Cat gap
Encyclopedia
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
. The cause of the "cat gap" is disputed, but may have been caused by changes in the climate (global cooling
), changes in the habitat and environmental
ecosystem
, the increasingly hypercarnivorous
trend of the cats (especially the nimravids), volcanic activity, evolution
ary changes in dental
morphology
of the Canidae
species present in North America, or possibly even attributed to patterns of periodicity of extinctions (climatic/floral cycles called "van der Hammen cycles")
All modern carnivorans
, including cats, evolved from miacoids
, which existed from approximately 65 to 33 million years ago. Miacoid species gave rise to Proailurus
(meaning "first cat"; also called "Leman's Dawn Cat"), which appeared about 30 million years ago, and is generally considered the first "true cat". However, there were other cat-like species before Proailurus, but not within the family of the order Carnivora.
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. All modern-day cats are descended from Pseudaelurus.
Nimravids were saber-toothed cat-like
animals of the family Nimravidae
. Although not "true cats" in the Felidae
family, Nimravidae are considered to be a sister taxon
to felids. They are basal feliforms, but their exact placement within the Carnivora group is still uncertain. Physically, Nimravidae resembled the Smilodon
(which would not evolve until many millions of years later). Nimravidae also became extinct in North America during the "cat gap."
) that lead to (hypercarnivory) dietary specialization. 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 the extinction of the nimravids there were no other feliform or Felidae
species until other felids arrived from Eurasia after crossing the Bering land bridge
18.5 million years ago. During this time there was great diversity among the other carnivorous mammals in North America - both hypocarnivory and hypercarnivory species - and other hypercarnivory species existed before, during, and after the cat gap.
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 forest
s to give way to savanna
s. 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. Fauna
l 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).
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 the 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 10000 km³ (2,399.1 cu mi) for a VEI
8+ magnitude), 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 in the Cretaceous–Tertiary extinction event. The resulting explosive volcanism probably ejected large amounts of dust and debris into the stratosphere
causing major cooling (see volcanic winter
). 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 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
species such as canids (dog-like species), mustelids (weasel-like species), and ursids
(bear-like species).
It has been suggested by some 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 niche
s that would otherwise have been filled by cats. This conclusion, however, is disputed.
Canid diversity
did increase during the cat gap period, but this diversity is not indicative that the diversity was due to the cat gap. Hypercarnivore feliforms (felids and nimravids) occupied an area that canids did not and where felids, nimravids, and hypercarnivorous creodonts are found. 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 carnivora
ns began to occupy hypocarnivorous
(non-meat-specialist) morphospace for the first time in North America. 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 the ursids.
However, other paleontologists take issue with this conclusions:
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
in which there are few fossils of cats or cat-like species
Feliformia
The Feliformia are a suborder within the order Carnivora and includes cats , hyenas, mongooses, civets and related taxa. The other suborder of Carnivora is Caniformia...
found in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. The cause of the "cat gap" is disputed, but may have been caused by changes in the climate (global cooling
Global cooling
Global cooling was a conjecture during the 1970s of imminent cooling of the Earth's surface and atmosphere along with a posited commencement of glaciation...
), changes in the habitat and environmental
Environment (biophysical)
The biophysical environment is the combined modeling of the physical environment and the biological life forms within the environment, and includes all variables, parameters as well as conditions and modes inside the Earth's biosphere. The biophysical environment can be divided into two categories:...
ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
, the increasingly hypercarnivorous
Hypercarnivore
A hypercarnivore is an animal which has a diet that is more than 70% meat, with the balance consisting of non-animal foods such as fungi, fruits or other plant material. Some examples include the big cats, dolphins, eagles, snakes, marlin, most sharks, and such invertebrates as octopuses and sea...
trend of the cats (especially the nimravids), volcanic activity, evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary changes in dental
Tooth
Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...
morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
of the Canidae
Canidae
Canidae is the biological family of carnivorous and omnivorous mammals that includes wolves, foxes, jackals, coyotes, and domestic dogs. A member of this family is called a canid . The Canidae family is divided into two tribes: Canini and Vulpini...
species present in North America, or possibly even attributed to patterns of periodicity of extinctions (climatic/floral cycles called "van der Hammen cycles")
Cat evolution
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
, including cats, evolved from miacoids
Miacoidea
Miacoidea is paraphyletic superfamily that had been traditionally divided into two families of carnivores: Miacidae and Viverravidae. Miacoids were primitive carnivores which lived during the Paleocene and Eocene Epoch about 33-65 million years ago...
, which existed from approximately 65 to 33 million years ago. Miacoid species gave rise to Proailurus
Proailurus
Proailurus was a prehistoric carnivore that lived in Europe and Asia approximately 25 million years ago in the Late Oligocene and Miocene. One recent phylogeny places it as a basal member of the Feloidea, the superfamily that includes mongooses, civets, hyenas, and cats; but other studies suggest...
(meaning "first cat"; also called "Leman's Dawn Cat"), which appeared about 30 million years ago, and is generally considered the first "true cat". However, there were other cat-like species before Proailurus, but not within the family of the order Carnivora.
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
Middle Miocene
The Middle Miocene is a sub-epoch of the Miocene Epoch made up of two stages: the Langhian and Serravallian stages. The Middle Miocene is preceded by the Early Miocene....
with the appearance of Pseudaelurus. Pseudaelurus crossed over to North America by way of the Bering land bridge
Bering land bridge
The Bering land bridge was a land bridge roughly 1,000 miles wide at its greatest extent, which joined present-day Alaska and eastern Siberia at various times during the Pleistocene ice ages. Like most of Siberia and all of Manchuria, Beringia was not glaciated because snowfall was extremely light...
from surviving populations in Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
18.5 million years ago. All modern-day cats are descended from Pseudaelurus.
Nimravids were saber-toothed cat-like
Saber-toothed cat
Saber-toothed cat or Sabre-toothed cat refers to the extinct subfamilies of Machairodontinae , Barbourofelidae , and Nimravidae as well as two families related to marsupials that were found worldwide from the Eocene Epoch to the end of the Pleistocene Epoch ,...
animals of the family Nimravidae
Nimravidae
The Nimravidae, sometimes known as false saber-toothed cats, are an extinct family of mammalian carnivores belonging to the suborder Feliformia and endemic to North America, Europe, and Asia living from the Eocene through the Miocene epochs , existing for approximately .-Morphology:Although some...
. Although not "true cats" in the Felidae
Felidae
Felidae is the biological family of the cats; a member of this family is called a felid. Felids are the strictest carnivores of the thirteen terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily pinnipedia are as carnivorous as the...
family, Nimravidae are considered to be a sister taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
to felids. They are basal feliforms, but their exact placement within the Carnivora group is still uncertain. Physically, Nimravidae resembled the Smilodon
Smilodon
Smilodon , often called a saber-toothed cat or saber-toothed tiger, is an extinct genus of machairodonts. This saber-toothed cat was endemic to North America and South America, living from near the beginning through the very end of the Pleistocene epoch .-Etymology:The nickname "saber-tooth" refers...
(which would not evolve until many millions of years later). Nimravidae also became extinct in North America during the "cat gap."
Hypercarnivorous tendency as a probable 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 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 ruleCope's rule
Cope's rule states that population lineages tend to increase in body size over evolutionary time. While the rule has been demonstrated in many instances, it does not hold true at all taxonomic levels, or in all clades...
) that lead to (hypercarnivory) dietary specialization. Hypercarnivory leads to increased vulnerability of extinction.
The nimravids were large cats that occupied this ecomorphic niche in the ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
until 26 Ma. It is highly likely that their hypercarnivory led to their extinction in North America. After the extinction of the nimravids there were no other feliform or Felidae
Felidae
Felidae is the biological family of the cats; a member of this family is called a felid. Felids are the strictest carnivores of the thirteen terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily pinnipedia are as carnivorous as the...
species until other felids arrived from Eurasia after crossing the Bering land bridge
Bering land bridge
The Bering land bridge was a land bridge roughly 1,000 miles wide at its greatest extent, which joined present-day Alaska and eastern Siberia at various times during the Pleistocene ice ages. Like most of Siberia and all of Manchuria, Beringia was not glaciated because snowfall was extremely light...
18.5 million years ago. During this time there was great diversity among the other carnivorous mammals in North America - both hypocarnivory and hypercarnivory species - and other hypercarnivory species existed before, during, and after the cat gap.
Changes in climate and habitat
Ecological succession
Ecological succession, is the phenomenon or process by which a community progressively transforms itself until a stable community is formed. It is a fundamental concept in ecology, and refers to more or less predictable and orderly changes in the composition or structure of an ecological community...
of the continent. Evidence from the geologic temperature record
Geologic temperature record
The Geologic temperature record are changes in Earth's environment as determined from geologic evidence on multi-million to billion year time scales...
shows that the earth was experiencing a period of global cooling
Global cooling
Global cooling was a conjecture during the 1970s of imminent cooling of the Earth's surface and atmosphere along with a posited commencement of glaciation...
, causing forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
s to give way to savanna
Savanna
A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses.Some...
s. 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
Entelodont
Entelodonts, sometimes nicknamed hell pigs or terminator pigs, is an extinct family of pig-like omnivores endemic to forests and plains of North America, Europe, and Asia from the middle Eocene to early Miocene epochs , existing for approximately .-Taxonomy:Entelodontidae was named by Richard...
gap", a period of some 7 million years when there were no nimravids, felids, or entelodonts in North America. Fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
l overturn at 25.8 Ma is the basis for division of the Arikareean
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
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
Harrisonian
The Harrisonian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 24,800,000 to 20,600,000 years BP, a period of . It is usually considered to overlap the Chattian and Aquitarian stages...
period (25.8–23.5 Ma).
Other possible causes of the "cat-gap"
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 CalderaLa Garita Caldera
La Garita Caldera is a large volcanic caldera located in the San Juan volcanic field in the San Juan Mountains near the town of Creede in southwestern Colorado, United States. It lies to the west of the town of La Garita, Colorado...
is a large volcanic caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
located in the San Juan Mountains
San Juan Mountains
The San Juan Mountains are a high and rugged mountain range in the Rocky Mountains in southwestern Colorado. The area is highly mineralized and figured in the gold and silver mining industry of early Colorado. Major towns, all old mining camps, include Creede, Lake City, Silverton, Ouray, and...
in southwestern Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
, United States, and is one of a number of calderas that formed during a massive ignimbrite
Ignimbrite
An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere....
flare-up in Colorado, Utah, and Nevada during the Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
Epoch
Series (stratigraphy)
Series are subdivisions of rock layers made based on the age of the rock and corresponding to the dating system unit called an epoch, both being formally defined international conventions of the geological timescale. A series is therefore a sequence of rock depositions defining a...
. The La Garita Caldera was the site of the 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 10000 km³ (2,399.1 cu mi) for a VEI
Volcanic Explosivity Index
The Volcanic Explosivity Index was devised by Chris Newhall of the U.S. Geological Survey and Stephen Self at the University of Hawaii in 1982 to provide a relative measure of the explosiveness of volcanic eruptions....
8+ magnitude), and was possibly the most energetic event on Earth since the Chicxulub impact
Chicxulub Crater
The Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named...
, which is thought by many paleontologists to have caused the extinction
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
of the dinosaurs in the Cretaceous–Tertiary extinction event. The resulting explosive volcanism probably ejected large amounts of dust and debris into the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
causing major cooling (see volcanic winter
Volcanic winter
A volcanic winter is the reduction in temperature caused by volcanic ash and droplets of sulfuric acid obscuring the sun and raising Earth's albedo after a large particularly explosive type of volcanic eruption...
). Climatic effects could also have been caused by sulphur ejected into the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
, which rapidly converts to sulphuric acid, an aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...
which cools the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
by blocking incoming solar radiation.
Another possible cause of the cat gap could have been the late Cenozoic ice age
Timeline of glaciation
There have been five known ice ages in the Earth's history, with the Earth experiencing the Quaternary Ice Age during the present time. Within ice ages, there exist periods of more severe glacial conditions and more temperate referred to as glacial periods and interglacial periods, respectively...
that began approximately 30 million years ago. This ice age caused glaciation in Antarctica that eventually spread to Arctic
Arctic
The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...
regions of southern Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
, Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
, and Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
. 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
Caniformia
Caniformia, or Canoidea , is a suborder within the order Carnivora. They typically possess a long snout and non-retractile claws . The Pinnipedia evolved from caniform ancestors and are accordingly assigned to this group...
species such as canids (dog-like species), mustelids (weasel-like species), and ursids
Ursids
The Ursids meteor activity begins annually around December 17th and runs for a week plus, until the 25th or 26th. This meteor shower is named for its radiant point which is located near the star Beta Ursae Minoris in the constellation Ursa Minor....
(bear-like species).
Evolution of Caniforms during the "cat-gap"
Caniformia
Caniformia, or Canoidea , is a suborder within the order Carnivora. They typically possess a long snout and non-retractile claws . The Pinnipedia evolved from caniform ancestors and are accordingly assigned to this group...
(dog-like species including canids, bears, weasels, and other related taxons) evolved a more carnivorous and hypercarnivorous
Hypercarnivore
A hypercarnivore is an animal which has a diet that is more than 70% meat, with the balance consisting of non-animal foods such as fungi, fruits or other plant material. Some examples include the big cats, dolphins, eagles, snakes, marlin, most sharks, and such invertebrates as octopuses and sea...
adaption to fill ecological niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s that would otherwise have been filled by cats. This conclusion, however, is disputed.
Canid diversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
did increase during the cat gap period, but this diversity is not indicative that the diversity was due to the cat gap. Hypercarnivore feliforms (felids and nimravids) occupied an area that canids did not and where felids, nimravids, and hypercarnivorous creodonts are found. 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
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
intermediates, and because carnivora
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
ns began to occupy hypocarnivorous
Hypercarnivore
A hypercarnivore is an animal which has a diet that is more than 70% meat, with the balance consisting of non-animal foods such as fungi, fruits or other plant material. Some examples include the big cats, dolphins, eagles, snakes, marlin, most sharks, and such invertebrates as octopuses and sea...
(non-meat-specialist) morphospace for the first time in North America. 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 the ursids.
However, other paleontologists take issue with this conclusions: