The first carnivoran was a carnivore, and nearly all carnivorans today primarily eat meat. Some, such as cats, pinnipeds, and weasels, are almost completely carnivorous. Others, such as bears, are more omnivorous, although the Polar Bear is predominantly carnivorous, with 90% of its diet consisting of seals. The Giant Panda is almost exclusively an herbivore but occasionally eats fish, eggs and insects.
Carnivorans have teeth and claws adapted for catching and eating other animals. Their eyes point forward. Many carnivorans hunt in packs and are social animals.
The Canoidea superfamily – Canidae (dogs and foxes), Mephitidae (skunks and stink badgers) Mustelidae (weasels), Procyonidae (raccoons), Ursidae (bears), Otariidae (eared seals), Odobenidae (Walrus), and Phocidae (earless seals) (the last three families formally classified in the suborder Pinnipedia) and the extinct family Amphicyonidae (bear-dogs) - are characterized by having a non-chambered or partially chambered auditory bullae, non-retractable claws, and well-developed baculum. Most species are rather simply colored, lacking the flashy spotted or rosetted coats of 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.
The Feloidea superfamily – Felidae (cats), Herpestidae (mongooses), Hyaenidae (hyenas), Viverridae (civets), and Eupleridae (Malagasy carnivores), as well as the extinct family Nimravidae (paleofelids) – often have spotted, rosetted or striped coats, and tend to be more brilliantly colored than their Canoidean counterparts. This is due to the fact that these species tend to range in tropical habitats, although a few species do inhabit temperate and subarctic habitats. Many are arboreal or semi-arboreal, and the majority are digitigrade. Diet tends to be more strictly carnivorous, especially in the Felidae family. They have fewer teeth and shorter skulls, with much more specialized carnassials meant for shearing meat. Felidae claws are retractile. The terminal phalange with the claw attached folds back in the fore-foot into a sheath by the outer side of the middle phalange of the digit, and is retained in this position when at rest by a strong elastic ligament. In the hind-foot the terminal joint or phalange is retracted on to the top, and not the side of the middle phalange. Deep flexor muscles straighten the terminal phalanges so that the claws protrude from their sheath, and the soft "velvety" paw becomes suddenly converted into a formidable weapon of offence. The habitual retraction of the claws preserves their points from wear.
The Pinnipedia superfamily (walruses, seals, and sea lions) are medium to large (to 6.5 m) aquatic mammals. Being homeothermic (warm-blooded) marine mammals, pinnipeds need a low surface area to body mass ratio. Otherwise they would suffer from excessive heat loss due water's high capacity for heat conduction. The body is usually insulated with a thick layer of fat called blubber and typically covered with hair. The digits are not separate, but connected by a thick web that forms flippers for swimming; thus the forelimbs and hindlimbs are transformed into paddles. This enables them to dive at extreme depths (600 meters for the Weddell Seal). They can remain underwater for long periods of time, sometimes over an hour or more, but most dives are usually short. The facial region of skull is relatively small, with pinnae very small or lacking and the vibrissae are well developed. The molariform teeth are mostly homodont and the canines are well developed. The tail is very short or absent, the ears are small or absent as well, and the external genitalia are hidden in slits or depressions in the body.
The Miacidae is not a monophyletic group, but a paraphyletic array of stem taxa. Traditionally, the Miacidae and the Viverravidae had been classified in a third, extinct paraphyletic superfamily, the Miacoidea, from which the direct ancestors of both Carnivora and Creodonta were thought to have arisen. Today Carnivora is restricted to the crown group, and Carnivora and miacoids are grouped together in the clade Carnivoramorpha, and the miacoids are regarded as basal carnivoramorphs. Based on dental features and braincase sizes, it is now known that Carnivora must have evolved from a form even more primitive than Creodonta and thus these two orders may not even be sister groups. The Carnivora, Creodonta, Pholidota, and a few other extinct orders are informally grouped together in the clade Ferae. Older classification schemes divided the order into two suborders: Fissipedia (which included the families of primarily land Carnivora) and Pinnipedia (which included the true seals, eared seals, and Walrus). However, it is now recognized that the Fissipedia is a paraphyletic group and that the pinnipeds were not the sister group to the fissipeds but rather had arisen from among them. Carnivora are generally divided into the suborders Feliformia (cat-like) and Caniformia (dog-like), the latter of which includes the pinnipeds. The pinnipeds are part of a clade, known as the Arctoidea, which also includes the Ursidae (bears) and the superfamily Musteloidea. The Musteloidea in turn consists of the Mustelidae (mustelids: weasels), Procyonidae (procyonids: raccoons), Mephitidae (skunks) and Ailurus. The oldest caniforms are the Miacis species Miacis cognitus, the Amphicyonidae (Bear-dogs) such as Daphoenus, and Hesperocyon (of the family Canidae, subfamily Hesperocyoninae). Hesperocyonine canids first appeared in North America and the earliest species is currently dated at 39.74 Ma, but they were not represented in Europe until well into the Miocene, and not into Asia and Africa until the Pliocene. Miacis and Amphicyonidae were the first of the caniforms to split from the others and are sometimes considered to be sister groups to Ursidae, but the exact closeness of Amphicyonidae and Ursidae, as well as Arctoidae to Ursidae, is still uncertain. The Canidae (wolves, coyotes, jackals, foxes and dogs) are generally considered to be the sister group to Arctoidea. The Ursidae first occur in North America in the Late Eocene (ca. 38 million years ago) as the very small and graceful Parictis that had a skull only 7 cm long. Like the canids, this family does not appear in Eurasia and Africa until the Miocene. The other caniform families Amphicyonidae, Mustelidae and Procyonidae occur in both the Old World and the New World by the Late Eocene and Early Oligocene.
The ancestor of all Feliformia evolved from the Caniformia-Feliformia split but the exact position of the Felidae, especially some extinct Felidae, in relation to the other families is somewhat disputed. Nandinia, the African Palm Civet, seems to be the most primitive of all the feliforms and the very first to split from the others. The Asiatic linsangs of the genus Prionodon (traditionally placed in the Viverridae) might form a family of their own as well, as some recent studies indicate that Prionodon is actually the closest living relative to the cats. The Nimravidae are sometimes seen as the most basal of all feliforms and the first to split from the others, but there is a possibility that Nimravidae might not even be Carnivora. Its position as a Carnivora is currently unstable. Other studies indicate that Barbourofelids forms a separate family, which is closely related to the true felids instead of being related to the Nimravids. Recognizable Nimravid fossils date from the late Eocene (37 mya), from the Chadronian White River Carnivora Formation at Flagstaff Rim, Wyoming. Nimravid diversity appears to have peaked about 28 mya. The hypercarnivorous (strictly meat-eating) nimravid feliforms were extinct in North America after 26 mya and felids did not arrive in North America until the early middle Miocene (16 mya).
It has been suggested that canids evolved hypercarnivorous morphologies because feliforms were absent during this period (the "cat-gap," 26-16 mya), 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. Hypercarnivorous canids were present before the disappearance of the nimravids, and all went extinct before the appearance of felids. Following the extinction of nimravids, only three 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 for the first time in North America. Procyonids did not arrive in North America until the early Miocene, and "modern" ursids (e.g., Ursinae), did not arrive until 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 occupied a morphospace generally shared with canids and not in close proximity to ursids. A large question remains as to why there was a progressive decline in hypercarnivorous carnivoramorphans during the late Oligocene/early Miocene. During this period all hypercarnivorous forms disappeared from the fossil record, including hypercarnivorous feliforms, canids, and mustelids. One possible explanation is climate change. Earth was gradually cooling after the late Paleocene, and over a period spanning the Eocene/Oligocene boundary there was a dramatic climatic cooling event occurred.
A recent study finally resolves the exact position of Ailurus: the Red Panda is neither a procyonid nor an ursid, but forms a monotypic family with the other musteloids as its closest living relatives. The same study also shows that the mustelids are not a primitive family, as was once thought. Their small body size is a secondary trait — the primitive body form of the arctoids was large, not small. Recent molecular studies also suggest that the endemic Carnivora of Madagascar, including three genera usually classed with the civets and four genera of mongooses classed with the Herpestidae, are all descended from a single ancestor. They form a single sister taxon to the Herpestidae. The hyenas are also closely related to this clade.
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