Caniformia

Carnivorans evolved from miacoids about 55 million years ago (Mya) during the late Paleocene.Some five million years later, the carnivorans split into two main divisions: caniforms (dog-like) and feliforms (cat-like).They soon split into cat-like and dog-like forms (Feliformia and Caniformia). Their molecular phylogeny shows the extant Carnivora are a monophyletic group, the crown group of the Carnivoramorpha.
The superfamily Canoidea (or suborder Caniformia) – Canidae (wolves, dogs and foxes), Mephitidae (skunks and stink badgers), Mustelidae (weasels, badgers, and otters), Procyonidae (raccoons), Ursidae (bears), Ailuridae (red panda), Otariidae (eared seals), Odobenidae (walrus), and Phocidae (earless seals) (the last three families formerly classified in the superfamily Pinnipedia) and the extinct family Amphicyonidae (bear-dogs) – are characterized by having nonchambered or partially chambered auditory bullae, nonretractable claws, and a well-developed baculum. Most species are rather plain in coloration, lacking the flashy spotted or rosetted coats like many species of felids and viverrids have. This is because Canoidea tend to range in the temperate and subarctic biomes, although Mustelidae and Procyonidae have a few tropical species. Most are terrestrial, although a few species, like procyonids, are arboreal. All families except the Canidae and a few species of Mustelidae are plantigrade. Diet is varied and most tend to be omnivorous to some degree, and thus the carnassial teeth are less specialized. Canoidea have more premolars and molars in an elongated skull.
By 40 Mya, the first member of the dog family proper had arisen. Called Prohesperocyon wilsoni, its fossilized remains have been found in what is now the southwestern part of Texas. The chief features which identify it as a canid include the loss of the upper third molar (part of a trend toward a more shearing bite), and the structure of the middle ear which has an enlarged bulla (the hollow bony structure protecting the delicate parts of the ear). Prohesperocyon probably had slightly longer limbs than its predecessors, and also had parallel and closely touching toes which differ markedly from the splayed arrangements of the digits in bears.
The canid family soon subdivided into three subfamilies, each of which diverged during the Eocene: Hesperocyoninae (about 39.74–15 Mya), Borophaginae (about 34–2 Mya), and Caninae (about 34–0 Mya). Caninae is the only surviving subfamily and all present-day canids including wolves, foxes, coyotes, jackals and domestic dogs belong to it. Members of each subfamily showed an increase in body mass with time and some exhibited specialized hypercarnivorous diets that made them prone to extinction.
By the Oligocene, all three subfamilies of canids (Hesperocyoninae, Borophaginae, and Caninae) had appeared in the fossil records of North America. The earliest and most primitive branch of the Canidae was the Hesperocyoninae lineage, which included the coyote-sized Mesocyon of the Oligocene (38–24 Mya). These early canids probably evolved for the fast pursuit of prey in a grassland habitat; they resembled modern civets in appearance. Hesperocyonines eventually became extinct in the middle Miocene. One of the early members of the Hesperocyonines, the genus Hesperocyon, gave rise to Archaeocyon and Leptocyon. These branches led to the borophagine and canine radiations.
Around 9–10 Mya during the Late Miocene, Canis, Urocyon, and Vulpes genera expanded from southwestern North America, where the canine radiation began. The success of these canines was related to the development of lower carnassials that were capable of both mastication and shearing.Around 8 Mya, the Beringian land bridge allowed members of the genus Eucyon a means to enter Asia and they continued on to colonize Europe. During the Pliocene, around 4–5 Mya, Canis lepophagus appeared in North America. This was small and sometimes coyote-like. Others were wolf-like in characteristics. Canis latrans (the coyote) is theorized to have descended from Canis lepophagus.
The formation of the Isthmus of Panama, about 3 Mya, joined South America to North America, allowing canids to invade South America, where they diversified. However, the most recent common ancestor of the South American canids lived in North America some 4 Mya and the likelihood is that there were more than one incursion across the new land bridge. One of the resulting lineages consisted of the gray fox (Urocyon cinereoargentus) and the now-extinct dire wolf (Canis dirus). The other lineage consisted of the so-called South American endemic species; the maned wolf (Chrysocyon brachyurus), the short-eared dog (Atelocynus microtis), the bush dog (Speothos venaticus), the crab-eating fox (Cerdocyon thous) and the South American foxes (Lycalopex spp.). The monophyly of this group has been established by molecular means.
During the Pleistocene, the North American wolf line appeared, with Canis edwardii, clearly identifiable as a wolf, and Canis rufus appeared, possibly a direct descendant of Canis edwardii. Around 0.8 Mya, Canis ambrusteri emerged in North America. A large wolf, it was found all over North and Central America and was eventually supplanted by its descendant, the dire wolf, which then spread into South America during the late Pleistocene.

Amphicyonidae

Amphicyonidae is an extinct family of large terrestrial carnivorans belonging to the suborder Caniformia which inhabited North America, Europe, Asia, and Africa from the Middle Eocene subepoch to the Pliocene epoch 42—2.6 Mya, existing for about 39.4 million years.
There were two main types of  bear dogs. Some, like Borocyon robustum, had long limbs that were ideal for running and looked much like modern wolves. Others, such as Amphicyon longiramus, were stocky and looked more like modern bears.Amphicyonids ranged in size from as small as 5 kg (11 lb) and as large as 100 to 773 kg (220 to 1,704 lb) and evolved from wolf-like to bear-like body forms.Early amphicyonids, such as Daphoenodon, possessed a digitigrade posture and locomotion (walking on their toes), while many of the later and larger species were plantigrade or semiplantigrade.The amphicyonids were obligate carnivores, unlike the Canidae, which are hypercarnivores or mesocarnivores.

Some of the oldest bear dog remains belong to Daphoenus and are dated to as far back as 40.4 to 37.2 million years ago, something that places them in the Bartonian period of the Eocene epoch. This is in contrast to the common statement that amphicyonid bear dogs originated in Asia since the daphoenine bear dogs like Daphoenus appear to have been exclusive to North America. A little later in the Priabonian period of the Eocene and you get the first amphicyonine bear dogs (those that belong to the Amphicyoninae) such as Cynodictis and Guangxicyon appearing in Eurasia. From the end of the Eocene and into the Oligocene bear dogs continued to diversify and spread out and become more common. Not only were both the Daphoninae and Temnocyoninae groups of bear dog well established in North America during the Oligocene period, but they were joined by the first Eurasian bear dogs of the Amphicyoninae, such as Ysengrinia in the Rupelian stage.
During the Oligocene period however most of the known bear dog genera have been estimated to be well below one hundred kilograms in weight, with some such as Drassonax and Paradaphoenus being estimated to be even below two kilograms in weight. For modern comparisons, the North American raccoon (Procyon lotor) can weigh between three and half to nine kilograms, while the red fox (Vulpes vulpes) can weigh from just over two to fourteen kilograms (though rare individuals may be heavier). Why the early bear dogs seem to have been restricted to such small sizes is actually quite simple to understand when you consider that they were not the only predators on the landscape.
The creodont mammals had evolved much earlier, and some such as Sarkastodon and certain species of Hyaenodon were already in the apex predator slots of the ecosystems. Wolf-like mesonychids (mammals similar to Mesonyx) were also abundantly active during the Eocene to Oligocene eras. Further competition would have also come from the nimravids, one of the groups of mammals that are also known as the ‘false sabre-toothed cats’ which also had their origins in the late Eocene. To further add to this mix you have the entelodonts that may have actively killed animals as well as challenged other predators for their kills. With so many predators already filling the top predator niches, the early bear dogs would have had a difficult time joining and displacing them from their position, and so adapted to fill and hunt in the predatory niches that the top predators simply could not fill. This would see the earlier bear dogs hunting small animals that were both too quick and offered too little sustenance for the larger predators to expend their energy on. They are also likely to have also supplemented their hunting by scavenging the left over remains of the larger kills of predators, but this would also be risky since the larger predators were easily capable of killing smaller bear dogs with little effort.
       Time however was ultimately on the side of the bear dogs since the Oligocene period was a time of gradual climatic change which would force a shift in the types of ecosystems and the animals that lived in them. The Eocene period started as a warm tropical period which saw lush forests covering most of the planet, but cooling during the latter half began to cause a decrease in the coverage of the tropical forests. This development that started in the Eocene would accelerate in the Oligocene with tropical forests becoming equatorially restricted, leaving increasingly large open areas in their wake. These areas began to be filled with grasses which during the later Miocene period would develop into vast grassy plains.
Many of the herbivorous mammals such as primitive horses like Mesohippus were still browsers of vegetation rather than grazers of grass and this meant that predators could approach and lurk in the undergrowth without being seen by their prey. It would then be a simple matter of the predator waiting for its target to come close enough for an ambush, quickly bursting out from cover and hitting their victim before it knew what was happening. However the on-going loss of much of the forests brought three problems for these predators. One is that they no longer had the cover to approach virtually unseen, something that meant that they would now have to wait for their prey to come to them rather than actively hunt it out. Second is that the browsing herbivores now had to cover more ground to find suitable plants to eat. This saw a drive towards more energy efficient locomotion which was simply achieved by evolving longer legs so that they could cover more ground with each step. A secondary side benefit from this of course is that they could now also run much faster than their ancestors, so when they spotted a nearby predator (something that was getting easier for them to do), they had a much easier time escaping them since the older predators were adapting for running short distances in an ambush, not long high speed pursuit over open ground. Third is that towards the Miocene more and more herbivores made the switch to eating grass instead of browsing upon vegetation, which meant that the numbers of herbivores presenting themselves as targets gradually became less and less.

As the older predator forms began to face harder times in the Oligocene, the bear dogs actually found themselves at an advantage especially the amphicyonine bear dogs that had the physical proportions to match the new herbivores for speed as well as retain the striking power to effectively take them down. With this predatory niche open the bear dogs quickly adapted to fill it and with the start of the Miocene period, the larger genera such as Amphicyon and Pseudocyon had appeared while the creodonts were but a shadow of their former success. The Miocene also brought a severe reduction in the number of nimravid genera, but these seem to have been largely replaced by the barbourofelids, another group of cat-like mammals that were once largely classed to be the same as the nimravids. Some such as Barbourofelis grew large enough to challenge some bear dogs, though it was still not as large as some of the largest species such as Amphicyon ingens. The entelodonts also survived for a time into the Miocene by becoming even larger as evidenced by the appearance of largest entelodont genera such as Daeodon and Paraentelodon.
The Miocene period was not the end of the changes however; it was simply another chapter in the on-going shift towards a cooler dryer planet. As types of animals evolve you often see a shift towards some of the species becoming larger and more specialised, something that is driven by the success of previous adaptations tempered by competition with other forms. In predators large size allows them to hunt larger prey as well as dominate other predators so that they could not challenge them. The problem is that while this secures short term survival, long term continuance of the species is always in doubt because these specialisations rely upon only a few factors at best. When these factors change the species is in a position where it must adapt or die. This happened to the creodonts (which actually managed to survive in very limited numbers until the later stages of the Miocene) the mesonychids, entelodonts, nimravids and barbourofelids. It would also happen to the amphicyonids.
The most commonly depicted bear dogs are the larger ones of the Miocene era such as Amphicyon ingens (though Pseudocyon may actually have been a bit larger). However, Miocene era bear dogs continued to come in a variety of sizes with some such as Ictiocyon being estimated to be less than one and a half kilograms in weight (similar to their Oligocene era relatives). As a group the amphicyonids seem to have filled every single predatory niche, but when broken down into specific genera and species they come across as being quite specialised for different roles. For example, the small Ictiocyon was probably a very efficient hunter of small rodent-like mammals, but it was beyond tackling large horses like Hipparion. Amphicyon was easily capable of taking down chalicotheres like Chalicotherium, but smaller animals simply wouldn’t do, even if Amphicyon managed to catch a small two kilogram animal, it would need to eat more than just one a day to keep its huge body going.
Towards the end of the Miocene and start of the Pliocene periods, all new groups of mammalian carnivores began to appear, and one of the significant things about these appearances is that they included the more direct ancestors to most of the mammalian predators that we know today. Many of these forms which included the ancestors to modern wolves and dogs as well as true cats of the Felidae all developed further refined bodies more suited to open environments such as plains and steppe. The main factor for the rise and success of these new mammals however does not just seem to have been a case of continued physical refinement, but the establishment of higher levels of intelligence (something that a tens of million years earlier had actually helped amphicyonids dominate many of the older mammal types).
Study of bear dog skulls leads to the reconstruction and study of the brain in terms of overall size and development. The results of brain studies for bear dogs is that in terms on mental development they were below the level that modern day predators are at. Caution should be used before declaring bear dogs to be stupid however; it’s just that most of their brain development seems to be orientated more towards senses such as smell and vision rather than higher level processes such as problem solving. What this means is that bear dogs were very capable of processing information from their surrounding environment, but they were more reliant upon physical ability than mental processing. This means that the bar dog thought process probably went along the lines of find prey, approach prey, catch, prey, kill prey and eat prey.
Other predatory mammals will of course operate to an identical thought process, but the newer predators were beginning to elaborate on this. Modern day dogs and some species of cat have considerably more developed brains than those attributed to older mammal types. One benefit of this development is a greater level of social interaction to the point that some types of animal form groups that hunt as a single unit. The exact point of when mammals began to hunt in packs is next to impossible to determine with certainty, but fossil evidence concerning wolves and even the sabre-toothed cat Smilodon combined with modern studies of living predators has concluded that mammals were pack hunting at least as far back as the Pleistocene. If these traits can be carried back to their ancestors in the Pliocene then we see a world where the numbers of older solitary predators were dwindling in the face of pack hunting mammals.
At the moment there is no definitive evidence that proves pack hunting in bear dogs. In fact most depictions of amphicyonids hunting in groups are actually based upon the link between the more common name of bear dog with actual modern dog behaviour. As it has already been stated at the top of this article bear dogs were not dogs. They were related to a certain degree, but they were still separate. A lack of fossil evidence combined with an underdeveloped brain when compared to modern mammalian predators leads to the conclusion that amphicyonids were probably solitary animals for most of their lives.
Another factor in the demise of the bear dogs would be continually changing prey species. Horses in the early Miocene were still very primitive in development, but by the end of the Miocene they had become one of the fastest animal groups. Other mammals continued to develop speed and agility as well as possibly strategy (for example, a pack of modern wolves will deliberately spook a herd to run in order to tire the weaker individuals), while some began to grow bigger and more solid to the point that even a bear dog like Amphicyon would have had difficulty in tackling them. The new predators however had the intelligence to adapt their behaviour to meet these new challenges, but the bear dogs as powerful as they had been were now relics of an older age. Surely enough by the end of the Miocene the bear dogs found themselves obsolete in a changing world with most bear dog genera gone by the end of the Miocene. The only possible exception to this is Arctamphicyon which seems to have lasted slightly longer till the early Pliocene.

Pseudocyon, also known as Amphicyonopsis, is an extinct genus of terrestrialcarnivores belonging to the suborderCaniformia, family of Amphicyonidae ("bear dog") and which inhabited Euroasia and North America from the Miocene epoch to the Late Miocene subepoch living 23.3—7.2 Mya. Pseudocyon existed for approximately 5.3 million years.

Very close contender is Amphicyon giganteus

Size: Up to 2.5 meters long. 1,4 meters tall at the shoulder.Weight estimated at up to  750 kg.
Amphicyon giganteus is an extinct species of Amphicyon, a large carnivorous bone cruhing mammal known as a Bear-Dog of the family Amphicyonidae, subfamily Amphicyoninae, from the Miocene endemic to Europe and Africa, living from 16.8—7.2 Ma existing approximately 9.7 million years An account of an ancient Rhinocerotidae (Iberotherium rexmanueli zbyszewskii) which may have been killed by a single individual or by a pack of Amphicyon giganteus in now what is Portugal, was written noted paleontologists Ginsberg et al. Amphicyon castellanus also shared its time period and habitat. Amphicyon giganteus was named by Kaup (1884). Amphicyon giganteus was the typical Bear-Dog Amphicyonid with morphology to both bears and dogs. Looking more like a bear than a dog, it had a large heavy tail, thick neck, robust limbs and teeth like a wolf. It was probably an omnivore with a lifestyle comparable to that of the brown bear

Amphicyon ingens is other very large bear-dog


Dimensions:length - 2,5 m, height - 120 сm, weight - 200-550 kg.
Amphicyon ("ambiguous dog") is an extinct genus of large carnivorous bone-crushing mammals, known as bear-dogs, of the family Amphicyonidae, subfamily Amphicyoninae, from the Aquitanian Epoch until the Tortonian. They ranged over North America, Europe, Asia, and Africa from 20.6—9 Ma ago, existing approximately 11.6 million years. Amphicyon was the typical bear-dog amphicyonid with morphology similar to both bears and dogs. With its robust build and maximum length of 2.5 m, the largest species looked more like a bear than a dog. It had a large heavy tail, thick neck, robust limbs and teeth like a wolf. It was probably an omnivore with a lifestyle comparable to that of the brown bear.
Amphicyon ingens lived from 20.06–13.6 Ma, approximately 6.46 million years. The species was originally described by W. Matthew in 1924 from specimens found in the Olcott Formation, Sioux County, Nebraska. Specimens attributed to this species have since been found in California, Colorado, and New Mexico. The largest known specimen to weigh 600 kg, making it the largest amphicyonid and one of the largest known carnivorous land mammals.