An artistic reconstruction of the Gansu hyena, perhaps a carnivore on the way to becoming an insectivore, like today’s aardwolf. (Image credit: Mick Ellison)
Of the hundred or so known species of hyenas—living and extinct—that roamed the earth, all were carnivores or omnivores, with the exception of the aardwolf, which mysteriously eats termites.
What happened in the story of the fearsome hyenas that caused one group to abandon raw meat and turn to insects?
Two fossil skulls from a 12- to 15-million-year-old hyena that once lived in the Chinese province of Gansu may shed light on this mystery.
According to Jack Tseng, assistant professor of integrative biology at the University of California, Berkeley, and curator at the University of California Museum of Paleontology, the aardwolf now living in eastern and southern Africa, Proteles cristata, is an anomaly.
“He (the aardwolf) is actually quite a mysterious animal,” he said. “They are hyenas, but they really are the strangest hyenas because they don’t do what other hyenas do, either living or extinct. The aardwolves are termite specialists who couldn’t be further from the other hyenas in terms of their ability to crush bone or slice through flesh. So the aardwolf has always been a really curious mammal that ecologists and paleontologists alike have been trying to learn more about.”
An aardwolf in Namibia in 2005. Aardwolfs are termite-eating hyenas found only in Africa, mainly southern and eastern Africa. (Image credits: Dominik Kauferle, Creative Commons license)
A specialist in bone-crushing carnivores, Tseng has examined many fossilized hyena bones throughout his career, but none had the aardwolf’s unique characteristics. Hyenas arose about 22 million years ago, and the aardwolf apparently arose about 15 million years ago based on an analysis of their genetic divergence from the three other living hyena species. But the only recognizable aardwolf fossils are at most 4 million years old.
Hence his interest in the new fossils from China, which fill in a long “ghost lineage” — a living species with little fossil evidence linking it to its ancestor.
The skulls – one of which is almost complete, including the front part of the lower jaw – were smuggled into the US but luckily acquired by collector and paleontologist Henry Galiano in 2002. In 2013, after Tseng confirmed arrangements to return the skulls to China, she agreed to join a small group of other paleontologists, including experts from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing, to analyze them.
Fossils halfway between hyenas and aardwolves
Tseng, then a postdoctoral fellow at the American Museum of Natural History in New York, has been working on the fossil skulls intermittently for the past eight years. What immediately struck him were characteristics that seemed halfway between those of bone-crushing hyenas and those of the aardwolf. In particular, the skulls had a broad palate, like the aardwolf, possibly to accommodate a larger, more muscular tongue that could be used to slurp termites. The teeth were more widely spaced, also like the aardwolf, suggesting the Gansu hyena was moving away from a meat-carving diet.
The fossilized upper jaw of the Gansu hyena, seen from above (left) and below. The palate is broad and resembles that of the aardwolf, which has a more muscular tongue for slurping termites. (Image credit: Mick Ellison)
Additionally, its middle ears had spacious, dome-shaped chambers, or bullae, found in animals like some desert rodents and aardwolves, which evolved to increase auditory sensitivity — perhaps to detect the buzzing of a termite colony.
“Relative to skull size, the bullae in the Gansu hyena are very enlarged compared to modern hyenas,” he said.
Similar to the extant aardwolf, the fossil also has larger eye sockets than other hyenas for its size.
While the research team does not claim that the Gansu hyena, which it calls Gansuyaena megalotis, was a direct ancestor of the aardwolf, it concludes that the hyena is the closest living aardwolf to date, and shows signs of evolving from have separated a meat-and-bones lifestyle into something closer to insect ivory. The age of the fossil also fits well with the molecular clock estimate of the aardwolf’s origin at around 15 million years ago.
“With these fossils, we can really get to the bottom of the question: ‘How does an otherwise very specialized meat-eating lineage have a member, an odd cousin, who took this completely different path, becoming a specialized insectivore, a termite specialist?’ he said. “Now we have the start and end point, which is today. The next step is to find out what happened in the past 10 million years of this line.”
Gray market fossils
Tseng pointed to the difficulty of properly analyzing fossils excavated by farmers that lack the local context typically recorded by paleontologists — particularly the geological context that is critical to attribution of an age. The age and place of origin of the Gansu hyena had to be inferred from the red rock still clinging to its fossilized bone.
Drawing of the skull of the Gansu hyena. Note the rounded and enlarged ear chamber to the right of the temporomandibular joint, indicating similar hearing ability to live aardwolf. (Image credit: Mick Ellison, 2020)
While many paleontologists – including Tseng in most cases – refuse to work with such “grey market” specimens, Tseng was willing to work with Galiano because they had the same goal: returning the fossils.
“I felt that we need to bring this back to its country of origin and bring it back into the scientific eye,” he said in collaboration with Chinese scientists. “My co-author, Zhan-Xiang Qiu from the Chinese Academy of Sciences, and I have agreed to collaborate on this project, on the understanding that we will first examine and publish the specimens, and then physically ship them back to China to be stored in their vertebrate paleontology collections to be kept safe.”
Tseng said Chinese fossils have a long history as “dragon bones” used in traditional medicine, and people in Gansu province in particular have long been digging in their fields to extract and sell fossils. The market shift from Chinese medicine practitioners to fossil collectors could lead to more commercial excavations, which can greatly reduce the scientific value of the fossils, he said.
“Paleontologists walk a fine line when they work with commercial collectors to place the fossils in a public trust such as a research institute or museum, and in this case some scholarly journals declined publication because the fossils were from the gray market “, he said.
The group eventually published in the bilingual journal Vertebrata PalAsiatica, sponsored by the IVPP in Beijing. The newspaper is now online, but publication of the print edition has been delayed by a coronavirus outbreak in Beijing.
An aardwolf. (Image credit: Greg Hume, Creative Commons License)
“There’s still a lot of debate about working with fossils by the gray market or by amateurs in the paleontology community, particularly among vertebrate paleontologists, because vertebrate fossils are rare compared to invertebrates or plants,” Tseng added. “Scientists have sometimes turned away collectors who have valuable specimens that they are willing to donate to museums because of the loss of scientific context and the attitude that when you work with people who don’t have an academic background, that’s kind of what it’s like Science.”
The publication is co-authored by Tseng, Galiano, Qiu, Nikos Solounias of the New York Institute of Technology, Xiaoming Wang of the Natural History Museum of Los Angeles County, and Stuart White of the UCLA School of Dentistry. The work was supported by the National Science Foundation and Galiano’s companies Dinosauria International LLC of Wyoming and Maxilla and Mandible Ltd. funded from New York.
