Werdelin & Solounias (1991) estimate that, at the peak of hyaenid diversity in the late Miocene, at least 24 different hyena species concurrently roamed Eurasia and Africa. Fortunately there is a fairly extensive fossil record of the Hyaenidae, and careful analysis of this record has revealed a great deal about the life-styles of extinct hyaenids, as well as about their distributions in space and time.
Although extant hyenas are rather dog-like in many aspects of their appearance, the family Hyaenidae actually belongs to the Carnivore suborder Feliformia, which also contains cats, mongooses, civets, and allies. Hyenas are thus far more closely related to cats and other Feliform taxa than to Caniform carnivores such as dogs, bears, or weasels. Fossil data suggest that members of the family Hyaenidae last shared a common ancestor with their Feliform sister taxon in the Oligocene, around 25 million years ago (MYA) (Werdelin & Solounias 1991), and recent molecular data suggest this divergence occurred even earlier, approximately 29 MYA (Koepfli et al. 2006). The molecular data further suggest that the sister group to the Hyaenidae is a Feliform clade containing the mongooses (family Herpestidae) and the fossa (genus Cryptoprocta), a small, civet-like Malagasy carnivore that was assigned to the family Viverridae until quite recently (Yoder et al. 2003). The history of the family Hyaenidae unfolded in Eurasia and Africa. Only one hyaenid (Chasmaporthetes) ever made it to North America, and no hyenas ever occurred in South America or Australia (Werdelin & Solounias 1991). Most of this evolutionary story unfolded during the Miocene epoch, which began 23 MYA, and spanned roughly 18 million years.
Although molecular data indicate that the divergence between the Hyaenidae and their sister clade (the group containing mongooses and the fossa) occurred 25-29 MYA, the first hyenas do not appear in the fossil record until several million years later. The hyena family was born in the warm, moist, forested habitat that covered most of Europe, Asia, and Africa during the Early Miocene. The earliest fossil hyaenaids are specimens found in France dating to c. 18 MYA (Werdelin & Solounias 1996). Fossil forms found in China are only slightly younger (Werdelin & Solounias 1991), suggesting that the hyaenids spread rapidly across Eurasia. However, they do not appear in the fossil record of Africa until roughly 14 MYA (Schmidt-Kittler 1987; Turner & Anton 2004). The hyaenids of the Early Miocene looked very little like the forms alive today, and none of them were capable of cracking large bones. Instead, the early hyaenids were generally small to medium-sized carnivores that resembled civets and mongooses in their body shape and life styles.
The early hyaenids clearly occupied radically different ecological niches than did later forms. They initially occupied omnivore and insectivore niches rather than meat- or bone-eating niches. Members of the genus Protictitherium were civet-like creatures with generalized dentition, suggesting they probably fed on small mammals, birds, insects and perhaps also some fruit (Werdelin & Solounias 1996). Their post-cranial skeleton was adapted for arboreal life, and in fact these animals even retained retractile claws that were lost during subsequent hyena evolution. Thus these forms were very similar in their ecomorphology to modern viverrids (Werdelin & Solounias 1991, 1996). The other early Hyaenid genus, Plioviverrops, included a group of small (approximately 2 kg) animals that resembled modern mongooses in body shape. Their teeth suggest that they relied mainly on insect food, although they also apparently consumed some of the same other food types as those eaten by Protictitherium. Plioviverrops had no retractile claws, and assumed a digitigrade stance, so these animals appear to have been terrestrial (Werdelin & Solounias 1996; Hunt 1996).
From their origins as civet- and mongoose-like creatures in Eurasia, the Hyaenidae diversified throughout the Middle Miocene. This first great radiation of the Hyaenidae produced a wide array of dog-like forms. Many of the hyaenids appearing during this first radiation were animals with body proportions, skulls and teeth much like those of modern canids (Werdelin & Solounias 1991, Werdelin 1996b; Hunt 1996), so the dog-like hyenas presumably occupied many of the same ecological niches as those occupied today by foxes, jackals, wolves, coyotes and hunting dogs. No species appeared in this first hyaenid radiation that were specialized for bone-cracking. Even though the cranio-dental characteristics of the aardwolf set it apart from other meat-eating forms, it is apparently the only surviving descendent of the once large clade of dog-like hyaenids. Koepfli et al. (2006) suggest that the aardwolf may have evolved from meat-eating ancestors to fill a termite-eating niche that was largely unexploited by other carnivorans during a period of evolutionary history when competition with other large carnivorans was probably extremely intense.The aardwolf is thus the only surviving representative of the once-large clade of “dog-like hyaenas.”
The Hyaenidae continued to diversify throughout the Late Miocene, achieving peak species diversity 11-7 MYA (Werdelin & Solounias 1991). A second group of dog-like hyenas appeared in the Late Miocene as the first great hyaenid radiation continued from the Middle Miocene. These new dog-like hyenas are referred to by many paleontologists as the “hunting hyenas.” The animals in this group were generally larger than the earlier dog-like forms, and their limb elements reveal greatly enhanced cursorial ability (Werdelin & Solounias 1991; 1996). The teeth of the hunting hyenas show markedly enhanced development of an extended dentition specialized for cutting and slicing, with a concomitant reduction in the bone-cracking portion of the dentition [leslie asks for an illustration]. Thus these animals were on a radically different evolutionary trajectory than the hyaenid forms that would eventually emphasize the bone-cracking portion of the dentition instead.
Hyaenids appeared in Africa for the first time in the Late Miocene, having immigrated from Eurasia (Janis 1993; Hunt 1996). Near the end of the Miocene, a wave of extinctions occurred among the Hyaenidae. Hardest-hit here were the civet- and dog-like hyenas.
The first bone-cracking hyena, Adcrocuta eximia, does not appear in the fossil record until the late Miocene. The skull of Adcrocuta bears a remarkably close resemblance to that of modern spotted, brown and striped hyenas. However, Adcrocuta had a very stocky build, with short, robust limbs, and it showed none of the cursorial adaptations seen in the extant bone-cracking hyenas (Werdelin & Solounias 1996), This species appears to have been highly successful as its fossil remains are found all over Eurasia and northern Africa (Werdelin & Solounias 1991). The appearance of Adcrocuta was followed by the second great hyena radiation, involving the appearance of several other hyaenid genera that were morphologically specialized for bone-cracking, including the lion-sized Pacyhcrocuta brevirostus. Weighing roughly 150 kg, Pacyhcrocuta was the largest hyaenid ever known. Because its muzzle was shorter than those in other hyenas, this species became commonly known as the ‘short-faced hyena.’ Most of the bone-cracking hyaenas became extinct before the Pleistocene.
The four extant genera of hyaenids apparently all originated in Africa, their ancestors having arrived earlier from Eurasia via the Gomphothere land bridge at what is now Saudi Arabia (Koepfli et al. 2006). The genera Hyaena, Parahyaena and Crocuta all first appear in the fossil record in the Late Pliocene, so these three genera are among the most recently evolved within the clade of bone-cracking hyaenids (Koepfli et al. 2006). Although brown hyenas (Parahyaena) are known from the Pleistocene in east Africa (Werdelin & Bartelone 1997), they occur only in southern Africa today. After originating in Africa and finding refuge there during the glacial periods of the Pleistocene, striped hyenas (Hyaena) dispersed out of Africa sometime within the last 130,000 years (Rohland et al. 2005). Today striped hyenas occur at low density in patches throughout eastern, western and northern Africa, the middle east and central Asia as far north as Georgia and as far east as Nepal (Mills & Hofer 1998). The aardwolf (Proteles) does not appear in the fossil record until the Pleistocene; these animals currently occur in two discrete populations, one in east Africa and one in southern Africa (Mills & Hofer 1998). The earliest members of the genus Crocuta first appear in the fossil record of Africa in the early Pliocene, dated at roughly 3.7 MYA (Lewis & Werdelin 2000). However, members of this genus soon dispersed out of Africa, and based on fossils from the period of its greatest range expansion in the Pleistocene, the genus Crocuta occupied virtually all of Europe and Asia, as well as all but the wettest jungles in sub-Saharan Africa (Werdelin & Solounias 1991).
In Europe, an early species in the genus Crocuta probably gave rise to C. spelaea, ‘the cave hyena’ (Lewis & Werdelin 2000). There is currently some debate in the literature regarding whether C. spelaea represents a single species or a complex of similar species. Fossil remains of these animals are widespread in Europe and Western Asia. Several taxa in the C. spelaea complex have been named in Europe, but the relationships among these are poorly understood (Lewis & Werdelin 2000). The C. spelaea complex became extinct toward the end of the Pleistocene. The same ancestral form (C. ultra) that gave rise to C. spelaea probably also gave rise to modern spotted hyaenas, although the extant form of Crocuta does not appear in the fossil record until very recently (only 900,000 to 200,000 years ago, Lewis & Werdelin 2000).
A comparison of the skulls of cave hyaenas (Crocuta spelaea) and modern spotted hyenas (Crocuta crocuta) using computer tomography in the laboratory of Dr. Doris Nagel can be seen at http://www.oeaw.ac.at/kfq/hyaenas/doris_oeaw.html. An entire master’s thesis on this subject can also be downloaded from this site.