Antediluvian Beasts of the East : Pliocyon robustus

The grass whistles in the wind as sea-spray is blown from the nearby Atlantic Ocean. This is Florida, but not the one we know today. It is the Miocene, and in the place of wetlands and palm trees are plains and small forests. These wide expanses of grassland are home to a variety of fauna, some just as seemingly alien as the biomes they live in. Among the more recognizable fauna is horse Astrohippus, a close relative of modern zebras and Przewalski’s horses. Astrohippus travel in large herds, up to 100,000 strong, which continuously roam the plains in search for fresh food. Males of this species have large, black manes which they use in territorial displays and courting rituals. Astrohippus are also very aggressive, and often kill or injure themselves in disputes.

Neohipparion and Hypohippus are also found on the plains in small herds, and browse on a variety of plants. These horses are most at home in the small groves which dot the grassland. Their hides, a mixmatch of grey, black, and white patches, camouflage the horses among the trees and bushes. Although not as aggressive as the Astrohippus, these horses will kick if threatened, and often an unlucky predator is killed in a dispute with a Hypohippus. Small herds of these horses will join the Astrohippus during the wet season as they migrate to coastal wetlands.

One of the weirder residents of the grasslands is the rhinoceros Teleoceras. These barrel-bodied grazers are armed with plate-like folds of skin that act as body armor, and a small nose horn. Their heavy size also poses a challenge to predators. Teleoceras also join the horses during the wet season, but are the most uncommon animals in the herds.

Evening, and a mixed herd of horses and rhinoceros stop at a waterhole surrounded by forest, intent on spending the night in the seemingly peaceful setting. Yet the herd is not so lucky as to have a peaceful evening. Suddenly, as if they were ghouls manifesting in the night, a dozen or more Borophagus, the aptly nicknamed “bonecrushing dogs”, emerge from the forest. The horses and rhinoceroses are thrown into a panic, and some flee into the water. The Borophagus start to harass the herd, picking off any animal they can get their teeth around. The Teleoceras are having none of it and run for the hills, flinging any bonecrushing dogs out of their way using nose horns. Unfortunately for the herd animals still caught in the ordeal, it’s about to get much worse.

Alerted by the commotion, marine predators flock to the violent scene. Among these are Alligators of the genus Alligator, which drag unlucky horses down into the muddy water. On land, several Epicyon, larger relatives of Borophagus, join the bloodbath, ripping flesh ad breaking bones. A few of the fox-like Vulpes stenognathus are attracted to the scene, and dodge the larger predators for their chance at a meal. Even worse for the horses, the largest predators of these lands have yet to arrive.

A few hours have passed, and the mass-predation event continues full-fledged. The Epicyon have dragged a couple carcasses away, and have left the scene altogether. The Borophagus, Alligator, and Vulpes stenognathus are joined by a large bear of the genus Agriotherium, who has already taken out a large male Hypohippus. 

As the predators enjoy the fruits of their labor, a roar, sounding like a mix between that of a wolf’s snarl and a bear’s growl, is heard from the forest. Just as the predators perk up their heads to see the sound’s origin, two streamlined forms jump from the thicket. These are Pliocyon robustus, the apex predator of these plains. The Borophagus, watching the two bear-dogs charge, anxiously wait the fight to come. As soon as they reach the herd, the two bear-dogs, each weighing around 450 pounds, start to take on a large male Astrohippus. One of the Pliocyon robustus jumps on the horse, holding the herbivore in place, while the other bear-dog severs the horse’s trachea with a bite to the neck. As one of the Pliocyon robustus digs into the fresh kill, the other charges toward a group of Borophagus feeding on the carcass of a Hypohippus. The bear-dog, growling at the bonecrushing dogs, leaps up on the carcass and lunges at the smaller predators. One Borophagus jumps for the bear-dog in a counterattack, but is knocked to the ground with a swipe of the female Pliocyon robustus’s arm. The other Borophagus, appalled at the sight of their comrade’s undoing, grab what horse meat they can and sprint away. At the sight of their departure, the female bear-dog gorges herself on the remaining carcass.

The Agriotherium, noticing the dispute between the Pliocyon robustus and the Borophagus pugnator, starts to growl at the bear-dogs, who roar back in retaliation. The bear, angered by the bear-dogs’ reaction, starts to charge, and is met by both bear-dogs near a carcass of a small Neohipparion. The bear rears up to a height of 10 feet as he tries to scare the bear-dogs. Undaunted, the Pliocyon fight back, and one scratches the nose of the bear, causing the Agriotherium severe pain. The bear does, however, manage to hit the male the bear-dog in the arm, leaving three small gashes which will, in time, scar. Enraged, the Pliocyon robustus slashes the thigh of the bear. The bear, bruised and beaten, growls and limps away. The bear will survive, but the encounter will leave a large, visible, discolored scar on the bear’s right leg.

Morning, and the sun sheds light on the outcome of the mass-predation event. The herd has moved on, leaving several carcasses on the shores of the waterhole, with many more dragged into the lake by Alligators. The Pliocyon robustus, who’ve continued to eaten throughout the night, rest under a small tree as a few Alligators bask in the sun behind them. Several hours later, the Pliocyon robustus wake from their slumber and run off. An hour later, they arrive at a small thicket, and are greeted by four small faces peeking out of the bushes. These Pliocyon robustus are a mating pair, and will raise their young to become the some of the most fearsome predators in Miocene North America.

Pliocyon robustus is a large amphicyonid (more commonly known as a “bear-dog”) from the Bone Valley Formation of Florida (Berta & Galiano, 1984). This large species of bear-dog is known from a mandibular ramus dating to the Clarendonian North American faunal stage of the Miocene epoch (Berta & Galiano, 1984).

Left mandibular ramus of Pliocyon robustus. From Berta & Galiano (1984).

Left mandibular ramus of Pliocyon robustus. From Berta & Galiano (1984).

As seen in the image above, the mandibular ramus of Pliocyon robustus is short in proportion compared to other amphicyonids, and also larger and more robust then that of Pliocyon medius (Berta & Galiano, 1984). So how big was this amphicyonid, exactly? The length of the mandibular ramus of Pliocyon robustus is around 87.3% of the length of the ~28.5 inch long SDSM 571, the left mandibular ramus of an Ischyrocyon gidleyi (See this post by the Museum of Geology for more on SDSM 571). SDSM 571 is from  Mission Pit, South Dakota, which is generally thought to be of Clarendonian age (Famoso & Pagnac, 2011). Therefore, I will use the mass estimate Figueirido et. al. (2011) found for Ischyrocyon individuals of Clarendonian age to estimate the mass of Pliocyon robustus. We multiply 546 kg (Figueirido et. al.’s mass estimate for Clarendonian Ischyrocyon) by 87.3/100 and voila, we get 476.658 kilograms, or 1050.85 pounds.

Of course, the mass estimate I found is tentative and was only calculated to give the reader an idea of the mass of Pliocyon robustus. That being said, the mass estimate just calculated shows that Pliocyon is among the largest amphicyonids, only beaten by Amphicyon ingens at 579 kilograms, Pseudocyon sp. from New Mexico at  773 kilograms, and Claredonian Ischyrocyon gidleyi at 546 kilograms (Figueirido et. al., 2011). If Pliocyon robustus was overall as robust as its left mandibular ramus is in comparison to other amphicyonids, Pliocyon robustus may have approached the masses of the largest amphicyonids.

The amphicyonids themselves were a diverse race of macropredatory carnivorans which existed for around 10 million years. They were extremely successful, and spread all over the world from North America to Asia. These killers were the top predators of their time, preying on the variety of herbivorous mammals present in the Miocene and Pliocene.

Amphicyon ingens, a relative of Pliocyon robustus. Photo by the author, 2015.

Amphicyon ingens, a relative of Pliocyon robustus. Photo by the author, 2015.

In fact, remains of the horses Astrohippus martini and Hypohippus chico are known from the same site that the holotype of Pliocyon robustus was recovered from (Berta & Galiano, 1984). It is probable that the large amhpicyonid preyed on these horses, chasing them down in the grasslands of the Miocene.The environment of Pliocyon robustus would have been similar to these plains. Photo by the author, 2015.

The environment of Pliocyon robustus would have been similar to these plains. Photo by the author, 2015.

The rule of the amphicyonids, however, was not to last. With time, they decreased in numbers, dwindling to extinction during the Pliocene. Perhaps the newly evolved forms of large felines and pack-hunting canids outcompeted the amphicyonids. Maybe the amphicyonids couldn’t cope with the newly evolved fast-moving fauna which replaced the slow, sometimes bulky mammals they so relied on for food. Whatever the reason, the amphicyonids became extinct. Out of them, Pliocyon robustus, the once-fearsome king of the Florida plains, would be remembered by the lower jaw of a single animal, bearing the killing canines of one of the most successful lineages of predatory mammals to have ever existed.

References

1. Berta, A.; Galiano, H. .1984. “A Miocene Amphicyonid (Mammalia: Carnivora) from the Bone Valley Formation of Florida.” Journal of Vertebrate Paleontology 4(1): 122-125.

2. Famoso, N. A.; Pagnac, D. .2011. “A Comparison of the Clarendonian Equid Assemblages from the Mission Pit, South Dakota and Ashfall Fossil Beds, Nebraska.” Transactions of the Nebraska Academy of Sciences and Affiliated Societies 32: 98-107.

3. Figueirido, B.; Pérez−Claros, J. A.; Hunt, R. M. Jr.; Palmqvist, P. .2011. “Body mass estimation in amphicyonid carnivoran mammals: A multiple regression approach from the skull and skeleton.” Acta Palaeontologica Polonica 56 (2): 225–246.

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