Sunday 22 January 2023

Miocene (Pt 37): The Miocene Oceans

Given their extreme adaptations to a life at sea, it should not be surprising that whales and dolphins have a long evolutionary history. Even at the dawn of the Miocene, when seals were at best, only very recent arrivals, and may still have been semi-aquatic, like otters, the cetaceans were already well-established. A time-travelling whale-watching expedition at almost any point during the Miocene would likely have been just as rewarding as one today and, especially so far as one would have been able to see them above the waves, the animals you would have been spotting would not have looked all that different.

At a more detailed anatomical scale, that would have been less true and many of the creatures would not have been as closely related to the modern forms as one might have supposed. Today, the largest number of cetacean species belong to the "true" or "oceanic" dolphin family, including not only all of the sea-going dolphins but some larger animals such as killer whales. But in the Miocene, true dolphins seem to have been comparatively rare and we know of very few fossils predating the subsequent, Pliocene, epoch.

For a long time, the oldest known fossil dolphin was Eodelphinus, which lived off the coast of Japan approximately 9 million years ago. First discovered in 1977, this was originally thought to be a close relative of the modern spinner dolphin of the Pacific, although a more detailed analysis in 2014 showed that it was more primitive than that, with a broad snout and heavy teeth. (This prompted a new name, which unfortunately turned out to have been already taken by a marsupial-like fossil, hence the current one). Since then, an even earlier example, Norisdelphis, has been described from the same area, dating back 11 million years to the Middle Miocene, not long after the family is thought to have originated.

Miocene porpoises were, if anything, even rarer. The few we know of are, like the early dolphins, all from the Pacific, with the most primitive being the 9-million-year-old Pterophocaena from Japan, distinguished by the fact that the bones around its blow hole had yet to migrate to their modern position. A few older fossils are known from the west coast of the Americas, but are not so obviously primitive, suggesting that the group has already diversified by this time.

Both porpoises and "true" dolphins appear to have originated during the Middle Miocene, probably around 15 million years ago as part of a rapid radiation of new forms, but the broader group to which they both belong is much older. Most of the dolphin-like animals we could have seen in the oceans at the time belonged instead to other members of the group, with multiple different families having been named, divided into the two lineages of the platanistoids and the inioids. Today, the few surviving members of these lineages are rare and endangered, having retreated to freshwater habitats as the dolphins and porpoises replaced them in the oceans, but in the Miocene, they were the dominant groups, and very much still living in saltwater.

Some, such as Pomatodelphis, known from both Maryland in the US and from Belgium, are similar enough to living forms to be placed in surviving families - its surviving relatives both live in the rivers of northern India. Unlike them, however, it had normal-sized eyes rather than being near-blind, and it probably lived just off the coast, rather than in murky, silt-filled rivers. Others are harder to place, with many clearly belonging to groups that have since gone extinct.

One of the most common, for example, appears to have been the shark-toothed dolphins, such as Squalodon, with multiple species known from both sides of the Atlantic and what is now the Mediterranean and Waipatea from New Zealand. Thought to be platanistoids, these still had identifiable incisor teeth, with the teeth further back in the jaw being serrated and resembling those of sharks - the teeth of all living dolphin-like animals are, of course, simple cones. On the other hand, the shape of the skull shows that it probably had the "melon" that modern dolphins use for echolocation and that the areas of the brain and nose devoted to the sense of smell were already greatly reduced. They were also larger than most living dolphins, perhaps four or five metres (12 to 15 feet) in length.

Another extinct family, the allodelphinids, were closer in appearance to modern dolphins but had flexible necks and long snouts that could have been useful for snapping up prey on shallow sea floors. With multiple species known from both sides of the North Pacific, in addition to Allodelphis itself, they also include the rather brilliantly named Ninjadelphis from Japan.

Among the inioids, Isthminia is thought to be a close relative of the living South American river dolphins. Living close to what is now Panama, but was then an open sea between the two American continents, around 6 million years ago. It's likely that this would have only shortly before its relatives first entered the Amazon and other South American rivers, possibly swimming into wide estuaries or bays when sea levels were higher, and then finding themselves in fresher water once they started falling again.

The placement of the long-snouted dolphins is less clear; they may well belong to a group outside any of those which survive today, or be closer to the beaked whales than the dolphins. As the common name suggests, they had an extraordinarily long and slender snout. This was formed from the upper jaw, and, while it had many sharp teeth lining most of the length, was toothless at the tip; the lower jaw was much shorter, giving the animals an appearance similar to that of swordfish. They seem to have been most common early on, and are known primarily from Europe, but some are known from the east coast of North America, and at least one seems to have reached as far as Peru. Probably the last example, Xiphiacetus, survived into the Late Miocene, when the line finally died out.

The beaked whales also appear to have originated in the Miocene, and are known from several fossils across the Atlantic, Pacific, and Southern Oceans. Relatively obscure even today, due to their habit of living in deep waters far from coastlines, most Miocene species already showed adaptations to the toothless suction feeding that characterises the modern forms. On the other hand, even the late surviving "Dagon's whale" Dagonodum of Belgium had strong, interlocking teeth, suggesting that it was still capable of a more dolphin-like hunting style although it could probably suck up prey as well. Another Late Miocene species, Messapicetus, seems to have still hunted in waters close to the surface, rather than relying on the deep dives that modern species do.

At least twenty different species of sperm whale have been identified from Miocene deposits, suggesting that the group was much more varied than today - although at least some are from fragmentary remains that may well be misleading. Many of them would have looked like smaller versions of the living species and so presumably had a similar lifestyle. 

The smallest were relatives of the living pygmy sperm whales, which appeared only towards the end of the epoch, around 7 million years ago. Examples include Nanokogia, which at 2 metres (6 feet) in length was closer in size to a dolphin than anything we'd now call a "whale". Its relative Scaphokogia was similar, and evidence suggests that it may sometimes have been prey for the local sharks. Orycterocetus, from the North Atlantic and Mediterranean, was probably closer to the true sperm whale of today. Another close relative, Aulophyseter also reached about 6 meters (20 feet) in length, which, while impressive in absolute terms, is only around a third the length of the modern animal.

Not all Miocene sperm whales fit this pattern, however. Although they do not appear to be closely related, suggesting that the pattern evolved more than once, some species of Miocene sperm whale had large teeth in both jaws and surely cannot have been feeding solely on deep-sea squid or other invertebrates. These include Zygophyseter, whose bite force has been estimated at around 10 kN, the same as a great white shark. Some fossils of another, Acrophyseter, had bony outgrowths on their jaw that also seem to be the result of biting down on something hard. All of this suggests that these animals fed on large vertebrate prey, most of which were probably other whales. The most fearsome of all appears to have been Livyatan, estimated to have been 14 to 17 metres (46 to 56 feet) long, and which could have weighed over 50 tons. It may have been the largest predatory air-breathing vertebrate ever to have lived.

Large though this is, it is still smaller than the living baleen whales, the great filter-feeding animals that prey on krill or other small animals - carnivorous from an ecological point of view, but not really what we think of as "predators". The earliest known definitive member of the right whale family is Morenocetus, dating back as far as the Early Miocene from the coast of Patagonia. It would have looked very similar to its modern relative, but was only about 5 metres (16 feet) long, suggesting that the modern style of feeding evolved before the gigantic size.

The rorquals, which include the living blue whale, likely originated in the North Atlantic during the Middle Miocene, and many of the known examples from the time were already of large size, reaching 15 metres (50 feet) or so. By 8 million years ago, during the Late Miocene, rorquals such as Archaebalaenoptera were found in cold waters of both hemispheres and so must have crossed the equator at some point despite this seemingly not being an ideal habitat for them. They may have done so by swimming through what is now Central America, and we do know of examples from the Pacific, including Incakujira from Peru and Miobalaenoptera from Japan. Judging from the shape of their jaws, however, Miocene rorquals had yet to develop the ram-feeding technique employed by the modern species, in which they simply sweep forward rapidly with their mouths open and would have to have relied on less efficient methods of feeding.

Eobalaenoptera, from the east coast of the US, lived around 14 million years ago, and likely predates the rorquals proper, having lived before they diverged from the grey whales, although superficially it would have looked rather similar. Many other baleen whales of the time are less easy to classify; for much of the 20th century, they were lumped together as "cetotheres" (literally "whale-beasts") although in 2004 that term was restricted to a more precisely defined clade including Cetotherium itself. They probably fed by suction feeding in the same way that the living pygmy right whale does. As a result, it has been suggested that the pygmy right whale is a "living fossil" cetothere, although it's fair to say this claim is controversial. Either way, many of the species are known from eastern Europe and the Caucasus, which seems to be where the cetotheres proper first originated during the Middle Miocene, in what is now the Black Sea.

Even before the arrival of the seals, however, cetaceans were not the only mammals that had taken to the sea. Today, the best-known sirenian is probably the Florida manatee but, if the number of fossils is anything to go by, manatees were not much more common and diverse in the Miocene than they are now. Ribodon is thought to have evolved in an inland lake in what is now western Brazil, before swimming down the Amazon to the coast; it is probably the direct ancestor of the modern species. It lived during the Late Miocene, but Miosiren is much older, dating to the Early Miocene of Belgium and southern England. Unlike Ribodon, it is thought that it probably fed on shellfish, rather than seagrass, although the fossils are rare enough that it is difficult to know for sure.

The dugongs, now restricted to just a single living species, seem to have been much more numerous in the Miocene. Examples are known from every continent except Australia and Antarctica, with the widespread Metaxytherium already having species in both Peru and Europe in the Early Miocene, around 20 million years ago. At least three different genera of dugong lived in Early Miocene India alone, including Bharatisiren, Domningia, and Kutchisiren. Unlike the known manatees of the day, these seem to have already been feeding on seagrass, although with a variety of specialisations suggesting some dietary variation between different species. 

The variation within the group at the time can be illustrated by comparing the sizes of some of the species. At one extreme the Late Miocene Nanosiren of Venezuela was just two metres (6 feet) long, much smaller than any living species of sirenian, meaning that their newborn young must have been about as small as it is possible for a fully aquatic animal to be and still remain warm-blooded. At the other, we have Rytiodus, known from both the northern and southern coasts of what is now the Mediterranean, at a whopping 9 metres (30 feet) long, larger than a killer whale.

And with that, I have finally reached the end of my survey of Miocene mammals. It started, and I can hardly believe I'm saying this, back in 2017. It's taken so long because of the great length of the epoch and the fact that it's still close enough to us that we have quite a lot of information on it. But long though it is, it still doesn't take us even half of the way back to the time of the dinosaurs. To get that far, we would have to head back to the preceding epoch, the Oligocene, a time when many of the modern groups of mammal simply didn't exist yet. 

Starting in March, I'll launch a new (and probably shorter) series that will take us back to that even earlier time.

[Photo by Claire H., from Wikimedia Commons.]

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