"Hyaenid" is the proper term for any member of the hyena family (the Hyaenidae), including the aardwolf.
Only four species of hyaenids still live today, but paleontologists know of at least 69 species from fossils -- not all of them even named yet. I cannot hope to describe them all here, so this is kind of a rough overview.
With their heavy jaws and sloping backs, modern hyenas may look vaguely "primitive," but in fact the first hyaenid that scientists can recognize as such dates back only between 18 and 17 million years ago -- the early Miocene Epoch. By that time, horses already had three toes instead of five and cats were recognizable, if only barely.
Of course, the ancestors of hyaenids must have separated from those of other carnivores millions of years before. Maybe they looked -- and lived -- much like civets such as this one, living in trees much of the time and eating almost anything, including fruit and insects. Most of the world was then covered with tropical or subtropical forests, with only a few areas of grassland.
At any rate, that first known hyena is Protictitherium gaillardi, which still basically looked like a civet. It could retract its claws like a cat and spent much if not most of its time in the trees. Its diet may have been birds, small mammals and even insects.
There were several species of Protictitherium after P. gaillardi. Even though they were quite primitive, they managed to survive in the trees above their more advanced kin for millions of years, at last dying out between 5.3 and 4.2 million years ago.
However, other hyaenids soon spread into Europe and began spending more time on the ground. The first known European hyaenid was Plioviverrops orbignyi, a small critter that has been called a "mongoose-like insectivore/omnivore." Plioviverrops was specialized for eating insects, rather than the wider diet of Protictitherium. Its claws couldn't retract, and it probably spent most of its time on the ground rather than in trees, like these dwarf mongooses. It lived from 17 million to 5.3 million years ago, -- basically all through the Miocene.
Shortly after the Plioviverrops genus evolved, the other hyaenids began getting bigger (along with changes in their teeth), as the warm forests and woods of the the early Miocene opened up into grasslands and parklands with definite seasons. Around 10 million years ago, some hyaenids evolved into "running hyenas" who were better adapted to running down their prey like wolves than for crushing bones and scavenging; others became more like small wolves or jackals. Finally, between 6 and 7 million years ago, some became fairly slow, bone-crushing animals (although the [spotted hyena] can run reasonably fast).
- One of the best-known jackal-like hyaenids is Ictitherium viverrinum. Its first remains in Europe turn up in Central Europe from 11 million years ago. It ranged over Western Europe until approximately 7 million years ago, when it began to vanish. It survived in southeastern Europe and southwestern Asia for another million years before it became totally extinct. It probably looked like a jackal, and not much like today's spotted hyena.
- Several genera of hyenas -- Belbus, Palinhyaena,
Ikelohyaena and Leecyaena --
started on the journey into the bone-crushing hyenas (the group to which the striped, [spotted] and [brown hyenas] belong). Their teeth
got larger and heavier for this job, but we know very little about them, because as yet we have only fragmentary skulls, jaws and teeth from them.
One of the most extreme bone-crushers that ever evolved was Adcrocuta eximia which lived in Greece, Austria and Spain a bit more than 9 million years ago. With its thick, short legs, it was not a good runner -- but it was a good bone-crusher and may have thus lived mainly by scavenging, even more so than any modern hyena.
- Three hyena genera took up running down prey for a living: Chasmaporthetes, Lycyaena and Hyaenictis. They had a long reign as important predators. Chasmaporthetes crossed the Bering land bridge during the Pliocene to enter North America, where it evolved a new species, Chasmaporthetes ossifragus -- North America's only native hyaenid. It did not die out until the first Ice Ages began, in the Pleistocene -- perhaps a victim of the climate changes that also triggered the growth of the great ice sheets.
By the end of the Miocene, hyaenids apparently ran into problems. The true canids (the ancestors of wolves, jackals, foxes and such) had entered Europe and were beginning to diversify, too, and perhaps the hyenas could not compete with them. The entire family almost died out in Europe, except for a few running hyenas like Chasmaporthetes and a giant version of the spotted hyena, called the "cave hyena".
Two known genera of extinct hyenas explored a whole new ecological niche -- Tongxinictes and Tungurictis. They may have been evolving to become like modern civets, such as the African civet shown here. Their teeth lost all ability to crush bone. But apparently this unique direction didn't work out, because they seem to have been rare and soon became extinct.
The [aardwolf], a termite-eating hyaenid, was once thought to have split off from other hyaenids very early, because its weak jaws and feeble teeth are so different. But there are no hyaenid fossils that seem related. A study in 2006 of its genetics showed that its ancestors may have split off from other hyaenids 10.6 million years ago. Oddly, its coat is colored just like the striped hyena's. This may mean the coat coloring is at least that old and was shared by other hyaenids of that time.
By the time the climate had truly warmed, perhaps 10,000 years ago, the only surviving hyaenids were the four species living today, and only the striped hyena still exists outside of Africa. Even the giant cave hyena of Europe became extinct. The hyaenid family appears to be in a state of irreversible decline, though the surviving species may well persist and evolve for many millions of years longer if humans do not exterminate them.
The Evolution of Western Eurasian Neogene Mammal Faunas, edited by Raymond L. Bernor, Volker Fahlbusch and Hans-Walter Mittman. Columbia University Press, New York. 1996. The chapter on hyaenid evolution was the chief source used for the information in this article.