Carnivorans have been the principal group of predaceous mammals throughout much of the Cenozoic, although in the Early Cenozoic creodonts were equally or more successful. Many extant carnivorans remain primarily meat eaters, but some lineages have evolved away from that regimen toward omnivory, frugivory, myrmecophagy, piscivory, and other specialized diets. Carnivorans occupy arboreal, scan-sorial, cursorial, and fossorial niches on land and have also invaded both freshwater and marine environments. They are naturally occurring throughout the world and its oceans, although terrestrial carnivorans never reached Australia (or Antarctica) without human intervention.
Many different classifications of Carnivora are in use (the one followed here is shown in Table 8.1). The order is usually divided into two large clades, the Feliformia (=Aeluroidea)— felids, viverrids, herpestids, hyaenids, and nimravids (cats, civets, mongooses, hyenas, and false sabertooths, respectively)—and the Caniformia—canids, ursids, amphicyonids, pinnipeds, procyonids, and mustelids (dogs, bears, bear-dogs, seals and walruses, raccoons, and weasels, respectively). All of the caniforms except canids are often united in a presumed monophyletic group Arctoidea. The oldest and most primitive known carnivorans are the Paleocene and Eocene
miacoids, comprising the families Viverravidae and Miaci-dae. Miacoids are a paraphyletic assemblage whose two families were previously considered to be basal members or sister taxa of Feliformia (Viverravidae) and Caniformia (Miacidae; Flynn and Galiano, 1982; Flynn et al., 1988; Hunt and Tedford, 1993; McKenna and Bell, 1997; Flynn, 1998). In more recent analyses miacoids are found to lie outside of the two crown clades (Wyss and Flynn, 1993; Flynn and Wesley-Hunt, 2005; Wesley-Hunt and Flynn, 2005; see Fig. 8.1). Those authors prefer to restrict Carnivora to the crown clades and use the term Carnivoramorpha for the larger group that includes miacoids.
The principal distinguishing characteristics of Carnivora concern the dentition and the basicranium (especially auditory structures). Like creodonts, carnivorans primitively retain a plesiomorphic placental dentition of 18.104.22.168/22.214.171.124, but reduction in this number is common. The hallmark of the Carnivora is the specialization of P4/Mj as bladelike carnassials (Fig. 8.2), a modification usually assumed to have occurred only once at these tooth loci (but see below).
Associated with this adaptation, the lower jaw moves mainly orthally (up and down), with little transverse motion possible; this restriction is further ensured by a tight-fitting cylindrical temporomandibular joint. Carnassials are present in most living and extinct carnivorans but have been lost or greatly modified in procyonids, ursids, and pinnipeds. When specialized for shearing, P4 is triangular, with the paracone centrally located on the buccal side of the tooth, the protocone shifted anterolingually (well anterior to the paracone), and a long, bladelike metastylar crest separated from the paracone by a carnassial notch. M1 is larger than the other molars and has a tall trigonid with a well-developed paracristid blade enhanced by a carnassial notch between the protoconid and paraconid. The second and third molars are reduced in size or lost.
In extant carnivorans the auditory bulla, which surrounds the middle-ear cavity, consists of three elements: the ecto-
tympanic (sometimes simply called the tympanic) and two entotympanics, rostral and caudal (Hunt, 1974b). There has been a widespread tendency among carnivorans to enlarge the middle-ear cavity, usually by expanding the bulla, which enhances hearing ability. Variations in the anatomy of the three bullar elements, particularly the caudal entotympanic, as well as in petrosal anatomy, have played an important role in deciphering carnivoran relationships (e.g., Hunt, 1974b, 1987, 1989, 1991, 1998c, 2001; Fig. 8.9). For example, the ec-totympanic is the largest bullar element in arctoids, whereas the caudal entotympanic tends to be larger in canids and fe-liforms. In addition, the internal carotid artery (ICA), which supplies the brain in many mammals, is usually reduced in carnivorans, many of which get their primary blood supply to the brain through the external carotid.
Caniforms and feliforms differ fundamentally in construction of the auditory bulla and in the carotid circulation
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