Cat gap
Gap in the fossil record
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The cat gap is a period in the fossil record of approximately 25 million to 18.5 million years ago in which there are few fossils of cats or cat-like species found in North America following the extinction of nimravids. 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 and environmental ecosystem, the increasingly hypercarnivorous trend of the cat-like predators (the nimravids), volcanic activity, evolutionary changes in dental morphology of the Canidae species present in North America, or a periodicity of extinctions called van der Hammen cycles.[1]
Cat evolution
All modern carnivorans, including felids, evolved from miacoids, which existed from approximately 66 to 33 million years ago. There were other earlier cat-like species but 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".[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, 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.[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 26 Ma and felids did not arrive in North America until the Middle Miocene with the appearance of Pseudaelurus. Pseudaelurus crossed over to North America by way of the Bering land bridge from surviving populations in Asia 18.5 million years ago.[3][4] All modern-day felids are descended from Pseudaelurus-grade felids.[5]
Nimravids, also known as false-sabertoothed cats, were a group of saber-toothed predators that occupied the cat-like niche prior to the appearance of felids. Despite their cat-like appearance, they aren’t actually true cats, but instead a distinct family of feliforms.[6] Nimravids first appeared in North America around 40 Ma, with the appearance of Pangurban.[7] Their diversification was due to the extinction of oxyaenids, which opened the niche of cat-like predators.[8] North American nimravids reached their highest diversity of 6 contemporary species during the Whitneyan and early Arikareean.[9] Including supplementary materials The last North American nimravid, Pogonodon, went extinct 26 Ma, which marked the beginning of the "cat gap".[10][11][9][12]
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 are replaced by phylogenetically distinct but functionally similar clades.[13][8] 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 to hypercarnivory dietary specialization. Hypercarnivory leads to increased vulnerability to extinction.[14]
The nimravids were large cat-like animals that occupied this ecomorphic niche in the ecosystem until 26 Mya.[15][16] Including supplementary materials 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 18.5 million years ago.[3][4]
Changes in climate and habitat
Another possible explanation for the extinction of feliforms in North America is changes in the ecology of the continent. Evidence from the geologic temperature record shows that the earth was experiencing a period of global cooling, causing forests to give way to savannas.[2][11] Nimravids favored forested environments,[17][18] with a few exceptions such as Eusmilus, showing adaptations for open environments.[19] Nimravids were significantly less agile and slower than their preferred prey, which was disadvantageous in expanding grasslands.[11] Additionally, competition with caniforms such as amphicyonids and canids may have prevented nimravids from occupying open environments.[12][20]
Climatic changes to arid conditions that muted variation at about 25.8 Ma coincides with the first appearance of hoglike creodonts and of pocket 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 the Arikareean time period (30.5–19 Ma), and the Arikareen NALMA (North American Land-Mammal Ages), into the Monroecreekian period (29.5–25.8 Ma), and then the Harrisonian period (25.8–23.5 Ma).[21]
Other
Another possible cause of the cat gap could have been the Late Cenozoic Ice Age that began 33.9 million years ago. This ice age caused glaciation in Antarctica that eventually spread to Arctic regions of southern Alaska, Greenland, and 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 caniform species such as canids (dog-like species), mustelids (weasel-like species), and ursids (bear-like species).[22]
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 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.[22]
Evolution of caniforms during the gap
It has been suggested by Valkenburgh (1999) that as a result of the cat gap several caniforms families (canids, bears, mustelids, and amphicyonids) evolved several hypercarnivorous characteristics such as reduced snouts, somewhat enlarged canines, and the extreme reduction of crushing molars.[13] However, Wesley-Hunt (2005) disagreed with this conclusion as their morphospace analysis found that canids never occupied morphospaces that consisted of felids, nimravids, and hypercarnivorous creodonts. Additionally, they found that most of the hypercarnivorous canids sampled were present before the disappearance of nimravids and went extinct before the appearance of felids which suggests a progressive decline of hypercarnivorous forms during the cat gap. The invasion of hypercarnivorous morphospace wouldn't be substantial until the immigration of felids into North America.[23]