Creodonta

Creodonta ("meat teeth") is a former order of extinct carnivorous placental mammals that lived from the early Paleocene to the late Miocene epochs in North America, Europe, Asia and Africa. Originally thought to be a single group of animals ancestral to the modern Carnivora, this order is now usually considered a polyphyletic assemblage of two different groups, the oxyaenids and the hyaenodonts, not a natural group. Oxyaenids are first known from the Palaeocene of North America, while hyaenodonts hail from the Palaeocene of Africa.<ref>Template:Cite journal</ref>

Creodonts were the dominant carnivorous mammals from Template:Ma, peaking in diversity and prevalence during the Eocene.<ref>Template:Cite book</ref> The first large, obviously carnivorous mammals appeared with the radiation of the oxyaenids in the late Paleocene.<ref name="Janis1998" /> During the Paleogene, "creodont" species were the most abundant terrestrial carnivores in the Old World.<ref name="Rose2005">Template:Cite book</ref> In Oligocene Africa, hyaenodonts were the dominant group of large flesh-eaters, persisting until the middle of the Miocene.

"Creodont" groups had an extensive range, both geographically and temporally. They are known from the late Paleocene through the late Oligocene in North America, the early Eocene through late Oligocene in Europe, from the late Paleocene through late Miocene in Asia, and from the late Paleocene to the late Miocene in Africa.<ref name="Gunnell1998" /> While most were small-to-medium sized mammals, among their number was Sarkastodon, one of the largest mammalian land predators of all time, weighing an estimated 800 kg.<ref name="Sorkin-2008" />

Though often assumed to have been outcompeted by carnivorans, there is little empirical support for this. The last genus, Dissopsalis, became extinct about Template:Ma.

Most modern paleontologists agree both "creodont" families are related to Carnivora, but are not their direct ancestors. It is still unclear how closely the two families are related to each other. In general, classification is complicated by the fact that relationships among fossil mammals are usually decided by similarities in the teeth, but the teeth of hypercarnivorous species may evolve similar shapes through convergent evolution, to deal with the mechanics of eating meat.<ref>Template:Cite journal</ref>

"Creodonts" share with the Carnivora, and many other predatory mammal clades, the carnassial shear, a scissors-like modification of upper and lower cheek teeth that was used to slice muscle tissue. This adaptation is also seen in other clades of predatory mammals.

Systematics and history

"Creodonta" was coined by Edward Drinker Cope in 1875.<ref name=Cope-1875/> Cope included the oxyaenids and the viverravid Didymictis but omitted the Hyaenodontidae. In 1880. he expanded the term to include families Miacidae (including Viverravidae), Arctocyonidae, Leptictidae (now Pseudorhyncocyonidae), Oxyaenidae, Ambloctonidae and Mesonychidae.<ref>Template:Cite journal</ref> Cope originally placed creodonts within the Insectivora. In 1884, however, he regarded them as a basal group from which both carnivorans and insectivorans arose.<ref>Template:Cite book</ref>

Hyaenodontidae was not included among the creodonts until 1909.<ref name="MatthewBridgerBasin">Template:Cite journal</ref> William Diller Matthew regarded Creodonta as a suborder of order Carnivora, divided in three groups:

• "Inadaptive Creodonta" (Creodonta inadaptiva), group that includes "Pseudocreodi" (oxyaenids and hyaenodontids) and the mesonychids,
• "Adaptive Creodonta" (Creodonta adaptiva), made up of the miacids and the taxa included in the wastebasket "Arctocyonidae",
• and "Primitive Creodonta" (Creodonta primitiva), made up of Oxyclaenidae.

Over time, various groups and species were removed from this order. It stabilized in the mid-20th century as representing oxyaenids, hyaenodonts, mesonychids, and arctocyonids,<ref>Template:Cite journal</ref> which were understood as the major groups of flesh-eating placental mammals that were not members of the Carnivora. It became increasingly clear that arctocyonids were a wastebasket taxon and mesonychids might be more closely related to ungulates. By 1969, Creodonta contained only the oxyaenids and the hyaenodontids.<ref name="Van Valen">Template:Cite journal</ref>

More recently, "Creodonta" had been considered to be a nonvalid polyphyletic assemblage of carnivorous placental mammals (and not a natural group), and members of Creodonta being sister taxa to Carnivoramorpha (carnivorans and their stem-relatives) within clade Pan-Carnivora (in mirorder Ferae), split in two groups: order Oxyaenodonta as one group and order Hyaenodonta plus its stem-relatives (family Wyolestidae and genera Altacreodus, Simidectes and Tinerhodon) in the other.<ref name="Matthew2017">Template:Cite journal</ref><ref name="Sandrine2017">Template:Cite journal</ref><ref name="Prevosti2018">Prevosti, F. J., & Forasiepi, A. M. (2018). "Introduction. Evolution of South American Mammalian Predators During the Cenozoic: Paleobiogeographic and Paleoenvironmental Contingencies"</ref><ref name="Simbakubwa">Template:Cite journal</ref><ref name="Geodiversitas4213">Template:Cite journal</ref> However, some phylogenetic analysis recover them as a natural group, such as a phylogenetic analysis of Paleocene mammals published in 2015 that supported the monophyly of Creodonta, and placed the group as relatives of clade Pholidotamorpha (pangolins and their stem-relatives).<ref name="HallidayUpchurch2015">Template:Cite journal</ref>

Polly has argued that the only available synapomorphy between oxyaenids and hyaenodontids is a large metastylar blade on the first molar (M1), but he believes that that feature is common for all basal eutheria.<ref name="Polly">Template:Cite journal</ref> Separating Oxyaenidae from Hyaenodontidae would also comport with biogeographic evidence, since the first oxyaenid is known from the North American early Paleocene and the first hyaenodontids are from very late Paleocene of North Africa.<ref name="Janis1998">Template:Cite book</ref>

Complicating this arrangement is the tentative endorsement by Gunnell<ref name="Gunnell1998">Template:Cite book</ref> of the erection of a third family, Limnocyonidae.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The group includes taxa that were once considered oxyaenids, such as Limnocyon, Thinocyon<ref name="MatthewBridgerBasin"/> and Prolimnocyon.<ref>Template:Cite journal In this paper the authors rename Marsh's Limnocyon protenus as Didymictis protenus and include it among the miacids</ref> Wortman had even erected a subfamily of Limnocyoninae within the oxyaenids.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Van Valen nests the same subfamily (including Oxyaenodon) within Hyaenodontidae.<ref name="Van Valen"/> Gunnell is agnostic whether Limnocyonidae is a group within Hyaenodontidae (although a sister group to the rest of hyaenodontids) or entirely separate.<ref name="Gunnell1998"/>

According to Gunnell, the defining features of the oxyaenids include: A small braincase low in the skull. The occiput wide at base and narrowing dorsally (to give it a triangular shape). The lacrimal bone makes a semicircular expansion on the face. The mandibles have heavy symphysis. M1 and m2 form the carnassials, while M3/m3 are absent. The manus and pes are plantigrade or subplantigrade. The fibula articulates with the calcaneum, and the astragalus articulates with the cuboid bone. The phalanges are compressed and fissured at the tip.<ref name="Gunnell1998"/>

Likewise, Gunnell's list of defining features of hyaenodontids includes: Long, narrow skull with a narrow basicranium and a high narrow occiput. The frontal bones are concave between the orbital regions. M2 and m3 form the carnassials. M3 is present in most species, while m3 is always present. Manus and pes range from plantigrade to digitigrade. The fibula articulates with the calcaneum, while the astragalar-cuboid articulation is reduced or absent. Terminal phalanges are compressed and fissured at the tip.<ref name="Gunnell1998"/>

The limnocyonids had the following features according to Gunnell: M3/m3 were reduced or absent, other teeth were unreduced. The rostrum was elongated. The animals themselves were small to medium-sized.<ref name="Gunnell1998"/>

Morphology

Dentition

Among primitive creodonts the dental formula is Template:DentalFormula, but later forms often had reduced numbers of incisors, premolars and/or molars.<ref name="Denison">Template:Cite journal (Subscription or payment required.)</ref> The canines are always large and pointed. The lateral incisors are large, while the medial incisors are usually small.<ref name="Gunnell1998"/> Premolars are primitive, with one primary cusp and various secondary cusps.<ref name="Rose2005"/>

Creodonts have two or three pairs of carnassial teeth. One pair performed the largest cutting function (either M1/m2 or M2/m3).<ref name="Gunnell1998"/> This arrangement is unlike modern carnivorans, which use P4 and m1 for carnassials.<ref>Template:Cite book</ref> This difference suggests convergent evolution among meat-eaters, with a separate evolutionary history and an order-level distinction,<ref>Template:Cite book</ref> given that different teeth evolved as the carnassials both between creodonts and carnivorans, and between oxyaenids and hyaenodonts. Carnassials are also known in other flesh-eating mammal clades, such as in the extinct bat Necromantis, as well as highly unrelated taxa such as the flesh-eating marsupial Thylacoleo.

Different molars were involved in the two major groups of creodonts. In the Oxyaenidae, M1 and m2 that form the carnassials. Among the hyaenodontids, it is M2 and m3. Unlike most modern carnivorans, in which the carnassials are the sole shearing teeth, other creodont molars have a subordinate shearing functions.<ref name="MatthewBridgerBasin"/> The difference in which teeth form the carnassials is a major argument for the polyphyly of Creodonta.

Cranium

Creodonts had long, narrow skulls with small brains. The skull narrowed considerably behind the eyes, producing a distinct splanchnocranium and neurocranium segments of the cranium. They had large sagittal crests and usually broad mastoids (which were probably derived features for the group).<ref name="Gunnell1998"/> Many creodonts had proportionately large heads.<ref name="Rose2005"/> In primitive forms, the auditory bullae was not ossified. Generally the temporal fossae were very broad.<ref name="Gunnell1998"/>

Postcranial skeleton

Creodonts had generalized postcranial skeletons. Their limbs were mesaxonic (with the axis of the foot provided by the middle of their five digits). Their method of locomotion ranged from plantigrade to digitigrade. The terminal phalanges were fused claws.<ref name="Gunnell1998"/> Template:Multiple image

Size

Creodonts ranged in size from the size of a small cat to the Template:Convert Sarkastodon. The larger animals, however, were not known until late in the Paleocene with the radiation of the oxyaenids,<ref name="Janis1998"/> such as the puma-sized Dipsalidictis and the probably bone-crushing scavenger Dipsalodon.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Certain creodonts (Arfia, Prolimnocyon and Palaeonictis) seem to have experienced the dwarfing phenomenon during the Paleocene-Eocene Thermal Maximum seen in other mammal genera. A proposed explanation for this phenomenon is that the increased carbon dioxide levels in the atmosphere directly affected carnivores through increased temperature and aridity and also indirectly affected them by reducing the size of their herbivorous prey through the same selective pressures.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The largest North American creodont is Patriofelis. A specimen of P. ferox collected in the Bridger Basin of southern Wyoming was the size of a full-grown black bear with a head almost the size of an adult male lion.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

During the Central Asia Expedition of 1930 by the American Museum of Natural History, the largest creodont ever discovered was collected: Sarkastodon mongoliensis. Its dimensions were described as 50% greater than the Patriofelis to which it bore many similarities.<ref name="Granger1938">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It has been estimated that Sarkastodon attained the body mass of twice the largest American lion.<ref name=Sorkin-2008>Template:Cite journal</ref>

Biology

Diet and feeding

Early creodonts (both oxyaenids and hyaenodontids) displayed the tribosphenic molars common for basal therians. Small forms had somewhat strong postmetacrista-metastellar crests<ref name="Gingerich1989">Template:Cite journal</ref> suggesting that they were probably opportunistic feeders, eating such things as eggs, birds, small mammals, insects and possibly plant matter as well,<ref name="Gunnell1998"/> possibly like extant viverrids.<ref name="Denison"/> Larger forms had greater shearing capacity and the capacity increased over time. Arfia, one of the most common carnivorous mammals in early Eocene North America, developed a more open trigonid on M3 over the course of the Early Eocene, increasing the shearing ability of the carnassials.<ref name="Gingerich1989"/> A similar development can be seen by comparing Oxyaena, Prototomus and Limnocyon with the smaller, more generalized feeders among the creodonts.<ref name="Gunnell1998"/>

Extinction

{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= Template:Ambox }} }} Several theories have suggested that hyaenodonts and oxyaenids became extinct because they were outcompeted by the newly-evolved Carnivora. However, there is no direct evidence that the existence of large Carnivora caused the extinction of these taxa, and in many cases (in Africa throughout the Early and Middle Miocene, and in North America and Eurasia during much of the Oligocene), hyaenodonts thrived in environments in which large carnivorans such as nimravids and (later) larger amphicyonids were also present as competitors. Theories that suggest they were outcompeted by the Carnivora include that their smaller brains limited their intelligence, but carnivoran brain sizes have not always been consistently large throughout their evolution, and the importance of brain size as a factor in intelligence has been vastly overestimated in the past when these ideas were published. Other speculations focus on their limb structure, which limited leg movement to a vertical plane, as in ungulates; they were unable to turn their wrists and forearms inward to trip, slash, or grab prey as some modern carnivores can. Creodonts had to depend entirely on their jaws to capture prey, which may be why creodonts generally had a larger head size in relation to their bodies than carnivores of similar stature. However, many carnivorans, such as large canids, are also dependent on their jaws alone to capture prey yet do so effectively even in situations where they must tackle large prey alone, so this also fails to provide a satisfactory explanation.

In the Carnivora, the last upper premolar and the first lower molar are the carnassials, allowing the rearmost molar teeth to evolve adaptations for feeding on non-meat foods. In creodonts, either the first upper and second lower molars, or the second upper and third lower molars, were the primary carnassials, and the rear teeth formed a carnassial series. This structure committed them to eating meat almost exclusively, which may have limited their ability to exploit mesocarnivore and omnivore ecological niches. These differences may have caused environmental changes to affect hyaenodonts and oxyaenids differently than they did many carnivorans, as the former would have been restricted to largely or entirely faunivorous diets, while many (though not all) carnivoran lineages were/are able to subsist on plant matter as well.

A study conducted on creodont and carnivoran dietary niches in the Cypress Hills Formation showed that only the smallest creodont in the faunal assemblage had any significant dietary overlap with carnivorans and that the niches of the other creodonts were extremely distinct from those of the carnivorans they lived alongside, with the study concluding that it was highly unlikely that the extinction of creodonts in North America can be attributed to competition with carnivorans.<ref>Template:Cite journal</ref>

Further reading

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