Saturday, September 20, 2014

Ailurids: the Lesser Pandas

The Red Panda (Ailurus fulgens) is the sole surviving representative of a family of small to medium-sized Carnivorans known as the Ailuridae (Gray, 1843), more commonly referred to as lesser or red pandas. This group, whose history dates back to the Oligocene, were once quite diverse and widespread in the forests and woodlands across the Northern Hemisphere, occupying niches that we today would associate with genets, civets, and foxes.

The extant Red Panda itself is a highly specialized animal that feeds almost exclusively on bamboo, a dietary preference it shares with the much larger, unrelated Panda Bear (Ailuropoda melanoleuca). The word "panda" is in fact derived from a Nepalese term that means "eater of bamboo"- nigalya poonya.

When French zoologist Frédéric Georges Cuvier described the Red Panda in 1825, he thought the animal was a type of raccoon, and so placed it in the Procyonidae. It was British zoologist John Edward Gray who placed it in its own family, Ailuridae, in 1843 based on differences in its physiology and behavior. At one time the Red Panda has even been considered to belong to a new and ultimately rejected family called the Ailuropodidae together with the Panda Bear. This theory was rejected after the recognition that the Panda Bear is, as its name suggests, a true bear within the family Ursidae.

Subsequent paleontological discoveries have now confirmed the Red Panda’s place in its own distinct family and recent DNA analysis has backed this up. The Ailuridae belong to a carnivoran clade known as the Musteloidea, which includes the Mephitidae (skunks), Procyonidae (raccoons and their relatives), and the Mustelidae (weasels and their relatives).
Schematic tree summarizing the phylogenetic relationships and temporal
ranges of Ailuridae and other related families within Carnivora.
Figure 3 from Salesa et al., 2006.
Notable Ailurid characteristics include canines with lateral grooves, an elongated second molar on the lower jaw with absent third molars, and the presence of mobile radial sesamoid bones, also known as “false thumbs”.

References & Further Reading
Glatston, Angela R. “Red Panda: Biology and Conservation of the First Panda”. Norwich, New York: William Andrew, 2010. ISBN 1437778135 <Book>

Salesa MJ, Anton M, Morales J (2006). “Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas”. PNAS 130(2): 379-382 <Full article>
Roberts MS & Gittleman JL (1984). “Ailurus fulgens”. Mammalian Species 222: 1-8 <Full article>

Thursday, June 19, 2014

Steller’s Sea Cow (Hydrodamalis gigas)

The Steller’s Sea Cow (Hydrodamalis gigas) was the largest of the sirenians to have lived in recent times, growing up  to 9 meters long and 10 tons. Described by and later named for Georg Wilhelm Steller in 1741, these slow-moving animals were easily captured and were hunted to extinction by 1768, 27 years after its discovery by Europeans. Its closest living relative is the Dougong (Dugong dugon) which lives in the south Pacific and Indian Ocean. 
Steller's Sea Cow skeleton on display at the Finnish Museum of
Natural History. Wiki
Habitat and Distribution
Pleistocene-Holocene age fossils indicate that Steller’s Sea Cows were formerly abundant throughout the north Pacific, reaching south to California and Japan and north to Alaska and Russia. By the time it was described by Steller in 1741, its range had apparently been drastically reduced to a single isolated population surrounded by the then uninhabited Commander Islands. They inhabited kelp forests and kelp beds near the coasts and were restricted to surface waters, not being able to dive deeply.

Physical Attributes
  • Total Length: 8 to 9m (26 to 30ft)
  • Weight: 8,000 to 10,000kg (8 to 10 tons)
Steller’s Sea Cow were the largest of the sirenians to have lived in recent times. The head was rather small compared to its huge body with a very broad upper lip that extended beyond the mandible, making it appear as though the mouth is located underneath the skull. The mouth itself is small and toothless with double lips on the upper and lower jaws. The space between the lips is filled with very dense and thick 38mm (1.5in) long bristles that take the place of teeth, used to hold and pull its food. The skin, according to Steller, was very thick, hairless, and wrinkled, reminiscent to the bark of an oak tree and nearly impervious to a blow from an ax or hook. For propulsion, Steller’s Sea Cows had a wide, whale-like tail flukes similar to that of dugongs. Also according to Steller, the sea cows could use their stout forelimbs for steering, “walking” along the ocean bottom, bracing themselves on rocks, embracing each other, and digging for algae and sea grasses, all of which are behaviors consistent with extant sirenians.
Feeding Ecology
Steller’s Sea Cow fed on a variety of the marine algae known as kelp, which they ate by ripping up out of the sea floor by the roots. Steller noted that wherever the animals had been feeding, the roots and stalks of kelp would wash up on the near shores in heaps. 

Kelp forests are among the most productive and dynamic ecosystems on Earth and the Steller's Sea Cow was the largest animal to have exploited this rich food source. This species' waste material likely enriched the kelp fields in which it lived and fed by adding extra nutrients. Steller's Sea Cows moved at a very slow and steady pace, and probably had a slow metabolic rate and could have subsisted on a relatively small food intake. Steller described them as being locally numerous and apparently very sociable.

Steller's Sea Cow skull. Wiki
Potential Predators
Prior to human exploitation, the Steller’s Sea Cow’s only predators were the Orca (Orcinus orca) and the giant shark Megalodon (Carcharodon megalodon). They were relatively slow swimmers and apparently unable to submerge, needing to rely on their rough skin and proximity to the shore to shield them from predators. When in danger, this animal could probably put on a sudden burst of speed of 15 to 20mph.

The early decline of the Steller’s Sea Cow may have been an indirect result of the harvest of Sea Otters (Enhydra lutris) by indigenous peoples along the continental shorelines. The otters play a key role in maintaining the sea urchin population, which would have competed with the sea cows for kelp. As observed in recent times, when the otter population is reduced, the sea urchin population explodes and the invertebrates decimate the kelp forests in which they live. This added competition for food combined with direct hunting of the sea cows themselves are the most likely reason for its drastic decline. Their tendency to live close to shore, slow swimming speed, and inability to dive deeply made them particularly easy to hunt, something which later European sailors would exploit to disastrous effect.
By the time Vitus Bering arrived to the north Pacific, the animals’ population had been limited to the coasts of islands that had been uninhabited by humans. After its discovery, Steller’s Sea Cow were quickly wiped out by sailors, seal hunters, and fur traders who hunted it for its meat, skin, and fat. The tough skin was suitable for making boats, and the oil from its blubber was particularly prized because it could be used as a butter substitute and as fuel for oil lamps for it did not emit odor or smoke and kept for a long time in warm weather without spoiling.

When formally described by Steller, Steller’s Sea Cows already had a very small and limited range. Their numbers were so low that zoologist Leonard Hess Stejneger estimated that when Steller found them, their population was around 1,500 individuals. Thus, it was already in immediate danger of extinction at the time. Unfortunately, this observation was made long after the fact, and overhunting led to this species extinction by 1768. There is further evidence that Steller’s Sea Cow also inhabited the Near Islands and oral tradition on the island of Attu suggests that they were still being hunted there after their extinction on the Commander Islands.
1988 restoration from the Soviet Union. Wiki
Scheffer, Victor B. (November 1972). "The Weight of the Steller Sea Cow". Journal of Mammalogy 53 (4): 912–914. doi:10.2307/1379236. JSTOR 1379236.

Steller, Georg Wilhelm (1899) [1751]. "De Bestiis Marinis, or, The Beasts of the Sea (1751)" (in English). Translated by Walter Miller and Jennie Emerson Miller, Transcribed and edited by Paul Royster. University of Nebraska Lincoln. Retrieved January 2014 <>

Turvey ST and Risley CL (2005). "Modelling the extinction of Steller’s sea cow". Biol. Lett. (2006) 2, 94–97 doi:10.1098/rsbl.2005.0415 <>

Self-Sullivan, Caryn (2007-02-25). "Evolution of the Sirenia". Sirenian International. Retrieved 2007-04-19. <>

D. G. Corbett, D. Causey, M. Clemente, P. L. Koch, A. Doroff, C. Lefavre, D. West (2008) "Aleut Hunters, Sea Otters, and Sea Cows", Human Impacts on Ancient Marine Ecosystems, University of California Press

Ellis, Richard (2004). No Turning Back: The Life and Death of Animal Species. New York City: Harper Perennial. p. 134. ISBN 0-06-055804-0.

Tuesday, June 10, 2014

American Lion (Panthera leo atrox)

The Lion (Panthera leo) evolved in Africa about 3.5 million years ago and quickly spread to Eurasia, where it remained abundant until historic times. At the start of the Pleistocene, Lions from Russia crossed Beringia and spread southward throughout North and South America. These New World Lions are referred to the subspecies Panthera leo atrox, or simply, the American Lion.
American Lion skeleton recovered from the Rancho La Brea tar pits, on
display at the George C. Page Museum, Los Angeles. Wiki
The American Lion is so named because of its New World distribution. The name Panthera may be derived from the Greek pan-, meaning “all”, and ther, meaning “prey”, translating literally as "predator of all animals”. Its species name, leo, is simply the Latin word for the Lion. The name atrox is a Proto-Indo-European word meaning “atrocious”, “fierce”, “savage”, or “cruel”.

Habitat & Distribution
Lions are typically found in more open habitats such as grassland, savanna, semidesert, scrubland, and open woodland, occasionally venturing into more forested areas where there are plenty of clearings in which large prey graze. Genetically, American Lions are identical to the Eurasian subspecies Panthera leo spelea, the Cave Lion. They descended from Lions that entered the New World from Russia via the Bering Land Bridge and eventually became isolated from the Old World population. Lions went on to colonize the whole North American continent from Florida to California east-to-west and Alaska to Honduras north-to-south.

Physical Attributes
The Lions that once lived in the temperate and subpolar lands of the north grew 8 to 10% larger on average than most modern Lions from Africa and India. This larger average size is a great example of Bergmann's rule, which states that animals native to colder climates at the northernmost or southernmost reaches of their ranges will grow larger than their close relatives in the warmer regions nearer the equator. Larger animals lose body heat at a slower rate than smaller ones, and so this is a useful adaptive change. Among extant felids, the Siberian Tiger (Panthera tigris altaica) best exemplifies this principle. Until their relatively recent extirpation over the last few centuries, the largest wild modern Lions were to be found in the southernmost part of Africa, which experiences freezing temperatures during winter.
Apart from this slight size difference, American Lions were anatomically identical to their living counterparts. However, because of their more northerly distribution these Lions would have needed a thicker and longer fur coat, particularly during winter. 32,000 year old cave paintings from Europe reveal that Pleistocene Lions had the same plain coats of their living relatives with some retaining a faint spotted pattern. Their fur is likely to have become paler in color during the winter months to better camouflage them against the snow. Also revealed through cave paintings, the male Lions of the Pleistocene lacked the exaggerated manes for which modern Lions are renowned for, having instead a smaller ruff of thick fur. The mane as we know it today is a recent evolutionary development among the species that originated sometime between 60,000 and 20,000 years ago.
Ecology & Behavior
Lions are the largest land predators in Africa today. In areas with particularly high prey density, prides may comprise 10 to as many as 20 adults. During the Pleistocene, however, Lions shared their habitat with other large and equally formidable predators, greatly increasing competition pressure. This made securing and maintaining a kill more of a challenge and such a large group would have been a handicap when it came to procuring enough calories to sustain each individual group member. American Lions are more likely to have lived in the smaller social groups similar to what is observed among living Asiatic Lions; female groups consisting of 2 to 6 adults and male coalitions of 2 to 4. Smaller group sizes are more efficient because they enable the cats to bring down prey too large for a single cat to handle (500 to 1,000kg) while at the same time enabling each individual to obtain a reasonable share when a smaller kill is made (100 to 200kg).
Modern Lions in Kenya. Groupings of this size would have been typical for
American Lions and would have rarely exceeded this number. Wiki
The most common prey animals taken by Lions fall between the 200 and 400kg range, which can be easily felled by one or two individuals with a meat yield sufficient enough to sustain all the group members for several days. Caribou (Rangifer tarandus), the Northern Hemisphere's equivalent to Africa's Blue Wildebeest, would have been among the most common prey, followed by various species of horses, llamas, and other large deer such as Wapiti (Cervus canadensis). In between these kills, the Lions would have opportunistically taken any smaller prey they could catch weighing 50 to 200kg. Such "snacks" would have included smaller deer like White-tailed (Odocoileus virginianus) and Mule Deer (O. hemionus), as well as the occasional pronghorn or peccary. 

When their regular prey was in short supply the Lions would have turned to larger animals within the 400 to 1,000kg range. Such prey would have included any of the various bison, musk ox, or larger camel species like Western Camel (Camelops hesternus) or Giant Camel (Titanotylopus nebraskensis). Hunting such big game required the collective strength of all the group’s adults. As with modern Lions, smaller prey animals would have been dispatched with a bite to the throat. Larger prey species, whose necks were too wide for an accurate throat bite, would have been dispatched with a muzzle-clamp bite, during which the Lion bites down on the prey's face sealing the mouth and nose, causing death by suffocation. Direct evidence of this killing method comes from Blue Babe, a 36,000 year old Lion-killed Steppe Bison (Bison priscus) unearthed in the permafrost of Fairbanks, Alaska in 1979. The specimen, a large bull, bears distinctive Lion bitemarks on his nose!

The modern African Lion is by far the largest land predator on the continent and it dominates all other carnivores with an iron fist. During the Pleistocene, however, this was not the case. Lions coexisted with many other great predators which individually could physically threaten it. Indeed if such predatory competition were present today, we would probably have to refer to the Lion as "the Prince of the Beasts". Interestingly, American Lions appear to have had slightly larger brains than their living relatives. This could mean that these Lions had more complex social behaviors and better problem-solving abilities, which would have helped it in its competitor-rich environment.

The American Lions' most substantial competition would have come from the Gray Wolf (taking the slot that Spotted Hyenas fill in Africa) and the Short-faced Bear, predators which shared the same habitat preference and presumably the same crepuscular-nocturnal activities. The latter of the two killers grew up to 4 times heavier than a Lion and could have easily dominated them at kills, possibly shadowing the hunting cats and then appropriating their kills. Modern Brown Bears employ this strategy in areas where they coexist with Gray Wolves.

Differences in habitat preference, prey selection, population density, and circadian rhythm helped other predators to minimize competition with Lions. The American Cheetah (Puma trumani) lived in much the same habitat, but it specialized in hunting smaller, more fleet-footed deer and pronghorns which Lions would have rarely tackled. They would have also preferred to hunt during the day while Lions were more selectively nocturnal. Jaguars (Panthera onca) and Cougars (Puma concolor) were solitary hunters that would have hunted in more wooded or bushy areas and would tend to avoid open areas in which the Lion was common and thus minimizing contact. Dire Wolves (Canis dirus) and Saber-toothed Cats (Smilodon fatalis) lived and hunted in riparian or marshland habitats while Lions prefer drier areas. Hunting Hyenas (Chasmaporthetes lunensis) and Scimitar Cats had coexisted successfully with Lions for 3 million years in the Old World. Scimitar Cats (Homotherium serum) in particular would have hunted the same large deer and horses as the Lion, however, they would have more frequently selected the larger bison, camels, ground sloths, and juvenile elephants, which the Lions would have generally hunted opportunistically or when their regular prey was unavailable. Also, these predators had lower population densities than Lions and so would not have encountered each other often.

References & Further Reading
Barnett R, Shapiro B., Barnes, I., Ho SYW, Burger J, Yamaguchi N, Higham TFG, Wheeler HT, Rosendahl W, Sher AV, Sotnikova M, Kuznetsova T, Baryshnikov GF, Martin LD, Harington CR, Burns JA, and Cooper A (2009). "Phylogeography of lions (Panthera leo ssp.) reveals three distinct taxa and a late Pleistocene reduction in genetic diversity". Molecular Ecology, 18: 1668–1677. doi: 10.1111/j.1365-294X.2009.04134.x <Full article>

Wheeler HT & Jefferson GT (2009). “Panthera atrox: body proportions, size, sexual dimorphism, and behaviour of the cursorial lion of the North American lains”. Papers on Geology, Vertebrate Paleontology and Biostratigraphy in Honor of Michael O. Woodburne, edited by L. B. Albright III. Flagstaff, Arizona: Museum of Northern Arizona Bulletin <Full Article>

Hayward MW (2006). "Prey preferences of the spotted hyaena (Crocuta crocuta) and degree of dietary overlap with the lion (Panthera leo)". Journal of Zoology 270: 606-614 <Full Article>

Radloff FG & Du Toit JT (2004). "Large predators and their prey in a southern African savanna: a predator's size determines its prey size range". Journal of Animal Ecology 73: 410-423 <Full Article>

Yamaguchi, N., Cooper, A., Werdelin, L. and Macdonald, D. W. (2004), Evolution of the mane and group-living in the lion (Panthera leo): a review. Journal of Zoology, 263: 329–342. doi: 10.1017/S0952836904005242 <Full article>

Turner A (1997). The big cats and their fossil relatives. New York: Columbia University Press. ISBN 0-231-10229-1 <Book>

Macdonald, David W. The Princton Encyclopedia of Mammals. Princeton, New Jersey: Princeton University Press, 2009. 750-751 <Book>

Friday, March 14, 2014

Land Crocs: Archosaur Predators in the Age of Mammals

When we think of modern crocodiles and their relatives, the image of an aquatic animal that is relatively slow-moving on land typically comes to mind. However, the animals that we know today are but one small (and highly specialized) part of a much larger and more diverse group of animals, most of which were better suited to life on dry land.

Land-based crocodylomorphs, the archosaur group which contains modern crocodilians and their extinct relatives, were very common throughout the Mesozoic Era and were incredibly diverse with a wide array of morphologies, families, and many species between them. Despite experiencing a considerable drop in diversity at the end of the Cretaceous, crocodylomorphs were among the first animals to take over the terrestrial predator niches that were left vacant by the theropod dinosaurs. As the Cenozoic Era progressed, competition from placental mammals has largely discouraged the long-term success of these animals on the continents of North America, Eurasia, and Africa. However, these predators remained prominent components of the fauna on the island continents of South America and Australia.

Planocranids appear in the fossil record immediately after the Cretaceous-Paleogene extinction event. They were a rather short-lived family with a temporal range spanning the early Paleocene to the middle Eocene. Closely related to modern crocodiles, these animals inhabited Eurasia and North America. They gradually faded into extinction as mammalian carnivores were becoming more diverse. In their heyday though, they were major predators of the small to mid-sized mammalian herbivores with which they coexisted. Three genera are currently recognized; PlanocraniaBoverisuchus, and Pristichampsus.
Heads of two Pristichampsids; Planocrania datangensis
(top) and Planocrania rollinattii (bottom).
Mekosuchines had a much longer run than the earlier planocranids, having existed from the Eocene all the way into the Holocene where they briefly coexisted with humans as recently as 3,000ya. These animals were closely relatives of modern crocodiles and are grouped as a clade within the family Crocodylidae. Their fossils mostly occur in Australia and its nearby islands. The group's longevity may be due to the absence of placental mammals in these regions. Most of them appear to have been at least partly aquatic. Others, such as Mekosuchus and Quinkana, evolved to become fully terrestrial. The latter of the two genera was once mistakenly interpreted as a late-surviving planocranid by early researchers.
Heads of two late-surviving Mekosuchids; Mekosuchus
inexpectatus (top) from Holocene New Caledonia and
Quinkana fortirostrum (bottom) from Pleistocene Australia.
The sebecosuchians were part of a much more diverse, mostly Gondwanan* group of crocodylomorphs known as the Notosuchia, which first appear in the fossil record during the early Cretaceous. The sebecosuchians comprise two families; the late Cretaceous Baurusuchidae, and the Cenozoic Sebecidae which we will be focusing on in this blog. Sebecids were a successful group that lasted from the early Paleocene (66mya) to the middle Miocene (11mya). They were an exclusively South American family that, together with the sparassodont marsupials and phorusrhacid birds, formed an important part of the predator community on that continent.
Restored head of Sebecus icaeorhinus, a Sebecid.
Living crocodilians are renowned aquatic predators that are typically more at home in the water than the land. However, the affinity these animals have to water varies between the species and some do have adaptations that make them better adapted for terrestrial locomotion. 

The Cuban Crocodile (Crocodylus rhombifer) evolved on the Caribbean islands where it had little or no competition from mammalian predators. It is one of the largest endemic carnivores in the region second only to the American Crocodile (Crocodylus acutus). Compared to others within its family, it has brighter adult coloration, larger teeth, rougher skin, longer legs, and reduced webbing between its toes. Though still amphibious by nature, it spends much more of its time on land than other crocodiles and it runs and leaps very well. Fossil evidence shows that these crocs once fed on extinct ground sloths, and the above attributes would have enabled them to hunt these animals on land. In fact, a colony of this species at Gatorland, Florida has exhibited possible pack-hunting behavior which could have helped it hunt such extinct megafauna. Other extant crocodilians are also known to hunt cooperatively, which raises implications into what hunting methods the extinct species above were capable of.
The Cuban Crocodile (Crocodylus rhombifer) spends more
time on land than any other extant crocodilian. Wiki
Another island crocodile, the extinct Horned Crocodile (Voay robustus) of Madagascar, exhibited characteristics that would have made it well-suited to terrestrial life. Compared to the Cuban Crocodile, this species had even more robust limbs, a shorter, deeper snout, and lower, more anteriorly-facing nostrils. On an island with few mammalian predators, this crocodile was one its top predators. It only became extinct after humans arrived a few thousand years ago.
The skull of the extinct Malagasy Horned Crocodile
(Voay robustus). Wiki
Gondwana: the more southerly of two ancient supercontinents that consisted of South America, Africa, Antarctica, Australia, India, Arabia, and Madagascar

References & Further Reading
Ross, Charles A. “Crocodiles and Alligators”. New York, NY. Facts on File, Inc. ISBN: 0-8160-2174-0

Brochu CA (2007). “Morphology, relationships, and biogeographical significance of an extinct horned crocodile (Crocodylia, Crocodylidae) from the Quaternary of Madagascar”. Zoological Journal of the Linnean Society 150(4): 835-863.

Molnar EM (2010). “A new reconstruction of the skull of Sebecus icaeorhinus (Crocodyliformes: Sebecosuchia) from the Eocene of Argentina”. Bazilian Geographical Journal: Geosciences and Humanities research medium 1(2): 314-330

Pol D, Powell JE (2011). “A new Sebecid mesoeucrocodylian from the Rio Loro Formation (Paleocene) of north-western Argentina”. Zoological Journal of the Linnean Society 163: 7-36

Paolillo A, Linares OJ (2007). “Nuevos Cocodrilos Sebecosuchia del Cenezoico Suramericano (Mesosuchia: Crocodylia)”. Paleobiologia Neotropical 3: 1-25 <Full Article>

Sunday, February 9, 2014

Oxyaenidae: Cat-like Predators of the Paleogene

The Oxyaenidae is a family of creodonts first described by renowned American paleontologist Edward Drinker Cope in 1877. While hyaenodonts are often described as dog- or hyena-like, oxyaenids are considered to be more cat-like or weasel-like in their general morphology with rather long bodies set on short, robust limbs. They were ambush predators with robust limbs, muscular bodies, shortened faces, and powerful jaws. They possessed enlarged, reinforced canines with blade-like molars, and some species even developed blunt premolars for cracking bones.

Mounted skeleton of Patriofelis ferox. Wiki
Evolutionary History
The earliest known oxyaenid was a domestic cat-sized animal called Tytthaena parrisi, known from the middle Paleocene (Tiffanian) of what is now the state of Wyoming. Throughout the rest of the Paleocene and Eocene, a number of other oxyaenid species existed across the Northern Hemisphere ranging from fox-sized to bear-sized. From the middle Eocene onward, however, oxyaenids would face mounting competition from another group of cat-like predators known as the Nimravidae, close relatives of true cats (Felidae) in the modern order Carnivora*. By the late Eocene, oxyaenids are absent from most faunal assemblages while nimravids are relatively common. The last known member of this group, Sarkastodon mongoliensis, became extinct early in the Oligocene epoch. Unlike the hyaenodonts, oxyaenids were restricted to North America and Eurasia but never appear to have invaded Africa. 

Stance & Locomotion
When Dr. Jacob L. Wortman first examined the fossils Oxyaena and Patriofelis in the late 1800s, he concluded that the animals' spreading paws were evidence that they once supported a web-like structure like that found in many semi-aquatic animals such as otters. He remarked “the broad, flat, plantigrade feet with their spreading toes suggest at first glance their use for swimming”. This in mind, he also theorized that oxyaenids were the likely ancestors of today’s pinnipeds (seals, sea lions, and walruses). This proposal led artists to reconstruct Patriofelis as an otter-like animal with webbed toes as shown in this 1896 reconstruction by Charles R. Knight.

Charles R. Knight's outdated and inaccurate illustration of
Patriofelis ferox (1896), depicted here as an otter-like animal
with webbed toes and plantigrade feet. Wiki
However, large paws with widely spaced toes are not unique to aquatic mammals: many of today's land predators like cats, bears, mustelids, and others have them as well. In fact, this characteristic has three functional advantages befitting a terrestrial carnivore;
  1. They enable silent movement through the animal’s environment by muffling any sounds made as the animal walks.
  2. Though not suitable for sustained running, they are well suited for short bursts of speed necessary for an ambush predator, which is a more conservative hunting method.
  3. Their wide surface area enhances grip, enabling greater manual dexterity for climbing and seizing prey.
A 1900 study comparing the feet of Oxyaena and Patriofelis to those of several modern opossums, raccoons, and cats revealed that the proportions of oxyaenid paws were comparable to those extant animals. The structure of the metapodials* also show that some oxyaenids may not have been fully plantigrade* as previously thought, but rather semi-digitigrade*. This would have enabled more efficient and agile movement on dry land. In life, oxyaenids would have possessed cushioning paw pads that would muffle their footsteps to further enable silent movement through their respective habitats.

Mounted skeleton of Patriofelis ferox, shown in a more accurate stance. Wiki
Oxyaenids were not the slow-moving, clumsy animals depicted in past writings and artwork. Rather, they were agile terrestrial predators adapted for long-distance walking, short bursts of speed, and perhaps some climbing ability.

Teeth & Jaws
Oxyaenids are described as being cat-like, while hyaenodonts are said to be dog-like. Indeed, the skulls do exhibit some “cat-like” characteristics designed to kill prey with a single bite. As in felids, shortening of the rostrum is achieved by the loss of the first premolars and the last molars. This reduction helps direct more of the animals’ bite force to the canine teeth, which in oxyaenids were extremely robust with deep roots for durability. The molars are highly specialized for shearing with the primary cutting teeth (the carnassials) comprising the first upper and second lower molars. Interestingly, among oxyaenids there seems to be a trend toward the reduction of the incisors, possibly an adaptation to enhance the puncturing aspect of the dentition.

Skull of Patriofelis ferox at Museum national d'Histoire
naturelle, Paris. Wiki
Oxyaenid skulls are exceptionally broad, strong, and appear quite large compared to the rest of the skeleton. For example; the middle Eocene predator Patriofelis ferox was the size of a Jaguar in terms of its head-and-body length. However, it had a skull that was just as long and considerably wider than that of a Lion, an animal nearly twice its size. High sagittal crests*, flared zygomatic arches*, and huge depressions in the mandible were the attachment sites of powerful jaw muscles which resulted in a tremendous bite force. In these aspects, the skull morphology of oxyaenids is more analogous to the extinct marsupial lions (Thylacoleonidae) of Australia, which were themselves superficially cat-like animals with heavily-built skulls and teeth adapted for piercing. 

Partial dentary and skull of Palaeonictis occidentalis.
Specimen on display at the Museum fur Naturkunde, Berlin.
Members of the genera Machaeroides and Apataelurus are special in that they are the earliest known mammals to have developed saberteeth; specialized upper canines that have become elongated and laterally-flattened. As with nimravid and felid sabertooths, their skulls are designed to increase their maximum gape and employ a downward stabbing bite with the upper canines. It should be noted that the placement of these genera within the Oxyaenidae is still a subject of debate; some authors place them within the Hyaenodontidae or Limnocyonidae.

Skull of the sabertoothed creodont, Machaeroides eothen, on
display at the American Museum of Natural History,
New York. Wiki
Feeding Ecology
Based on their cat-like morphology, we can assume that oxyaenids were mostly solitary predators that did not require the assistance of a pack to bring down relatively large prey items. Although they lacked the ability to retract their claws like nimravids and felids, their broad forepaws were clearly capable of grasping and could have easily have been involved in the seizing of prey in the initial stages of the kill. Other predators like mustelids, ursids, and dasyurids* do this as well. Their powerful skulls, meanwhile, could then deliver precise and devastating bites to captured prey. The most likely killing method would have been a piercing bite to the head or neck. In addition to sectorial* dentition, some oxyaenids also possessed blunt, heavy premolars similar to those of modern bone-cracking hyenas. Many oxyaenids were most likely hypercarnivores which fed almost exclusively on animal matter. However, at least some, like the genus Palaeonictis, seem to have been more mesocarnivorous: having a diet mostly comprised of animal matter with small doses of plant matter.

Head sketches of three Oxyaenids from the early, middle, and late Eocene respectively.
From left-to-right: Oxyaena lupina, Patriofelis ferox, and Sarkastodon mongoliensis.
Genera & Species
Tytthaena (Gingerich, 1980)
The genus Tytthaena contains a single species T. parrisi, the earliest and smallest known oxyaenid. This species from the middle Paleocene of Wyoming was about the size of a modern domestic cat and was similar in morphology to later oxyaenids such as Oxyaena and Dipsalodon.

Oxyaena (1874)
This animal’s genus name is a combination of the Greek words oxys and hyaena, literally translating to “Sharp Hyena”. Oxyaena lived from the late Paleocene to early Eocene of North America with most of its fossils being known from the state of Colorado. The last upper premolar and the second lower molar formed the carnassials. In contrast to others of its family, O. lupina was a nimble and lightly-built predator that may have been able to hunt both on the ground and in the trees. It had a dental formula of I3/3, C1/1, P4/4, M2/2 x 2 = 40 teeth.

Palaeonictis (De Blainville, 1842) 
The genus name is derived from the Greek words palaio, meaning “old” or “ancient”, and ictis or ictidis, a type of weasel. Palaeonictis lived from the Paleocene to the early Eocene and are known to have inhabited North America and Europe. The genus is characterized by their relatively blunt premolars that were apparently adapted for crushing bones. Four species have been described ranging in size from that of a Wolverine to a Black Bear; P. peloraP. occidentalis, and P. wingi from North America, and P. gigantea from Europe.

Patriofelis (Leidy, 1872) 
The genus name translates literally to “Father Cat”, derived from the Latin words patrius, “father”, and feles, “cat”. Compared to Oxyaena the legs are stockier and the lumbar vertebrae are much larger and more robust. The caudal (tail) vertebrae are also larger and bear pronounced chevrons on their ventral surfaces. This suggests that Patriofelis was adapted to grapple with large prey animals by using its tail prop itself up while it stood up on its hind legs. The dental formula was I2/2, C1/1, P3/3, M1/2 x 2 = 30 teeth. Two species have been described based on body size, possibly representing different sexes of the same animal. P. ferox is the larger of the two, the size of a large Jaguar or a female Lion. P. ulta is smaller, at about one-third the size and may simply represent the female morph of P. ferox rather than its own species.

Sarkastodon (Granger, 1938) 
The name Sarkastodon is derived from the Greek words sarx, “flesh”, and odous, “tooth”, translating to “Flesh-tearing Tooth”. The last and largest of the oxyaenids Sarkastodon mongoliensis lived during the middle to late Eocene of Asia. This animal is known only from a single skull with a dental formula of I2/1, C1/1, P3/3, M1/2 x 2 = 22 teeth. However, if its body proportions are similar to its close relative Patriofelis, Sarkastodon would have been an exceptionally heavily-built, powerful predator comparable to the largest felids such Smilodon populator and Xenosmilus hodsonae. Among contemporary predators, only Andrewsarchus mongoliensis was larger. 

Other oxyaenid genera include: Ambloctonus, Dipsalodon, Dipsalodictis, Dormaalodon

Carnivora: the mammalian order which contains dogs, cats, bears, hyenas, etc.
Dasyurid: any member of a family of small, predatory marsupials native to Australia.
Digitigrade: a type of locomotion in which only the toes touch the ground.
Metapodial: the bones which comprise the palms of our hands and the soles of our feet.
Plantigrade: a type of locomotion in which the whole foot is planted on the ground.
Sagittal crest: the ridge of bone that runs down the midline of the skull in many mammals.
Sectorial: adapted for cutting.
Zygomatic arch: commonly known as the “cheek bones”.

References & Further Reading
Chester SGB, Bloch JI, Secord R, Boyer DM (2010). “A new small-bodied species of Palaeonictis (Creodonta, Oxyaenidae) from the Paleocene-Eocene Thermal Maximum”. Papers in the Earth and Atmospheric Sciences 298: 227-243 <Full Article>

Gingerich PD (1980). “Tytthaena parrisi, oldest known oxyaenid (Mammalia, Creodonta) from the late Paleocene of western North America”. Journal of Paleontology 54(3): 570-576 <Full Article>

Gazin LC (1946). “Machaeroides eothen Matthew, the saber-tooth creodont of the Bridger Eocene”. Proceedings of the United States National Museum <Full Article>

Granger W (1938). “A giant oxyaenid from upper Eocene of Mongolia”. American Museum Novitates 969: 1-5 <Full Article>

Osborn HF (1900). “Oxyaena and Patriofelis restudied as terrestrial creodonts”. Bulletin American Museum of Natural History 13(20): 269-279 <Full Article>

Wortman JL (1899). “Restoration of Oxyaena lupina Cope, with descriptions of certain new species of Eocene creodonts”. Bulletin American Museum of Natural History 12(6): 139-149 <Full Article>

Wortman JL (1894). “Osteology of Patriofelis, a middle Eocene creodont”. Bulletin American Museum of Natural History 6(5): 129-165 <Full Article>