When Giant Hypercarnivores Prowled Africa

We would like to thank Raycon wireless headphones for supporting PBS. In 2013, a researcher working on the vast collections of the National Museums of Kenya made a surprising discovery. Hidden in a cabinet marked "hyenas," he noticed a large fossilized lower jaw of some type of carnivore. And it was big, much bigger than the jaw of a lion, the largest carnivore in Africa today. Six years later, it was revealed to the public as a huge beast completely new to science. It was called Simbakubwa kutokaafrika and although its name literally means “great lion of Africa” in Swahili, this creature was not a big cat.

And although it was found in a drawer labeled “hyenas,” it wasn't a hyena either. It was a hyenodont, an extinct family of carnivorous mammals that lived from the Paleocene to the Miocene, and inhabited Africa, Europe, Asia and North America. But Simbakubwa was not only unique for its large size. The jaw fossil was excavated in the late 1970s at a site called Meswa Bridge in western Kenya that dates back to between 26 and 23 million years ago. It was from this jaw that researchers were able to estimate a range of potential body sizes for Simbakubwa, based on molar size and comparisons with living carnivores.

And it seems that, in its smallest form, Simbakubwa was probably the size of a large lion, and in its largest form, it might have been larger than a polar bear. And he was hypercarnivorous, meaning: he got more than 70% of his calories from meat! Because of its age and monstrous size, Simbakubwa is the oldest known

giant

member of its hyaenodont subfamily. These hyaenodonts gave the world some of the largest carnivorous land mammals ever known. And while these

giant

s were the top predators of their time, they no longer exist. It turns out that becoming the biggest, baddest beast on the landscape can have serious consequences

when

that landscape suddenly changes.

Hyenodonts are members of an extinct order of carnivorous mammals called creodonts. And although they were supercarnivores, creodonts evolved independently of the order Carnivora, which includes all the modern felids and canids we have today, among other creatures. Creodonts are older and more primitive than carnivores, and they also looked different. Their skulls were low with small skulls and their limbs were generally short and heavy. Early in their evolutionary history, about 65 million years ago, creodonts branched into two lineages, one of which was the hyenadons. But the exact place where they emerged remains 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 Hyainailourinae. This particular group probably originated in the region known as Afro-Arabia, which includes the entire Arabian Peninsula and North Africa, about 48 million years ago. From there, they spread to Europe, Asia, and North America, where their large size and powerful bite ensured their position at the top of the food chain. But by the middle Oligocene, about 30 million years ago, they appear to have become extinct on the northern continents, perhaps 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 recently evolved carnivores in Eurasia by a sea route. And this isolation could have contributed to the rise of giant hyenodonts, like Simbakubwa. Because it was not until the early Miocene, about 23 million years ago, that evidence of the largest members of this group appeared, approaching the size of a modern rhinoceros. What we know about Simbakubwa and its close relatives comes from a very fragmentary fossil record, consisting mostly of dental remains and some bones from the rest of the skeleton. This limited fossil record has made it difficult to determine what their family tree is, or even their own! -- 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. These are modified premolars and molars with self-sharpening edges that cross each other with a cutting movement, as if they were scissors. Now, living carnivores normally have only one set of these scissor-shaped teeth... but Simbakubwa and his relatives had three, turning their jaws into meat-cutting machines. But understanding the rest of its anatomy is somewhat difficult. To sink those teeth into all that meat, they first had to catch it. That's why researchers are really interested in finding out how these giant predators moved.

And the only clue we have about Simbakubwa's locomotion is quite intriguing. Many modern carnivores are digitigrade walkers. This means they walk 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 most common in open grassland environments, where this posture helps them conserve energy and increases their stealth, speed, and hunting success. And Simbakubwa's well-preserved heel bone, called the calcaneus, was similar to that of one of his relatives, which has been reconstructed as semi-digigrade: not completely up to the toes, but not completely flat either.

Researchers also believe that this relative was capable of powerful leaps. So based on the similarities between the two heel bones, it is possible that Simbakubwa was also semi-digigrade, making him a faster and more agile hunter. All of these advantages—a large body, three sets of teeth for cutting meat, and the ability to move quickly—probably led to the success and survival of hyenodonts during the Miocene epoch. They probably hunted really large herbivores, like relatives of modern elephants and rhinos, filling a niche that remains vacant today. But if they were such powerful beasts that were so well adapted to their ancient landscapes, why don't they still exist?

Well, hyenodonts disappeared in the late Miocene, about 5 million years ago, thanks to an even more powerful force... a rapidly changing environment. This period of change began about 23 million years ago,

when

the Arabian Peninsula moved closer to Eurasia, closing the sea lane that had kept Simbakubwa and its relatives separate 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 immigrants were hyaenodonts from Afro-Arabia heading north and carnivores from Eurasia arriving in Africa. This exchange of animal populations occurred at a time when the Earth was undergoing major changes, such as the formation of the East African Rift System, where two parts of the African tectonic plate began to move away from each other.

These changes caused dramatic transformations in the Afro-Arabian landscape, with forests becoming drier and converting to more open habitats. All of these changes spelled disaster for hyaenodonts, but researchers are still trying to figure out exactly how. There are two current hypotheses for the extinction of the Simbakubwa group: changes in food webs, as environments changed and large herbivores disappeared, or competition from modern carnivores. The first suggests that when the environment began to change, it was bad news for the large herbivores that the hyaenodonts depended on. Large herbivores tend to reproduce slowly, which can lead to a rapid decline in their numbers if ecosystems change too quickly, because they cannot adapt quickly enough.

Fewer large herbivores meant less food for large hyaenodonts, which were not adapted to hunting smaller prey. This would have made hyenodonts more vulnerable to extinction than smaller carnivores, which had a broader diet. The second hypothesis involves competition from members of the order Carnivora when they moved from Eurasia to Afro-Arabia about 23 million years ago. Until that time, hyenodonts had been dominating predatory niches in Afro-Arabia. Fossil evidence suggests that the first Afro-Arab carnivores were fairly small mesocarnivores, animals that got at least half their calories from meat and the rest from other things (think raccoons and coyotes). And they are generally less affected by environmental changes than large

hypercarnivores

.

So, as the new carnivores dispersed into Africa from Eurasia, they became more diverse and began to compete with hyaenodonts. Additionally, carnivores with complex cooperative behaviors, such as living in packs, may have been able to steal prey from solitary hyaenodonts. Between the changing environment, lack of food, and the introduction of intelligent and cunning competition, Simbakubwa and his relatives might simply have met their match, after triumphing across three continents for at least 15 million years. The evolution and extinction of these hypercarnivores offers rare insights into an era that saw massive changes in climate, oceans, tectonic activity, and the dynamics of many ecosystems.

And as more fossils are found, we will be able to recreate and better understand the African landscape they lived in, which is also the landscape where our early ancestors evolved. Ultimately, Africa's giant hyaenodonts 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 earbuds that come with Bluetooth pairing, a portable charging pod, and 6 hours of playtime. Instead of traditional single-channel layout, 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, which is why Raycon has 6 different sized gel tips designed to fit any size, with a noise-isolating fit and no wires or stem. The earbuds are also sweat-resistant if working out is your thing. For more information, visit buyraycon.com/eons. We hope you enjoyed this episode of Keeping Up with the Carnassials. But I must always thank this month's eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng, and Steve! 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, subscribe at youtube.com/eons.