While we often tend to think that prehistoric animals tended to be larger than those alive today, this, was however, rather less true of whales, which have grown more or less steadily in size over the course of their evolution, perhaps in part to make it increasingly difficult for anything else to eat them. So, for example, the Pliocene killer whale (Orcinus citoniensis) was around 4 metres (12 feet) in length, barely more than half that of the modern species - although still quite impressive on a human scale.
This species was first identified in the 1980s from a fossil site in Tuscany, although, given that killer whales today live just about anywhere that there's salt water, the Pliocene ones were probably quite widespread, too. Apart from their smaller size, some of what we know of their anatomy resembles that of the much smaller dolphins from which killer whales evolved, and there's a pretty good chance that this might be the direct ancestor of the modern animal.
The same Tuscan deposits have also revealed several specimens of another, similarly sized, dolphin, Hemisyntrachelus. Evidently rather common in the Pliocene Mediterranean, closely related species have also been found elsewhere since, so we know that they reached at least as far north as the Dutch coast, and that an animal almost (but not quite) indistinguishable from them lived off the coast of Peru. In appearance, they were similar to, but larger than, modern bottlenose dolphins, as well as having some similarities with killer whales. The latter, in this case, is probably a case of parallel evolution, a feature of them being particularly large dolphins and adopting a generalist diet similar to that of the modern false killer whale.
One of the more bizarre dolphin fossils so far found was recovered from the Vestfold Hills in Antarctica. One of many cetacean fossil species discovered at the site, Australodelphis mirus is the only one to be formally described so far, and is, indeed, the only named cetacean fossil from Antarctic coasts at all. It was a fairly small dolphin, but what was so strange about it was that it had no teeth in its upper jaw, and a remarkably long snout. In another case of parallel evolution, in many respects, it resembled the relatively mysterious modern beaked whales more than it did its fellow dolphins, and the best bet is that it fed by sucking up moderately sized invertebrates, such as squid.
A similarly weird-looking small cetacean lived off the coast of southern California during the Pliocene, although this one is thought to be a large porpoise, rather than a dolphin. Its lower jaw was twice the length of its upper one, extending into an 85 cm (2' 6") toothless probe that stretched well beyond the snout. Semirostrum probably fed by using this projecting jaw to poke about in the mud and scoop up bottom-dwelling animals.
Outside of the dolphin and porpoise families, a number of other whales are known from the Pliocene. Bohaskaia, discovered as recently as 2012, was related to modern narwhals and belugas, both of which are Arctic species. That it lived off the coasts of Virginia and North Carolina may suggest that the modern species are descended from some that once lived further south, in warmer waters. Relatives of the great filter-feeding baleen whales of today were generally much smaller in the Pliocene, with many, such as Balaenula, a close relative of the 16-metre right whale, being only around 5 metres (16 feet) in length.
Somewhat larger were primitive rorquals, which reached around 8 metres (26 feet), although this is still much smaller than their modern relatives, the blue and humpback whales. Many of these, such as Diunatans, from the North Sea, had straight jaws, unlike the curved forms seen in modern rorquals. As a result, they probably couldn't expand their throats in the same way, and instead of gulping down vast quantities of krill with a single lunge, would have had to rely on less efficient suction feeding.
After Duniatans died out, these primitive rorquals survived longer in the Mediterranean than they did elsewhere, and modern style "lunge feeding" had appeared among ocean-going rorquals and right whales by the end of the epoch, with animals very similar to the living sort being known from the North Sea at that time (as are sperm whales, for that matter).
Seals and their relatives were also common animals during the Pliocene, being found at various coasts around the world. Most would have looked quite similar to modern species, with examples including Pliophoca from the Mediterranean, which is probably related to monk seals. However, some more "primitive" forms of seal did survive into at least the early Pliocene, such as Acrophoca from Chile and Peru, which seems to have been less streamlined than modern animals.
Walruses, however, were much more varied in the Pliocene than they are today, and didn't typically have the large tusks that we so associate with the sole living species. Instead, many, such as Dusignathus, had four enlarged canine teeth, but nothing that could truly be described as "tusks" in the modern sense. At the opposite end of the spectrum, Valenictus, from California, had nothing but, being entirely toothless apart from the tusks; it probably fed by sucking up small animals. Protodobenus from Japan may have been the ancestor of the living walrus, although, it too, had yet to develop the full tusks of its putative descendant.
Finally, we know of fossil sirenians from the Pliocene, too, including unusually large bodied, strong-tusked dugongs that once lived in the Mediterranean, but that died out as the climate cooled, living no direct descendants.
All of which wraps up my survey of the mammals of the Pliocene world. Aside from the coasts I've just mentioned above, the only region I've really ignored has been the Antarctic, as cold and inhospitable then as it is now, and equally devoid of anything much, apart from penguins. The Pliocene came to an end as the Arctic too, became permanently locked in ice and began to grow, leading, not much later, to the true dawn of the Ice Ages. But, if the Pliocene was, in a sense, the multi-million year autumn that led into the winter of the Pleistocene, then the epoch before it must have been the summer.
The Pliocene, and this series, is over. But, starting in June, I'll be launching a series that takes a step further back, to look at the Miocene.
[Picture by Nobu Tamura, from Wikimedia Commons.]