The Caninae lineage, present from the early Oligocene, finally made its big move during the late Miocene, as the open grasslands continued to expand. One distinctive feature of the subfamily, which had slender, elongated limbs, is that the front and hind big toes became progressively smaller, and ceased to be functional. This cursorial feature, not found in the other two canid subfamilies, became an advantage when the landscape opened up. By the late Miocene, early precursors of the modern “true” foxes (tribe Vulpini) had emerged, as well as a genus, Eucyon, that was ancestral to the tribe Canini. The latter group comprises the “canines” in the narrow sense of the term, and includes dogs, wolves, coyotes, jackals, certain foxes, and other species.
A key development in Caninae history was the spread of the subfamily out of North America, beginning about 7 million years ago, when some groups crossed the Bering land bridge into Asia. With the exception of a single species in the middle Miocene of China, hesperocyonines never escaped the dog family cradle, nor did any borophagines. Records of the Caninae appeared in Europe first, and almost immediately thereafter in Asia and Africa. The first member of the genus Canis—to which the gray wolf, coyotes, jackals, and the domesticated dog belong—loped onto the scene about 6 million years ago.
During the subsequent epoch, the Pliocene (5.3 million to 1.8 million years ago), a further opportunity opened up for the Caninae. About 3 million years ago, the Panamanian Isthmus formed, linking North and South America. Carnivorans that arrived in South America generally trumped the native predators, and the Caninae were part of that success story, radiating explosively out of a few lineages in Central America and southern North America. Members of the subfamily constitute the largest group of carnivoran predators in South America today. Indeed, with eleven species, South America is home to almost one-third of the entire canid diversity on the planet.
Just as the intercontinental flux led to a new peak of diversity among the canids—one that continues through the Pleistocene epoch and down to the present time—so, too, did it influence the array of prey. Ancestral horse species, which had lost their two outer digits but retained three, were eclipsed in North America by single-digit horses. By Pliocene to early Pleistocene times, the modern horse genus, Equus, had spread to Eurasia and South America, along with members of the camel family (mostly llamas and their extinct relatives), which, like canids, had been confined to North America during much of their existence. While the Caninae subfamily thrived, however, borophagines dwindled to one or two species of highly specialized bone-cracking dogs, which became extinct by the end of the Pliocene.
The third canid expansion brought dogs into contact with hyaenids, which, with one brief exception during the Pliocene, had never expanded into North America. By the Pliocene, however, the competitive landscape had changed significantly for both families, and their members weren’t fighting for the same fare. The foxes and jackal-like dogs that arrived in the Old World were much smaller than most hyaenids, which by now were all large, bone-cracking animals.
If we look around today at the major terrestrial carnivoran families—canids, felids, ursids, mustelids, and others—we see that each has a balanced spectrum of small and large species, but not the hyaenids. Apart from the aardwolf, which is a highly specialized termite-eater, there are only three living species of hyenas, all large carnivores. In the major carnivoran families, if the large-size species become extinct in the future, smaller forms could evolve to replace them. But if the large hyenas one day become extinct, their great evolutionary lineage will end.