When Giant Hypercarnivores Prowled Africa

When Giant Hypercarnivores Prowled Africa

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We'd like to thank Raycon wireless earphones for supporting PBS. In 2013, a researcher working in the vast collections of the National Museums of Kenya made a surprising discovery. Tucked away in a cabinet marked “hyenas,” he noticed a large fossilized lower jawbone from some kind of carnivore. And it was big -- much bigger than the jawbone of a lion, the largest carnivore in Africa today. Six years later, it was revealed to the public as an enormous beast that was entirely new to science. It was named Simbakubwa kutokaafrika And while its name literally means ‘big lion from Africa’ in Swahili, this creature was not a big cat. And even though it was found in a drawer labelled “hyenas”, it was not a hyena, either. It was a hyenodont, an extinct family of carnivorous mammals that lived from the Paleocene to the Miocene Epoch, and inhabited Africa, Europe, Asia, and North America. But Simbakubwa was not only unique because of its large size. The fossil jawbone was excavated in the late 1970s at a site called Meswa Bridge in western
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Kenya that dates back about 26 million to 23 million years ago. It was from this jawbone that researchers were able to estimate a range of potential body sizes for Simbakubwa, based on the size of the molars and comparisons to living carnivores. And it looks like, at its smallest, Simbakubwa was probably the size of a large lion, and at its largest, it could’ve been larger than a polar bear. And it was a hypercarnivore - meaning: it got more than 70% of its calories from meat! Based on its age and its monstrous size, Simbakubwa is the oldest known giant member of its subfamily of hyaenodonts. These hyaenodonts gave the world some of its largest terrestrial, carnivorous mammals ever known. And while these behemoths were the apex predators of their time, they’re not around anymore. It turns out that becoming the biggest, baddest beast on the landscape can have serious consequences, when that landscape suddenly changes. Hyaenodonts are members of an extinct order of carnivorous mammals called the creodonts. And even though they were super-carnivorous, creodonts evolved independently from the order
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Carnivora, which includes all the modern felids and canids we have today, among other critters. Creodonts are older and more primitive than Carnivora, and they looked different, too. Their skulls were low with small braincases, and their limbs were generally short and heavy. Early on in their evolutionary history, about 65 million years ago, the creodonts branched into two lineages, one of which was the hyenadonts. But the exact place they arose is still kind of a mystery. Some experts think they originated in Africa; others say Asia, and some suggest both, with a distinct group of hyaenodonts evolving on each of the two continents. But the fossil and phylogenetic evidence we have seems to suggest that they arose somewhere in Eurasia. And Simbakubwa was part of a subfamily of hyaenodonts called the Hyainailourinae This particular group probably originated in the region known as Afro-Arabia, which includes the whole Arabian Peninsula and northern Africa, around 48 million years ago. From there, they spread into Europe, Asia, and North America, where their large size
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and powerful bite ensured their position at the top of the food chain. But by the middle Oligocene, roughly 30 million years ago, it looks like they went extinct on the northern continents, maybe due to competition from other groups of carnivores that evolved during that time. But in Afro-Arabia, they persisted, evolving in isolation, separated from the newly-evolved carnivores in Eurasia by a seaway. And this isolation could have contributed to the rise of giant hyaenodonts, like Simbakubwa. Because, it wasn’t until the early Miocene, around 23 million years ago, that evidence of the largest members of this group appeared, approaching the size of a modern-day rhinoceros. What we know about Simbakubwa and its close relatives comes from a very fragmentary fossil record, which is mostly made up of dental remains and a few bones from the rest of the skeleton. This limited fossil record has made it hard to figure out what their family tree -- or even they themselves! -- looked like. What we do know from their teeth is that these hypercarnivores had to be able to process a lot of meat, which they did with specialized teeth called carnassials.
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These are modified premolars and molars with self-sharpening edges that pass by each other in a shearing motion - like a pair of scissors. Now, living carnivores usually have only one set of these scissor-like teeth...but Simbakubwa and its relatives had three, turning their jaws into meat-slicing machines. But making sense of the rest of their anatomy is kind of hard. In order to sink those teeth into all that meat, they had to catch it first. So researchers are really interested in figuring out how these giant predators moved around. And the one clue we have about Simbakubwa’s locomotion is pretty intriguing. Many modern carnivores are digitigrade walkers. This means that they walk up on their toes with their heels in the air, making them faster than plantigrade walkers like us, who walk with the soles of our feet flat on the ground. Digitigrade carnivores are more common in open grassland environments, where that posture helps them save energy, and increases their stealth, speed, and hunting success. And Simbakubwa’s well preserved heel bone -- called the calcaneum -- was similar to
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that of one of its relatives, which has been reconstructed as semidigitigrade - not all the way up on its toes, but not totally flat-footed, either. Researchers also think that relative was capable of making powerful leaps. So, based on similarities between the two heel bones, it’s possible that Simbakubwa was also semi-digitigrade, making it a faster, more agile hunter. All of these advantages -- a large body, three sets of meat slicing teeth, and the ability to get around quickly -- probably led to the success and survival of hyaenodonts through the Miocene Epoch. They likely hunted really large herbivores, like the relatives of modern elephants and rhinos, occupying a niche that remains vacant today. But, if they were such powerful beasts that were so well-adapted to their ancient landscapes, then why aren’t they still around? Well, the hyaenodonts met their demise by the end of the Miocene, around 5 million years ago, thanks to an even more powerful force… a rapidly changing environment. This period of change started back around 23 million years ago, as the Arabian Peninsula
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approached Eurasia, closing the seaway that had kept Simbakubwa and its relatives separated from the carnivores of Eurasia. From then on, waves of animals were able to migrate back and forth between the two regions. And among these migrants were hyaenodonts from Afro-Arabia heading north, and carnivorans from Eurasia coming into Africa. This exchange of animal populations happened at a time when Earth was undergoing major changes, like the formation of East Africa Rift System, where two parts of the African tectonic plate started to move away from each other. These changes led to dramatic transformations in the Afro-Arabian landscape, with forests becoming drier and turning into more open habitats. All of these changes spelled disaster for the hyaenodonts, but researchers are still trying to figure out exactly how. There are two current hypotheses for the extinction of Simbakubwa’s group: changing food webs, as environments changed and large herbivores disappeared, or competition from modern Carnivorans. The first one suggests that when the environment started to change, it was bad
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news for the large herbivores that hyaenodonts relied on. Large herbivores tend to reproduce slowly, which can lead to a rapid decline in numbers if ecosystems change too quickly, because they can’t adapt fast enough. Fewer large herbivores meant less food for the large hyaenodonts, which weren’t adapted to hunting smaller prey. This would’ve made hyaenodonts more vulnerable to extinction than the smaller carnivorans, which had a broader diet. The second hypothesis involves competition from members of the order Carnivora as they moved from Eurasia into Afro-Arabia around 23 million years ago. Up to that point, hyaenodonts had been dominating the predatory niches in Afro-Arabia. Fossil evidence suggests that the earliest Afro-Arabian carnivorans were pretty small mesocarnivores, animals that get at least half of their calories from meat and the rest from other things - think raccoons and coyotes. And they’re usually less dramatically affected by environmental changes than large hypercarnivores are. So, as new Carnivorans dispersed to Africa from Eurasia, they became more diverse and
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started to compete with the hyaenodonts. Plus, Carnivorans with complex cooperative behaviours, like living in packs, may have been able to steal prey from lone hyaenodonts. Between the changing environment, lack of food, and the introduction of smart and sneaky competition, Simbakubwa and its relatives might have simply met their match, after succeeding on three continents for at least 15 million years. The evolution and extinction of these hypercarnivores offers rare insights into a time that saw massive changes in the climate, oceans, tectonic activity and the dynamics of many ecosystems. And as more fossils are found, we may be able to recreate and better understand the African landscape in which they lived -- which is also the landscape where our early ancestors evolved. Ultimately, the giant hyaenodonts of Africa serve as a stark reminder that even the most powerful beasts are no match for a changing world. Thanks to Raycon for supporting PBS Digital Studios. Raycon makes wireless earphones that come with Bluetooth pairing, a portable charging
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capsule, and 6 hours of playtime. Instead of traditional single-channel distribution, Raycon uses True Wireless Audio which uses independent frequencies for the left and right channels. Raycon's latest E25 model comes in six different color options. Every ear is different, so Raycon has 6 differently sized gel tips designed to fit any size, with a noise-isolating fit and no wires or stem. The earphones are sweat-resistant too if working out is your thing. For more information go to buyraycon.com/eons. We hope you enjoyed this episode of Keeping Up with the Carnassials. But I've always gotta give a shout out to this month’s Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng, and Steve! Go pledge your support at patreon.com/eons and become an Eonite! And also thanks for joining me in the Konstantin Haase Studio. If you like what we do here, then subscribe at youtube.com/eons.

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