Creodonta

Also known as Pseudocreodi or "archaic carnivores," creodonts are a group of extinct carnivorous mammals that thrived during the Eocene and Oligocene in North

Amphicyonidae

Fig. 8.1. (A) Relationships of creodonts and carnivorans. Alternatively, Amphicyonidae could be the most basal branch of Caniformia, and Pinnipedia could be the sister taxon of Musteloidea. Canidae constitute the Cynoidea. (B) Alternative view of basal carnivoran relationships. (A based on Flynn and Wesley-Hunt, 2005; B modified from McKenna and Bell, 1997, and Flynn and Galiano, 1982.)

Fig. 8.1. (A) Relationships of creodonts and carnivorans. Alternatively, Amphicyonidae could be the most basal branch of Caniformia, and Pinnipedia could be the sister taxon of Musteloidea. Canidae constitute the Cynoidea. (B) Alternative view of basal carnivoran relationships. (A based on Flynn and Wesley-Hunt, 2005; B modified from McKenna and Bell, 1997, and Flynn and Galiano, 1982.)

America and from the Eocene into the Miocene in the Old World. Indeed, they were the most abundant carnivorous mammals in the Old World during the Paleogene (Muizon and Lange-Badre, 1997). They coexisted with true carnivores, order Carnivora, during their entire duration, but were eventually replaced by members of the "modern" order. The reason for this replacement remains a mystery. The available evidence does not particularly support competitive displacement, as creodonts were declining in diversity through most of the Eocene, and carnivorans did not diversify morphologically to fill the vacant niches (Van Valkenburgh, 1999; Wesley-Hunt, 2005). Because they were similarly adapted, creodonts are usually considered together with Carnivora (in the higher taxon Ferae) and could be their sister group, although as noted above, the evidence for this alliance is weak.

Some popular and general accounts still include a diversity of primitive carnivorous mammals, such as mesony-chids and arctocyonids, under the heading of creodonts. But virtually all current paleomammalogists have abandoned this antiquated view and restrict the name Creodonta to two families, Oxyaenidae and Hyaenodontidae. These two families are united by the presence of more posterior car-nassials (specialized shearing teeth) than in the Carnivora, as well as by various primitive aspects of the skeleton. The teeth modified as carnassials—often two or even three teeth in each jaw, although the more posterior tooth is typically the principal carnassial—usually differ in the two families, how ever; and there are few, if any, skeletal synapomorphies. These factors suggest that Creodonta, although a convenient and widely used term, is not a natural group (Polly 1996), and that the two families may have emerged independently from cimolestan ancestors. Cimolestes (see Chapter 7) is a plausible morphotype for both families of creo-donts (Lillegraven, 1969).

The dental formula of creodonts is primitively 3.1.4.3/ 3.1.4.3, but many forms have reduced the number of molars (oxyaenids, most limnocyonine hyaenodontids), incisors, or premolars (Denison, 1938). The canines are always large. Creodont premolars are simple, with one primary cusp and variable anterior and posterior accessory cusps; a low protocone is sometimes present on the posterior upper premolars. The molars are primitively tribosphenic. The lower molars have tall trigonids and narrow talonids, the trigonids often wider than the talonids, and the talonids often reduced, especially on the more posterior molars. The lower carnassial typically has a reduced metaconid and a sectorial paracristid. The upper molars are triangular, with connate (closely joined) paracone and metacone, small conules, and a well-separated and often reduced protocone. The metastyle of the upper carnassials is large and joined to the metacone by a bladelike crest that occludes against the paracristid. The last upper molar is usually reduced, but the last lower molar ranges anywhere from very large to very small in different taxa.

Table 8.1. Classification of Creodonta and Carnivora

Superorder FERAE

Order fCREODONTA

fHyaenodontidae fOxyaenidae Order CARNIVORA

f Viverravidae1 fMiacidae2 Suborder FELIFORMIA

fNimravidae Felidae Viverridae Nandiniidae Herpestidae Hyaenidae Suborder CANIFORMIA Infraorder CYNOIDEA

Canidae Infraorder ARCTOIDEA Parvorder URSIDA

Superfamily Ursoidea Ursidae fAmphicyonidae3

Superfamily Phocoidea (=Pinnipedia) fEnaliarctos Otariidae Phocidae Odobenidae Parvorder MUSTELIDA Mustelidae Procyonidae

Notes: Modified after McKenna and Bell (1997). The dagger (f) denotes extinct taxa. Families in boldface are known from the Paleocene or Eocene.

1 Previously considered stem feliforms; now considered stem carnivorans.

2 Previously considered stem caniforms; now considered paraphyletic stem carnivorans.

3 Sometimes placed in a separate superfamily fAmphicyonoidea; relationship to Ursidae is uncertain; could be the sister taxon of other caniforms (Wesley-Hunt and Flynn, 2005).

Many of these dental features, as well as some of the postcranial traits mentioned in the next paragraph, are also characteristic of the most primitive carnivorans (miacoids). The overall dental morphology of creodonts is also similar— strikingly so in some cases—to that of various borhyaenid and dasyuroid marsupials, but in this case the resemblances are certainly convergent. As observed by Muizon and Lange-Badre (1997), the recurrence of this complex of dental features in multiple mammalian groups, many of which are unequivocally only distantly related, constitutes strong evidence of its frequent homoplasy. This observation is further cause to be suspicious of a close relationship between cre-odonts and Carnivora.

As in many mammals, the enamel of creodont teeth exhibits a pattern of decussating prisms called Hunter-Schreger bands (HSB). The HSB have an unusual zigzag arrangement in taxa whose gross dental morphology suggests bone-crushing habits (Stefen, 1997), but the significance of this pattern is uncertain, as a similar pattern occurs in some herbivorous Paleogene mammals (Koenigswald and Rose, 2005).

Many creodonts had disproportionately large heads. Primitive characteristics of creodonts compared to Carnivora include strong sagittal and lambdoidal crests on the skull, an unossified auditory bulla, relatively short and generalized limbs (in early representatives), carpus with a centrale and separate scaphoid and lunate bones, and pentadactyl feet. The terminal phalanges are fissured at the tip and tend to be broader in oxyaenids than in hyaenodontids. Most cre-odonts were generalized terrestrial animals, but some early representatives were scansorial, and some later hyaenodontids were specialized cursors. Despite their primitive features, however, creodonts were not subordinate to car-nivorans in dental adaptation for carnivory.

Creodonts were widespread and successful during the Early-Middle Tertiary, so why did they become extinct? Many were larger than contemporaneous carnivorans, and equalled or surpassed them in carnivorous adaptation. Although they were once thought to have had smaller brains than did contemporary carnivorans, it is now known that creodonts and carnivorans generally had similar-sized brains, and that both groups showed comparable brain expansion during the Early Tertiary (Radinsky, 1977). Thus competitive exclusion does not appear to be the explanation (Van Valken-burgh, 1999). Whatever the cause, oxyaenids became extinct by the late Eocene, whereas hyaenodontids persisted almost to the Pliocene (in parts of the Old World; only through the Oligocene in North America). Possibly contributing to their demise were their generally more conservative postcranial skeletons, but the full explanation is unknown.

Oxyaenidae

Oxyaenids were a mainly North American group of cre-odonts with relatively short, broad skulls and deep, robust dentaries. The primary carnassial teeth are M1/M2 (often with assistance from P4 and M1), and the third molars are absent, a derived feature relative to most hyaenodontids (Figs. 8.2, 8.3). All show clear specializations for meat eating, although the carnassials are only weakly developed in the earliest forms, and some seem to have been better adapted for crushing bones than cutting flesh. Certain later types even achieved catlike or hyena-like hypercarnivorous dental adaptations.

Most oxyaenids had rather long bodies and short, robust limbs with relatively short middle and distal segments (Fig. 8.4D). The weakly grooved astragalus articulates distally with both the navicular and the cuboid. The stance was probably plantigrade. Oxyaenid locomotor capabilities have proven difficult to assess, in part because they are so generalized, and in part because there are no obvious modern analogues. Oxyaenids are often described as wolverinelike, "terrestrial ambulatory" predators (e.g., Matthew, 1909; Denison, 1938; Gunnell and Gingerich, 1991). Prominent crests on the humerus and a well-developed olecranon process on the ulna suggest digging ability as well. Some features of the skeleton (e.g., divergent first digit, robust humerus, nearly flat astragalar trochlea indicating a mobile upper ankle joint) suggest an arboreal ancestry, and it is even possible that some early oxyaenids were scansorial.

Creodont Eocene Europe
Fig. 8.2. Carnassial positions in carnivorans (A, B) and creodonts (C, D): (A) viverravid Didymictis; (B) miacid Miacis; (C) hyaenodontid Hyaenodon; (D) oxyaenid Oxyaena. Carnassials are P4/M1 in carnivorans, M2/M3 in hyaenodontids, and M1/M2 in oxyaenids. (Modified after Matthew, 1909.)

Oxyaenids were less diverse and of much shorter duration than hyaenodontids. About ten genera are recognized. The earliest genus, Tytthaena (Fig. 8.3A), appeared in North America in the late Paleocene (middle Tiffanian; Gunnell and Gingerich, 1991)—before hyaenodontids but after the oldest Carnivora—perhaps as an immigrant from Asia. Tytthaena was a rare, housecat-sized oxyaenid known only from teeth found in Wyoming. By the end of the Paleocene (Clarkforkian) several oxyaenid genera representing three different subfamilies were present, but their pedigree is uncertain. Late Paleocene Dipsalodon, Palaeonictis, and Dipsali-dictis were the first big carnivorous mammals of the Early Cenozoic except for mesonychids, some species attaining the size of small bears. The latter two genera survived into the Eocene, but were much less common than their relative Oxyaena (Figs. 8.3B, 8.4A,C), which was one of the most prevalent carnivorous mammals in western North America during the Wasatchian. It, too, reached the size of a small bear—much larger than any contemporary true carnivoran. Both Oxyaena and Palaeonictis dispersed to Europe during the early Eocene. They were evidently terrestrial animals, whereas Dipsalidictis had more gracile limbs and more flexible elbow and ankle joints, suggesting scansorial habits (Gunnell and Gingerich, 1991).

One lineage of oxyaenids (Ambloctoninae), typified by Palaeonictis (Fig. 8.3E), widened the premolars, deempha-sized the carnassials, and reduced M2, perhaps reflecting a more omnivorous or durophagous diet. A relative of Palaeonictis, Wasatchian Ambloctonus, evolved in a somewhat different direction, achieving catlike shearing by reducing M2 to a bladelike paracristid, although this molar was smaller than M1. Hypercarnivory was achieved by the bear-sized middle Eocene oxyaenines Patriofelis (from North America and Europe; Fig. 8.4B,D) and Sarkastodon (Asia; Fig. 8.3D), which were probable descendants of Oxyaena. They had very robust premolars and independently evolved a bladelike M2 consisting only of the paracristid, but here it is the largest cheek tooth, giving them a distinctly hyena-like dentition. With such slicing molars and crushing premolars they could have consumed both meat and bones (Gazin, 1957; Gunnell, 1998).

Carnivory reached its peak in the late Wasatchian-Bridgerian Machaeroides and Apataelurus (Machaeroidinae; Fig. 8.3C), which not only evolved a huge bladelike M2 consisting of a hypertrophied paracristid, as in Patriofelis and Sarkastodon, but also had long upper canines protected by a prominent ventral flange at the front of the jaw (Denison, 1938; Scott, 1938; Dawson et al., 1986). These specializations made machaeroidines the oldest known saber-toothed predators (not to be confused with machairodontines, true saber-toothed cats, which did not appear until the Miocene). Their affinities remain problematic; they have been variously associated with oxyaenids (Dawson et al., 1986; Gunnell, 1998) or with limnocyonine hyaenodontids (Denison, 1938; McKenna and Bell, 1997).

Hyaenodontidae

Hyaenodontids (Figs. 8.5-8.8) were much more diverse than oxyaenids, consisting of about 50 genera found in Eocene-Oligocene deposits of North America and Eocene-

Oxyaena Teeth
Fig. 8.3. Dentitions of oxyaenids (all left side except D): (A) Tytthaena; (B) Oxyaena; (C) Apataelurus; (D) Sarkastodon (right mandible, medial view); (E) Palaeonictis. (A from Gingerich, 1980; B from Matthew 1915a; C from Scott, 1938; D from Denison, 1938; E from Rose, 1981.)

Miocene beds of Eurasia and Africa (Lange-Badre, 1979; Gingerich and Deutsch, 1989; McKenna and Bell, 1997). They are variously placed in two to four subfamilies, whose relationships are not well understood. The Proviverrinae is a paraphyletic assemblage of the most primitive genera, whereas more derived genera are assigned to Hyaenodonti-nae, Limnocyoninae, or Pterodontinae (Polly, 1996). Proviverrinae and Pterodontinae are sometimes subsumed in Hyaenodontinae (e.g., McKenna and Bell, 1997). Hyaen-odontids were particularly successful in the early and middle

Eocene of North America and Europe, which had several genera in common. In Europe oxyaenids disappeared after the early Eocene, and hyaenodontids filled many of the vacant niches. Hyaenodontids are one of several higher taxa that generally herald the beginning of the Eocene across Laurasia (Gingerich, 1989; Smith and Smith, 2001). An exception is Prolimnocyon, the earliest member of the Limno-cyoninae, which was evidently present, but rare, in the latest Paleocene of Asia (Meng et al., 1998). This occurrence hints at an Asian source for the group.

Raul Martin Thylacosmilus
Fig. 8.4. Oxyaenids: (A) skull of Oxyaena; (B) skull of Patriofelis; (C) right forefoot and hind foot of Oxyaena; (D) skeleton of Patriofelis. (A-C from Denison, 1938; D from Gregory, 1951.)

If Hyaenodontidae form a clade with Oxyaenidae, as is commonly assumed, it implies the existence of Hyaenodontidae by no later than the Tiffanian, perhaps in some as-yet unsampled region. Furthermore, the derived dental state of oxyaenids (loss of third molars) relative to hyaenodontids, suggests that stem creodonts, which probably would have resembled primitive hyaenodontids, were present well before the Tiffanian.

Hyaenodontids generally had longer, narrower skulls and shallower jaws than those of oxyaenids (Figs. 8.5, 8.6), which are probably primitive conditions. They differ further from oxyaenids in primitively retaining three molars, with the principal shearing usually taking place between

M2 and M3, though more anterior molars often were also involved. In fact, these characteristics apply only to the subfamilies Proviverrinae, Hyaenodontinae, and Pterodon-tinae. In contrast, limnocyonine hyaenodontids greatly reduced or lost the third molars; hence their carnassials were always developed at M1/M2, as in oxyaenids. In addition, many limnocyonines had shorter, broader skulls, similar to those of oxyaenids. Other aspects of the dentition and postcranial anatomy, however, imply closer relationship to proviverrines than to oxyaenids (Denison, 1938; Polly 1996; Gunnell, 1998). Their early dental reduction implies a ghost lineage of proviverrines well back into the Paleocene.

Compared to oxyaenids, hyaenodontids tended to have longer, more gracile limbs, with more mediolaterally compressed but typically fissured terminal phalanges. At least some species were digitigrade (Fig. 8.7). Several early Eocene genera (the limnocyonine Prolimnocyon and the proviver-rines Prototomus, Tritemnodon, and Pyrocyon) show varying degrees of adaptation for scansorial locomotion (Gebo and Rose, 1993; Rose, 2001a). Their contemporary, Arfia (also a proviverrine), has characteristics more typical of incipient cursors: a humerus with a prominent greater tuberosity and a supratrochlear foramen, and a femur with a high greater trochanter and a well-defined patellar groove. Paradoxically, Arfia seems to have had a very flexible ankle that allowed limited hind foot reversal (Gingerich and Deutsch, 1989), a specialization found in arboreal mammals that descend trees headfirst. Presumably this flexibility would have made the ankle less stable on the ground. Late early Eocene Gazino-cyon was more clearly cursorial, based on ankle modifications that kept movement in the parasagittal plane (Polly, 1996). Bridgerian Sinopa shows many of the same features and was probably incipiently cursorial (Matthew, 1906). These two genera foreshadow the specializations that are better developed in Hyaenodon. The mixture of scansorial and terrestrial features found in many early hyaenodontids, however, suggests that they were generalists that spent significant amounts of time both on the ground and in the trees, not being restricted to either habitat. Many of these early members were not as big as typical oxyaenids, being fox-sized or smaller (ranging from 1 to 15 kg; Egi, 2001), and their dental modifications for carnivory were less well developed.

Quercitherium (Fig. 8.5C) from the middle and late Eocene of Europe was one of several genera that evolved swollen premolars probably used for crushing hard food items, such as bones or mollusks (Lange-Badre, 1979). Middle Eocene Lesmesodon (Fig. 8.8) from Messel is known from complete skeletons with soft-tissue impressions, revealing a bushy tail like that of a squirrel or a fox (Morlo and Habersetzer, 1999). It is also distinctive among hyaenodontids in having unfissured terminal phalanges. The skeleton indicates that Lesmesodon was a generalized terrestrial animal.

Later genera, such as Hyaenodon (later Eocene-early Oligocene of North America and Eurasia), included species with a head ranging in size from that of a housecat to that of a lion (Fig. 8.6B; Mellett, 1977). As mentioned earlier, however, the head was out of proportion with the body, so most species of Hyaenodon probably weighed only 10-40 kg, while the largest species of the genus weighed about 120 kg (Egi, 2001). Hyaenodon was widespread in the northern continents during the middle Tertiary, evidently occupying the vacant ecological niche left by the extinction of oxyaenids. Although small species of Hyaenodon may have been scan-sorial, larger species were relatively long-legged for creo-donts and had many cursorial specializations (Fig. 8.7D).

Mustela Vison Mandible Anatomy
Fig. 8.5. Right dentitions of hyaenodontids: (A) Gazinocyon upper teeth; (B) Prolimnocyon upper and lower teeth; (C) Quercitherium lower teeth; (D) Tritemnodon lower teeth. (A from Gingerich and Deutsch, 1989; B from Gebo and Rose, 1993; C from Lange-Badré, 1975; D from Matthew, 1915a.)
Prolimnocyon
Fig. 8.6. Skulls of hyaenodontids: (A) Sinopa; (B) Hyaenodon. (A from Matthew, 1906; B from Mellett, 1977.)

They were digitigrade, with a reduced deltopectoral crest on the humerus; perforated olecranon fossa; deep humeral trochlea articulating with a broad proximal radius (which limited forearm supination); a long, deep patellar groove on the femur; a compact tarsus with a relatively deeply grooved astragalar trochlea; and closely appressed metatarsals (Scott and Jepsen, 1936; Mellett, 1977).

Hyaenodon had robust premolars and specialized bladelike carnassials (M2/M3) analogous to those of hyenas (which did not appear until the Miocene). The bladelike structure of the molar trigonids, formed by the paracristid, was enhanced by the loss of the metaconid. Late Eocene Pterodon achieved hypercarnivorous molars in the same way, which was long considered an indication of close relationship to Hyaenodon. Polly (1996) argued, however, that the loss of the metaconid occurred independently in the two genera.

Mellett (1977) documented a chain of events that led to changes in the skull and dentition of Hyaenodon, eventually resulting in larger body size, greater gape, and more efficient shearing. He postulated that lengthening of the principal shearing blades, the metacrista (=postmetacrista) of the upper molars and the paracristid of the lowers, and reorientation of these crests from oblique to almost mesio-distal enabled Hyaenodon to exploit larger prey and therefore to increase in size. Efficient function of the deciduous car-nassial (dP4) necessitated an occluding tooth below and led to selection for precocial eruption of a lower carnassial, M1. This tooth is typically heavily worn or prematurely lost in Hyaenodon. Subsequent loss of M3 allowed the posterior expansion of the M2/M3 carnassials so characteristic of Hyaenodon, and eventually caused further changes in the jaw joint and chewing muscles. These traits surely made Hyaen-odon one of the most formidable predators of the middle Tertiary.

Yet even Hyaenodon was dwarfed by the later Eocene Hemipsalodon, the largest North American hyaenodontid at more than 400 kg (Egi, 2001), and the early Miocene hyaeno-dontid Megistotherium, one of the largest carnivorous mammals ever, with a skull about 66 cm long—nearly twice as big as that of a bear or a lion. Megistotherium may have weighed as much as 800 kg (Savage, 1973).

As already noted, limnocyonines differ from other hyaenodontids in skull shape, reduction or loss of third molars, and development of more anterior carnassials. For these reasons they are sometimes placed in their own family (e.g., Gun-nell, 1998). Wasatchian Prolimnocyon, the oldest limnocyo-nine, seems to have been at least partly scansorial, whereas Bridgerian Thinocyon was similar in size and postcranial anatomy to a mink (Mustela vison)—about 1-1.5 kg. The larger (8-16 kg) middle Eocene Limnocyon was a generalized, perhaps semifossorial, terrestrial form (Matthew, 1909; Gebo and Rose, 1993; Egi, 2001).

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    How comon are third molars?
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    When did the Oxyaena evolve?
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