Modern beluga whales and narwhals are the only living representatives of the monodontid lineage, found only in cold Arctic and sub-Arctic waters. But this whale family actually first evolved in much warmer climates – and some of them were downright tropical.
It seems to have had a larger number of functional teeth than modern monodontids, and probably didn’t suction feed like its modern close relatives. Instead it may have fed more like most porpoises and dolphins, relying more on speed and snapping jaws to capture prey.
It inhabited the Mediterranean at a time not long after the sea there had mostly dried up and then been rapidly refilled. The presence of warm-water marine species such as bull sharks, tiger sharks, and dugongs in the same fossil beds as Casatia indicates the local climate at the time was hotter than it is today, with tropical temperatures – and suggests that this whale’s ancestors must have originally moved into the replenishing Mediterranean from lower latitudes alongside these other warmth-adapted animals.
This tropical monodontid was also much closer related to modern belugas than modern narwhals are, which raises the possibility that the two living monodontid species actually specialized for colder conditions completely independently of each other rather than descending from a cold-adapted common ancestor. Instead modern belugas and narwhals may have originated from separate warm-water monodontid ancestors who evolved similar cold-tolerant adaptations in parallel as the climate cooled during the onset of the Quaternary ice age, while the rest of their relatives all went extinct.
Known just from fossilized lower jaws and teeth, with some teeth up to nearly 13cm long (~5″), its full life appearance and size are uncertain – but it may have been slightly larger than a modern bottlenose dolphin at around 4.5m long (~14’9″). It’s traditionally been considered to be part of the kogiid family, closely related to modern pygmy and dwarf sperm whales, but some studies disagree with that classification and instead place it in the true sperm whale lineage.
I’ve reconstructed Kogiopsis here as a kogiid-like animal, with a similar sort of shark-like head shape and “false gill” markings. In the background a second individual is depicted displaying “inking” behavior, releasing a defensive cloud of reddish-brown fluid from a specialized sac in its colon.
Living during the mid-Eocene, about 43 million years ago, in a shallow sea-covered region that is now part of Egypt‘s Western Desert, Phiomicetus was an early protocetid – an amphibious foot-powered swimmer, at a transitional point in the evolution of whales from deer-like terrestrial animals to fully aquatic screaming torpedoes.
About 3m long (~10′), it had large jaw muscles and sharp teeth with wear patterns that suggest it was a raptorial hunter grabbing and snapping at prey with powerful bites. It would have probably tackled fairly big prey compared to other protocetids, hunting things like large fish, turtles, and even smaller whales in an ecological role similar to that of modern orcas.
Along with the distantly-related long-snouted Rayanistes it’s one of the earliest known whales from Africa, giving us further glimpses at a time period when early cetaceans were first dispersing out from the South Asian subcontinent via the ancient Tethys Sea.
They had a mixture of anatomical features similar to true seals, sea lions, and walruses, but weren’t actually the ancestors of any of those modern groups. Instead they seem to have just been their own separate thing, a very early diverging “cousin” lineage of pinnipeds that convergently developed close resemblances to their later relatives.
It would have been a sea lion-like animal, able to walk on all fours when hauled out on land, and showed distinct sexual dimorphism, with males growing to sizes of around 4m long (13′) and females being somewhat smaller. It powered its swimming using its front flippers, and may have mostly foraged in deep dark waters, using both keen vision and sensitive whiskers to locate prey.
The nasal region of its skull also shows some similarities to modern elephant seals, and some reconstructions depict males with the same sort of large proboscis.
Unlike modern baleen whales it was small, about the size of a modern porpoise at around 2m long (6’6″), and the wear on its multi-cusped teeth suggest it was a predator taking slicing bites of fish – possibly using suction-assisted feeding like its close relatives the aetiocetids.
Its fossilized remains are also a rare example of an ancient whale fall, with characteristic bore holes in its bones from Osedax worms.
Early members of this group swam like otters, using a combination of undulating their bodies and paddling with large hind limbs, but somewhere in the Late Eocene they switched over to propelling themselves entirely with their tails and gave rise to even more whale-like forms like the basilosaurids.
Discovered in the Wadi Al-Hitan (“Valley of the Whales”) fossil site in Egypt, Aegicetus lived around 37-35 million years ago. It was similarly-sized to earlier protocetids like Georgiacetus, measuring about 3.5m long (11’6″), but its hind limbs were proportionally smaller. Its hips were also completely disconnected from its vertebrae, giving it much more flexibility to undulate its body and tail – and preventing it from supporting its weight on land, suggesting that it spent its entire life in the water.
It wasn’t a direct ancestor to more “advanced” cetaceans, since it lived alongside several species of basilosaurids. Instead it seems to represent a late-surviving example of what the earlier protocetid-basilosaurid transitional forms would have looked like.
Since the last coupleof weeks have featured marine mammals, let’s have one more! This time not a cetacean but a member of the other group of fully aquatic mammals still alive today: the sirenians.
Although commonly known as “sea-cows” due to their herbivorous grazing habits, sirenians’ closest living relatives are actually modern elephants. They’re thought to have originated in Africa over 50 million years ago, starting off as pig-like or hippo-like semi-aquatic animals — but they must have been good swimmers capable of crossing oceans very early in their evolutionary history, since some of the earliest known sirenian fossils actually come from the other side of the Atlantic on the Caribbean island of Jamaica.
Sobrarbesiren cardieli here extends some of our knowledge of early four-legged sirenians to Europe, dating to the mid-Eocene about 42 million years ago. Hundreds of bones were found in Northeastern Spain, representing at least six different individuals and giving us a fairly complete idea of this species’ anatomy.
It was smaller than modern sea-cows, reaching about 2m long (6’6″), and seems to represent a transitional point between the semi-aquatic ancestral sirenians and fully aquatic later forms. It had a head very similar to its modern relatives, and probably a tail fin, but also still retained small functional hind limbs.
It was initially thought to still be somewhat semi-aquatic and capable of quadrupedal locomotion on land, but a later analysis of its hind limb bones suggests that it may actually have been much more aquatic than that. Its hind legs had a wide range of motion and were probably used for otter-like swimming, undulating the body while paddling, but might not have been capable of supporting its weight on land. So if Sobrarbesiren did still haul out of the water, it may have had to move more like a seal.
Last week’s weird-snouted Furcacetus wasn’t the only recently-discovered ancient platanistoid dolphin that deserves some attention.
Ensidelphis riveroi was described in the same paper, and also lived in the coastal waters around Peru during the early Miocene, about 19 million years ago. It was a little less closely related to its modern river-dwelling cousins than Furcacetus, and was slightly larger, estimated to have measured about 3m long (9’10”).
But what made it weird was its incredibly long snout, lined with around 256 tiny sharp teeth, which also curved markedly to the right side along its 55cm (1’10”) length.
With only one known skull of Ensidelphis it’s impossible to tell if this was a natural condition for the species or if it was some sort of anomalous individual. It doesn’t seem to be a deformation of the fossil, at least.
Similar unusual right-side bending has been seen in the skulls of a few individuals of modern South Asian river dolphins, franciscanas, and Amazon river dolphins, possibly caused by injuries at a young age being exaggerated as the animals grew. However, many other platanistoid dolphins (especially squalodelphinids) are known to have naturally had similar bends in their snouts – but always to the opposite side, curving to the left instead of the right.
But naturally bent or not, what might Ensidelphis have been doing with that incredibly lengthy snoot?
Its long slender jaws would have had a fairly weak bite, so it probably wasn’t able to catch large prey, and it had a very flexible neck. Possibly it swam along near the seafloor using its snout to probe and sweep around in the sediment for buried small prey.
Modern South Asian river dolphins swim along on their sides while doing this – almost always on their right sides, interestingly enough – and if Ensidelphis did the same sort of thing then a snout bent in that direction might have been an advantage.