The Hyena Who Saw the Canyon

“Has the Earth’s sixth mass extinction already arrived?” This question – the title of a review published in last week’s Nature – immediately sparked a flurry of news reports about an impending ecological catastrophe on a scale not seen in 65 million years. We are not witnessing a die-off as severe as any of the “Big Five” prehistoric cataclysms just yet, but the continued, gradual loss of threatened species is bringing us ever closer to the tipping point. We can either take action and stave off this large-scale disaster, or simply wait for it to happen.

Of course, extinction is the inevitable fate of every species. Species do not only disappear during worldwide disasters. Extinction greatly outpaces the origin of new species on a global scale during rare crises, but the character of life on earth is constantly shifting as some lineages dwindle as others speciate and continue to change.

You do not need to look very far back into the fossil record to appreciate the ongoing ebb and flow of life. When I traveled through Utah and Wyoming for the first time in the summer of 2009, I saw the iconic elk, pronghorn, bison, and bears that symbolize the American wilderness. But these animals are only the inheritors of a landscape that has been inhabited by a changing cast of megamammals for millions of years. The great mammoths, ground sloths, deep-snouted bears, and sabercats of Pleistocene North America represent a lost world that disappeared only yesterday in geological terms, but they, too, were preceded by what we might perceive as strange assemblages of creatures, including North America’s only hyena.

In 1901, workmen at the Val Verde Copper Mines in Anita, Arizona were prospecting around an ancient limestone fissure when they discovered a cache of ancient mammal bones. The fragments were badly broken, but the bone material itself was well-preserved, and a number of specimens were soon collected by B.C. Bicknell. The site also piqued the interest of globe-trotting fossil hunter Barnum Brown, who collected a few additional specimens in 1904. Pieces of prehistoric horses and camels were found among those of pronghorn, squirrels, groundhogs, and pocket gophers, as well as what appeared to be jaw fragments from a large cat.

It would take a decade and a half for the Anita bones to be fully described. Brown had intended to do it himself – he even sent for the specimens Bicknell had collected – but he never got around to it. Eventually the bones were given to what is now the Smithsonian National Museum of Natural History, where Oliver Perry Hay finally reported on them in 1921.

Most of the initial identifications Brown had made in his notes turned out to be correct. The mammals appeared to represent a time in the not-too-distant past when forms still living in North America today mixed with lineages that have since been extirpated. What stood out were two parts of “cat” jaw that didn’t correspond to any known feline. With the exception of a tiny part of a molar, the crowns of the teeth were entirely gone, but together the two pieces comprised most of the mandible of a carnivorous mammal. Though this was not much to work with, Hay was able to determine that the jaw had belonged to a hyena – a type of carnivore never before found in North America – and he named it Chasmaporthetes ossifragus.”The name of this [genus] makes allusion to the Grand Canyon,” Hay wrote, “whose beginning this animal may have witnessed.”

Yet Chasmaporthetes was not unique to North America. The species Hay described – C. ossifragus – turned up in deposits between 3 and 1.5 million years old at other sites in Mexico, the American southwest, and Florida, but other species of the same genus were also discovered in Europe, Africa and Asia. Rather than being entirely unique to North America, Chasmaporthetes had initially evolved elsewhere and eventually spread over the Bering Land Bridge into North America. It was a long-lived variety of hyena that was just part of a radiation of now-extinct forms.

Although Africa’s spotted hyena is the most iconic member of the group, there are three other species of living hyena: the striped hyena, the brown hyena, and the aardwolf. They are all that’s left of a once-more widespread and diverse lineage that traces back about 20 million years to small, civet-like forms such as Plioviverrops. Now, based upon appearances alone, it might seem reasonable to lump all four modern hyenas into a single evolutionary subgroup tied together by common ancestry, but this wouldn’t be right. The aardwolf, a strange and small hyena that primarily eats termites, is actually a relatively distant cousin of other modern hyenas and represents what some of the early members of the group may have been like. Likewise, the extinct giant Pachycrocuta was a closer relative of the spotted hyena than the striped and brown hyenas, and there was an entire array of extinct forms with no living representatives. Chasmaporthetes was among these now-extinct hyena lineages, and it was significantly different from the hyenas we know today.

Chasmaporthetes has often been called the “hunting hyena.” By itself, this isn’t a very helpful moniker. Despite their reputation as scavengers, for example, spotted hyenas actually obtain much of their meat through hunting, with carrion making up as little as five percent of their diet in some populations. Even so, the nickname is meant to highlight the long-legged and relatively graceful build of Chasmaporthetes. This was a hyena well-adapted to running and chasing down prey.

For decades, most of what was hypothesized about the North American hyena was based on Chasmaporthetes specimens found elsewhere. Jaw fragments and teeth were all that had been recovered in the southwest and Mexico. This changed in 1981, when Annalisa Berta described parts of the skull and limbs of the hyena found in Florida. No single skeleton was found, but by looking at the accumulated pieces Berta determined that the Florida hyenas had strongly-muscled, flexible upper arms and long, slightly curved tibiae which indicated that the hyenas had very powerful hindlimbs. The fact that premolars of Chasmaporthetes resembled the meat-slicing teeth of the spotted hyena rather than the crushers of the brown and striped hyenas was taken as an indication that it was more of a predator than a scavenger, and a fleet-footed one at that.

Though the North American Chasmaporthetes specimens were differentiated from other species by their relatively robust limbs, deep lower jaws, and slightly curved tooth rows, their general anatomy was consistent with finds in the Old World. These hyenas were hunters that ran down their prey. This might have put them in competition with speedy cats that evolved about 1.8 million years ago – namely North America’s false cheetah Miracinonyx – but some researchers stressed caution in drawing conclusions about diet on the basis on anatomy alone. In a 1994 paper about Chasmaporthetes and Hyaenictis, paleontologists Lars Werdelin, Alan Turner, and Nikos Solounias wrote:

It should be noted, however, that our suggestion of adaptations towards a cursorial and active hunting mode of life for Chasmaporthetes does not mean that it did not scavenge, nor that it was necessarily in competition with extremely cursorial hunters such as Acinonyx [true cheetahs] and Miracinonyx. However, relative to other hyaenas[,] it has clearly evolved in that direction.

Although many researchers stressed that the cheek teeth of the several Chasmaporthetes species were better suited to shearing than crushing, this did not mean that the hyenas were incapable of cracking bone. After all, modern spotted hyenas are formidable hunters as well as accomplished bone-crackers, and a complete skull of the European species C. lunensis found in Spain exhibited patterns of tooth wear consistent with breaking open bones. Much like the modern spotted hyena, Chasmaporthetes was a hunter that could make full use of a carcass, as well as scavenge when the opportunity presented itself.

But paleontologists have been able to do more than propose hypotheses on the gross anatomy of the Chasmaporthetes bones alone. The rediscovery of the skull from Spain – which had been found in the 1970’s and studied by Dolores Soria for her doctoral thesis before fading from view until 2007 – finally provided scientists with an opportunity to see what kind of stresses and strains the hyena’s skull was capable of withstanding. Paleontologists have carried out these tests for a variety of bone-crunching mammals over the years, and so there was already plenty to compare the C. lunensis skull with.

Zhijie Tseng, Mauricio Antón, and Manuel Salesa published the results of their study in Paleobiology earlier this year. Like many other bone-cracking carnivorans, the skull of Chasmaporthetes exhibited a mosaic of features that gave it a powerful bite – a short snout, massive premolars, a large sagittal crest on the top of the skull for muscle attachment, deep lower jaws, and teeth modified at the microscopic level to resist fracturing. These traits are present to relatively lesser or greater degrees among carnivorous mammals adapted to crack bones, but the scientists proposed that the skull of Chasmaporthetes would have had suffered greater stress while breaking through bone than the skull of a modern spotted hyena.

The scientists turned out to be wrong. Based upon the computerized models created for the study, Tseng and colleagues concluded that the skull of “Chasmaporthetes was just as adapted for handling stress incurred during bone-cracking behavior as the modern Crocuta [spotted hyena].” Yet this does not necessarily mean that the extinct hyena hunted and fed in the exact same way that spotted hyenas do. Chasmaporthetes still had comparatively slender teeth better suited to cutting through fresh than breaking through bone, and so the authors of the paper suggest that the stress-absorbing features of the skull might be adaptations to withstanding forces generated by struggling prey. The way the hyenas caught prey has to be taken into account, and future studies that model stresses created by prey may help scientists identify skull characteristics related to hunting rather than fracturing bone. Chasmaporthetes certainly could have been a competent bone-cracker, but whether the anatomy of its skull can be attributed to this kind of behavior is another question.

Unfortunately, no one has yet found a complete skull from an American hyena. Perhaps some lucky paleontologist will, but, for now, the skull from Spain provides the best available information about the possible feeding habits of these “hunting hyenas.” I can only imagine a pack of Chasmaporthetes chasing down a prehistoric pronghorn through the grasslands – a scene that still echoes in Africa, but occurred during a distant part of North America’s prehistory.

Top Image: A spotted hyena carries the defleshed leg of a wildebeest. Image from Flickr user kibuyu.

References:

M. Antón, A. Turner, M. J. Salesa, J. Morales (2006). A complete skull of Chasmaporthetes lunensis (Carnivora, Hyaenidae) from the Spanish Pliocene site of La Puebla de Valverde (Teruel) Estudios Geológicos, 62 (1), 375-388

Barnosky, A., Matzke, N., Tomiya, S., Wogan, G., Swartz, B., Quental, T., Marshall, C., McGuire, J., Lindsey, E., Maguire, K., Mersey, B., & Ferrer, E. (2011). Has the Earth’s sixth mass extinction already arrived? Nature, 471 (7336), 51-57 DOI: 10.1038/nature09678

Berta, A. (1981). The Plio-Pleistocene hyaena Chasmaporthetes ossifragus from Florida Journal of Vertebrate Paleontology, 1 (3), 341-356 DOI: 10.1080/02724634.1981.10011905

Cooper, S., Holekamp, K., & Smale, L. (1999). A seasonal feast: long-term analysis of feeding behaviour in the spotted hyaena (Crocuta crocuta) African Journal of Ecology, 37 (2), 149-160 DOI: 10.1046/j.1365-2028.1999.00161.x

Ferretti, M. (2007). Evolution of bone-cracking adaptations in hyaenids (Mammalia, Carnivora) Swiss Journal of Geosciences, 100 (1), 41-52 DOI: 10.1007/s00015-007-1212-6

Hay, O.P. 1921. “Descriptions of species of Pleistocene vertebrata, types or specimens of most of which are preserved in the United States National Museum.” Proceedings of the National Museum. 59 (2391): 599-642

Kurten, B., & Werdelin, L. (1988). A review of the genus Chasmaporthetes Hay, 1921 (Carnivora, Hyaenidae) Journal of Vertebrate Paleontology, 8 (1), 46-66 DOI: 10.1080/02724634.1988.10011683

Tseng, Z., Antón, M., & Salesa, M. (2011). The evolution of the bone-cracking model in carnivorans: cranial functional morphology of the Plio-Pleistocene cursorial hyaenid Chasmaporthetes lunensis (Mammalia: Carnivora) Paleobiology, 37 (1), 140-156 DOI: 10.1666/09045.1

TURNER, A., ANTON, M., & WERDELIN, L. (2008). Taxonomy and evolutionary patterns in the fossil Hyaenidae of Europe Geobios, 41 (5), 677-687 DOI: 10.1016/j.geobios.2008.01.001

WERDELIN, L., TURNER, A., & SOLOUNIAS, N. (1994). Studies of fossil hyaenids: the genera Hyaenictis Gaudry and Chasmaporthetes Hay, with a reconsideration of the Hyaenidae of Langebaanweg, South Africa Zoological Journal of the Linnean Society, 111 (3), 197-217 DOI: 10.1111/j.1096-3642.1994.tb01483.x