Oligocene (Pt 18): Sawfish-Dolphins and Baleen Whales with Teeth

Olympicetus, a simocetid whale

Although the oldest fossils of seal-like animals may date back to the end of the Oligocene, there are very few of these, and the dating may not be wholly reliable. Whales, however, are a different matter and were already well established even at the beginning of the epoch, 33 million years ago. In fact, the Oligocene marks an important phase in their evolution, since it was at this time that the oldest living groups first appeared and that, potentially, the last common ancestor of all living whales roamed the seas.

Even so, especially towards the end of the epoch, it is possible to place some Oligocene cetaceans into groups we are familiar with today. For example, there was Kentriodon, which is better known from the Miocene, but first appeared in the southern oceans at the tail end of the Oligocene. Although it is not placed in any living family, it is the oldest member of a branch that diverged from the common ancestor of dolphins and porpoises around this time. It likely looked rather similar and had a similar fish-and-squid-based diet.

Arktocara lived earlier on, dating back perhaps 29 million years to the middle of the epoch. It, too, does not belong to any living family, although later relatives survived well into the Miocene before dying out. Its closest living relatives are the river dolphins of India, a pair of endangered species quite distinct from the true dolphins. It was only slightly smaller than they are, with an estimated length of 2.26 metres (7' 5"), although it had, for a dolphin, an unusually long neck. It was found in deposits from the south coast of Alaska that would, at the time, have been the floor of a shallow sea. Certainly a far cry from the tropical rivers that its relatives just about survive in today; it's possible that the ancestors of the modern group were driven there by the success of the true dolphins at occupying their original habitat.

The "shark-toothed dolphins", such as Squalodon and Eosqualodon, may also have belonged to the same general branch (although this has been disputed). Like many others, they first appeared towards the end of the Oligocene, surviving well into the following epoch. Otherwise fairly normal-looking dolphins, they still had triangular, flesh-slicing teeth at the back of the jaw, rather than the fish-grasping cones that all modern toothed whales have; it's likely some of their teeth were long enough to project over their lips when their mouths were closed, giving them a snout loosely resembling that of a crocodile. They are thought to have eaten relatively large prey.

But these relatives of modern forms were not alone. Many other groups, apparently with less of a connection to living animals, also prospered during the Oligocene, early experiments in dolphin-like evolution that ultimately failed to reach our present time. Waipatia from New Zealand is sometimes identified as another distant relative of the Indian river dolphins, but more commonly classified as a side branch to all modern toothed cetaceans. At about 2.1 metres (6' 11") in length, this had the same sharp molar teeth as Squalodon and its relatives, but the teeth in general were smaller and more delicate, and some unusual features of the skull also distinguish the two groups.

The family in which Waipatia is placed did not, so far as we know, outlast the Oligocene, but it briefly prospered, providing us with a range of fossils, mostly also from New Zealand. Among the strangest was Nihohae, whose name means "slashing tooth" in Maori. In other respects, much like a typical dolphin, this had a long snout ending with a set of three long teeth splayed out on each side. These created visible "tusks" somewhat reminiscent of the teeth of a sawfish, although, unlike the "true" tusks of, say, a narwhal, they didn't grow continuously throughout life. Our best guess is that they were used in the same manner as a sawfish uses its blade - that is to say, the animal whipped its head from side to side to slash at and stun fish swimming nearby.

Moving further back to the Early Oligocene, it becomes harder to place cetacean fossils into any living group. Those we do have therefore end up in a host of small families, or simply on their own, examples of an early stock from which the better-known toothed cetaceans later emerged. Most were originally placed in a family called the agororophiids, but in recent decades, it has become clear that this is a "wastebasket" for anything too primitive to place elsewhere. With the original family now reduced to a single genus, others have been split off. The simocetids, for example, are unusual in that they may have still had a vomeronasal organ - used to detect pheromones and other scents in many non-human terrestrial mammals, but absent in all living whales.

The xenorophids are one of the more diverse families in this broad assemblage. They seem to be a very early offshoot of other toothed whales but, despite this fact, features of the skull suggest that Cotylocara and Echovenator at least (and therefore, presumably all the others) were capable of echolocation, indicating that this must have evolved surprisingly early, perhaps even at the end of the preceding epoch. 

Most xenorophids were between 2.5 and 3 metres (8 to 10 feet) in length, but Inermorostrum was unusually small, at less than 2 metres (up to 6' 2"), about the size of a modern porpoise. Like porpoises, it had a very short snout but, more surprisingly, it had no teeth in its upper jaw. Other features of the skull imply that it hae sufficient nerves in its snout to supply functional, muscular lips, or even whiskers. While we can't know that for certain, the lack of teeth suggest an animal that fed by suction, slurping up soft squid and small fish from shallow waters.

On the subject of toothed whales, the largest animals alive today are the baleen whales, feeding on krill and other plankton rather than pursuing larger prey. The baleen whales form a separate lineage from their toothed cousins, and this, too, predates the Oligocene. At the dawn of the epoch, the aetiocetids were the dominant family of baleen whales, and they had a key difference from their modern relatives: they still had teeth.

Aetiocetus itself, for example, which is known from Japan, Mexico, and Oregon, had the mammalian full set of 44 teeth. Indeed, some species had even more, beginning to lose the distinction between incisors, molars, and so on, a feature commonly seen in modern toothed cetaceans such as dolphins. Significantly, however, the upper jaw also had channels for enlarged blood vessels that, in modern toothless whales, would supply the growing baleen plates. While baleen itself does not fossilise, this strongly suggests that Aetiocetus had both teeth and filtering plates, placing it partway along this key evolutionary transition. How it used this combination of features is not entirely clear, although it probably fed on fish rather than plankton, and at least partially chewed its food after capturing it.

The aetiocetids are only known with confidence from the North Pacific, where they likely originated, perhaps from ancestors that had swum through what was then the gap between North and South America from the Atlantic. Willungacetus, from Australia, may also belong to the same family, but its remains are too fragmentary to be sure, and it still does not push the animals beyond the Pacific. The earliest known is Fucaia, which is also the smallest, no larger than a dolphin, where its relatives - if not the size of modern baleen whales - often reached 4 metres (13 feet) in length. It seems to have been a reasonably active hunter, judging by the apparent flexibility of its skeleton, supporting the idea that these animals still fed on fish.

Even by this point, however, some whales had developed further along the path to modern filter feeding, indicating that the aetiocetids cannot be the ancestors of any living animals. The eomysticetids date back to at least the middle Oligocene, and, while some species may have had small teeth, these were vestigial and useless, possibly falling out when they reached adulthood. (Today, baleen whales do develop tooth buds, but these disappear before birth). They were also smaller than their giant modern relatives, although with the head alone measuring around 1.5 metres (5 feet), animals such as Eomysticetus were not exactly small either.

Waharoa, from New Zealand, is known to have had baleen plates only towards the rear of the mouth, but could probably feed by skimming the surface of the water for plankton, trapping it in the plates as the whale moved forward. The more sophisticated "lunge-feeding" of many modern baleen whales was, however, probably not yet possible. Another primitive feature is the blowhole, which, in these early whales, was still positioned in front of the eyes, closer to where the regular nostrils would have been in their ancestors.

The simple pattern of toothed to toothless is further muddled by the presence of the mammalodontids which, so far as is currently known, were unique to the Late Oligocene. Only two genera are known, Mammalodon and Janjucetus, both originally discovered in Australia, although fossils from New Zealand have since also been uncovered. Although many features of the skull suggests that these do, indeed, belong to the baleen whale lineage, there is no evidence that they had baleen at all, with Janjucetus in particular having a rather fearsome set of teeth.

Although relatively small, at not much more than two metres (6'6") in length, this likely made it an active predator, not unlike a modern leopard seal. Mammalodon may have been less fearsome, with one suggestion being that it clenched its teeth together, leaving just enough of a gap between the hind ones to suck water through them, trapping fish in the mouth. It may have fed by sucking up creatures from the floor of shallow seas.

Coronodon, from the Early Oligocene of South Carolina, was another "baleen whale" with large teeth instead of baleen. It is not currently placed in any particular family, most likely being a representative of a very early branch in the wider group. It was unusually large for the time, around five metres (16 feet) in length and so probably weighing over a ton, and had unusually pointed incisor teeth, which appear ideal for piercing the bodies of slippery fish. Although the idea that it might have used its oddly shaped cheek teeth to strain food from the water has been proposed, it is perhaps more likely that it really was the active predator that it looks like, at least some of the time.

Cetaceans were not the only aquatic mammals in the Oligocene. The sirenians have an almost equally long history and have changed less over the intervening millions of years. Manatees such as Anomotherium from Germany and dugongs such as Stegosiren from the eastern US were superficially similar to modern forms and, for example, already subsisted on a diet of seagrasses.

The oldest desmostylians date from the Early Oligocene, but survived alongside the sirenians until the end of the Miocene. The two groups were related, although the desmostylians retained a full set of four legs, although these are thought not to have been strong enough to fully support their weight on dry land. They had a similar diet, rooting up seagrasses with their tusks, and it is probably this that eventually led to them being outcompeted by the manatees and dugongs. The best-known Oligocene example is also the earliest and most primitive, Behemetops. It was larger than most of the later forms, too, with one estimate suggesting that it may have weighed as much as two tons, similar to a rhinoceros.

And with that aquatic behemoth, I have finished my survey of the mammals of the Oligocene epoch. It was a time when the modern forms of many mammals were just beginning to take form, a transition between the more distant archaic past and the Neogene period dominated by more familiar types of creature. It started 33.9 million years ago, but what's significant about that date is that it's barely more than half the way back to the time of the non-avian dinosaurs. In this and the preceding three series taken together, I have only covered the second half of the history of the Age of Mammals.

Next time, I will take a look at the epoch that covers most of the first half: the Eocene, when strange early mammals still held sway.

[Picture by Cullen Townsend, from Wikimedia Commons.]