Cat gap

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Proailurus may have been the first true feline. It evolved shortly before the beginning of the cat gap.

The cat gap is a period in the fossil record of approximately 25 to 18.5 million years ago in which there are few fossils of cats or cat-like species found in North America. It is thought that all cat and cat-like species went extinct in North America during this time period. The cause of the cat gap is disputed, but may have been caused by changes in the climate, changes in the environmental ecosystem, dominance of hypercarnivore cats (nimravids), global cooling, volcanic activity, evolutionary changes in dental morphology, or possibly even variations in the North American mammalian ecomorph cycle.

Cat evolution

Feliform evolutionary timeline

All modern carnivores, including cats, evolved from miacoids, which existed from approximately 65 to 33 million years ago. Miacids gave rise to Proailurus (literally meaning "first cat" or "dawn cat"), which existed approximately 26 million years ago, and is generally considered the first true feline.

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

The increase in disparity through the early Miocene occurs during a time when no feliforms were present 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. All modern-day cats are descended from Pseudaelurus.

Nimravids were saber-toothed cat-like animals of the family Nimravidae. Although not cats, Nimravidae were distantly related to the true cats. Physically, Nimravidae resembled the Smilodon (which would not evolve until many millions of years later). Nimravidae also went extinct in North America during the cat gap.

Hypercarnivorous tendency as a possible cause of the "cat-gap"

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. Over the past 50 million years, successive clades of 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 dietary specialization (hypercarnivory). Hypercarnivory leads to increased vulnerability of extinction.

The nimravids were large cats that occupied this ecomorphic niche in the ecosystem until 26 Ma. It is highly likely that their hypercarnivory led to their extinction in North America. After their extinction their were no other felines to fill their gap until other felines arrived from Eurasia after crossing the Bering land bridge 18.5 Ma.

Other possible causes of the "cat-gap"

Many cats tend to be arboreal hunters. The disappearance of forests in North America may have caused the mass extinction.

One 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. 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’’ and the ‘‘entelodont gap’’, a period of some 7 million years when there were no nimravids, felids, or entelodonts in North America. Faunal overturn at 25.8 Ma is the basis for division of the Arikareean time period (30.5–19 Ma), or 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).

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.

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 is a large volcanic caldera located in the San Juan Mountains in southwestern Colorado, United States, and is one of a number of calderas that formed during a massive ignimbrite flare-up in Colorado, Utah and Nevada during the Oligocene Epoch. The La Garita Caldera was the site of enormous eruptions 28–26 million years ago. The scale of the La Garita volcanism was far beyond anything known in human history, and was possibly the most energetic event on Earth since the Chicxulub impact, which is thought by many paleontologists to have caused the extinction of the dinosaurs (see: Cretaceous–Tertiary extinction event). The resulting explosive volcanism could have ejected large amounts of dust and debris into the stratosphere causing major cooling. Climatic effects could also have been caused by sulphur ejected into the stratosphere, which rapidly converts to sulphuric acid, an aerosol which cools the troposphere by blocking incoming solar radiation.

Another possible cause of the cat gap could have been the late Cenozoic ice age that began approximately 30 million years ago. This ice age caused glaciation in Antarctica that eventually spread to Arctic regions of southern Alaska, Greenland, and Iceland. Glaciers and ice sheets on the North American continent, as well as the cooling trend, could have made the ecosystem unhabitable for feliformia cat-like species, yet habitable for cold-weather caniformia species such as canids (dog-like species), mustelids (weasel-like species), and ursids (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 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.

Evolution of Caniforms in response to the cat gap

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Some paleontologists argue that caniforms like Amphicyonidae - "Bear dogs" - responded to the cat gap by evolving to become more cat-like, to fill the hypercarnivore ecological niche

Some paleontologists have theorized that as a result of the cat gap, caniforms (dog-like species including canids, bears, weasels, and other related taxons) evolved a more carnivorous and hypercarnivorous adaption to fill ecological niches that would otherwise have been filled by cats., however recent data does not support this hypothesis. Hypercarnivore feliforms (felids and nimravids) occupied an area that canids did not and where felids, nimravids, and hypercarnivorous creodonts are found. Further, hypercarnivorous canids were present before the disappearance of the nimravids. Following the extinction of nimravids, only three new taxa originated, two of which were relatively small in body size. Disparity increased during the "cat-gap" even with the extinction of the hypercarnivorous extremes. This was due to the extinction of morphological intermediates, and because carnivorans began to occupy hypocarnivorous (non-meat-specialist) morphospace - not hypercarnivorous morphospace - for the first time in North America. For example, Procyonids began to arrive in North America in the early Miocene, and "modern" ursids arrived in the late Miocene. Extinct lineages of Ursidae were present in North America from the late Eocene through the Miocene, and Amphicyonid (Bear-dogs) were present during this period as well, but they occupied a morphospace generally shared with canids and not in close proximity to ursids.

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 molars. In North America the first caniform group of moderate body size to move in the direction of hypercarnivory were the endemic hesperocyonine canids, with three genera (Parenhydrocyon, Enhydrocyon, and Mesocyon), ranging in size from jackals to small coyotes, appearing in the early Arikareean (circa 28 MYA). Notably, these three evolved alongside the last hyaenodont and the remaining three nimravids, two of which were puma-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 amphicyonids, one endemic (Daphoenodon) and two from the Old World (Temnocyon and Mammocyon), a leopard-sized mustelid (Megalictis) as well as two hypercarnivorous bears, the hemicyonines Cephalogale and Phoberocyon.

However, other paleontologists dispute this:

It has been suggested that canids evolved hypercarnivorous morphologies because feliforms were absent during this period (the ‘cat-gap’’, 26–16 Ma) (Van Valkenburgh 1991). The data presented here do not support this hypothesis. In the calculated morphospace... Canids never occupy the area of morphospace in which felids, nimravids, and hypercarnivorous creodonts 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 went 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."