Bast, Sekhmet, and the Egyptian Hyena-cats

Megistotherium, a Miocene hyena-cat

The majority of the land-based mammalian predators we are familiar with today belong to the order Carnivora. This is a diverse order, including such mammal families as the cats, bears, dogs, weasels, and seals. In fact, if we ignore the cetaceans and a few kinds of marsupial, they are the only large carnivorous mammals alive. But, as so often, this was not always the case, and they once shared the world with at least three other orders of predatory placental mammal (plus some marsupials far more fearsome than any Tasmanian devil). 

Two of these orders died out relatively early on, but one of them survived for much longer, producing multiple diverse species that lived across Eurasia, North America, and Africa. These were the hyaenodonts, named for Hyaenodon itself, first identified from a fossil all the way back in 1838. With so many species, they must have been successful in their day, but their numbers declined until the last two species died out in Africa and India around 9 million years ago, perhaps due to competition from the carnivorans, perhaps due to long-term climate change. Or, more likely, both.

Although hyaenodonts are thought to have first appeared in Asia, Africa is key to understanding much of their evolutionary history, not least because they lived there for so long. The first African hyaenodont fossils were described in the 1900s, and were assigned to the previously known European genera Pterodon and Apterodon. This made sense at the time, given the relatively fragmentary nature of the fossils and the fact they had been discovered in Egypt which is quite close to Europe.

Or, at least, it is now. The 1900s were before Alfred Wegener proposed the modern theory of continental drift, let alone before anyone had come up with a plausible mechanism and been able to prove him right. We now know that, for most of the hyaenodonts' time in Africa, the continent was an island, separated from Eurasia by the wide Tethys Sea (now the Mediterranean Sea/Persian Gulf, Arabia being on the African side). This at least raises questions as to how closely related the European and African forms really are and in which direction they may have moved.

But then, there is still no clear agreement as to how the different subgroups of hyaenodont should be named. The hyenodontid family was first named in 1869, and promoted to full order level almost a hundred years later. In 1996, it was divided into two subfamilies, one including Hyaenodon itself, and restricted to North America and Eurasia and another that includes Pterodon, and therefore the African species (among others). 

The name originally suggested for this second subfamily has since changed, and many consider it to be a full family in its own right, separating it off from the hyaenodontids proper as the Hyainailouridae. The name translates as "hyena-cat family", although, just as with Hyaenodon, there is no real connection with hyenas (or cats) beyond some similarity in the shape of the teeth.  Regardless of what rank we happen to give it, it's significant in that it suggests the hyaenodonts more widely developed their highly carnivorous habits twice, the hyainailourids doing so separately from their kin.

However, even this may be overly simplistic, with the hyena-cats possibly representing more than one lineage - depending, of course, on exactly how you define them. Although we do know of examples from Europe, North America, and India, the great majority of known species assigned to the group are  North African. Most of these, however, are known from little more than a few teeth, which makes it difficult to get a detailed picture of how they related to one another or, perhaps more significantly, what they were really like.

Recently, however, an unusually complete fossil cranium from a hyena-cat has been described from the Fayum Depression, a major fossil site in northern Egypt. From the teeth, the researchers were able to conclude that it belonged to the same species as a fragmentary specimen described in the 1990s, and placed, for lack of anywhere better, in Pterodon. Now that we can see more of it, that has changed, and the creature has been given the new name of Bastetodon - "Bastet's tooth" from the Egyptian goddess of cats. 

The deposits in which the skull was found have been dated to approximately 30 million years ago, during the early Oligocene. African hyaenodonts, along with the local primates and rodents, suffered a major die-off during the Oi-1 glaciation event at the dawn of the Oligocene, then a further drop off in diversity that was bottoming out by about the time the fossil was deposited. This second die-off is too early to have been caused by the Oi-2 glaciation, which is the only other major burst of worldwide climate change in the epoch, but does correspond with a series of huge volcanic eruptions in northeastern Africa that helped raise the Ethopian Plateau and are connected with the creation of the Red Sea; while this had little effect worldwide, it's hard to imagine that it didn't have a major effect on Africa through pumping sulphur into the atmosphere and changing the arrangement of the Nile's tributaries.

The skull is about 19 cm (7½") long. Making some assumptions about bodily proportions we can estimate that the animal it belonged to weighed somewhere around 27 kg (60 lbs), comparable to a smallish leopard or a striped hyena. The teeth were sharp and adapted for shearing meat, indicating that Bastetodon was a purely carnivorous predator with a powerful bite. Moreover, it had reduced the number of its teeth during evolution, losing both the first premolar and the last molar. Although the exact details are different, this is the same pattern we see in cats, where a smaller number of teeth allows for a shorter snout and a stronger, flesh-slicing bite. The parts of the skull that would anchor the jaw muscles are also enlarged and strengthened, reinforcing the suggestion of a highly predatory lifestyle.

Comparison with other known hyena-cats identifies the closest known relative as Falcatodon, which lived in the same area a couple of million years earlier and has the same dental adaptations. Beyond that, its close relatives include Hyainailouros itself, for which the group is named, and the powerfully built lion-sized Simbakubwa, one of the largest African predators of the early Miocene.

The skull also allows us to take another look at the genus Pterodon more widely. This was formerly considered to have four species, one of which is that now renamed as Bastetodon. Of the other three species, two were African, but the other was European, although all of them did at least live in roughly the same time period, a few million years before Bastetodon at the end of the Eocene epoch. The new study interprets the two African species as very close relatives of each other, but not at all of the European species. Thus, the latter, having been discovered first, is now the only one to keep its original name. 

The African species were both larger than Bastetodon, but slightly less adapted to a purely carnivorous diet. Their new name is Sekhmetops, translating as "Sekhmet-face" from Bastet's more fearsome counterpart, the Egyptian lion-faced goddess of vengeance. 

All of this makes it likely that the hyena-cats, as a group within the hyaenodonts more widely, originated in Africa, dispersing at least three times to the north, creating the known European, Asian, and North American species in the process. Their ancestors had probably been in Africa since shortly after the extinction of the non-avian dinosaurs, placing them about as far back as we can go in the Age of Mammals.

[Picture by "ДиБгд" from Wikimedia Commons.]