three evolved alongside the lasthyaenodont and the remaining three nimravids, two of which werepuma-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 adaptedamphicyonids, one endemic (Daphoenodon) and two from the Old World (Temnocyon andMammocyon), a leopard-sizedmustelid (Megalictis) as well as two hypercarnivorous bears, thehemicyoninesCephalogale andPhoberocyon.[12]
However, other paleontologists take issue with this conclusion:
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 calculatedmorphospace ... Canids never occupy the area of morphospace in which felids, nimravids, and hypercarnivorouscreodonts 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.[13]
