Once again it’s a PBS Eons commission roundup day!
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.
The two living subspecies of the South Asian river dolphin are the last surviving members of a lineage known as the Platanistoidea, an early evolutionary branch of the toothed whales. This group was once much more diverse and widespread than their modern representatives, found in oceanic habitats around the world from the Oligocene to the mid-Miocene.
Many of them had forward-pointing protruding teeth at the tips of their snouts, resembling those of some plesiosaurs or pterosaurs, suggesting they were a convergent adaptation used for snagging hold of slippery soft-bodied aquatic prey.
Furcacetus flexirostrum is one the newest additions to this group, named and described in late March 2020. It lived in Pacific coastal waters around Peru during the early Miocene, about 19-18 million years ago, and was about the same size as modern South Asian river dolphins at around 2.3m long (7’7″).
And it had a uniquely-shaped snout for a cetacean, curving upwards for most of its length but then turning downwards right at the tip, which along with large forward-pointing teeth gave its jaws a vaguely crocodilian appearance.
Cetaceans are just weird animals in general. Fully aquatic mammals best described as “fat screaming torpedoes“, with bizarre head anatomy and their nostrils pulled up to the top of their heads behind their eyes. Some of them are among the largest animals to ever exist, some of them can live to over 200 years old, and some can dive to incredible depths below the ocean surface.
Living during the mid to late Miocene, about 14-7 million years ago, Eurhinodelphis ranged across the Mediterranean and the North Atlantic, with fossil remains known from Western Europe, Turkey, and the East Coast of the United States. It was a fairly small dolphin-like cetacean about 2m long (6’6″), and was part of a lineage of early toothed whales called eurhinodelphinids.
Its upper jaw was around five times longer than the rest of its skull, and toothless past the point where the lower jaw ended. Much like the modern billfish it resembled, it probably used its snout to slash at fast-moving fish, stunning them and making them easier to catch.
It was thought to have been a fairly simple linear process from toothed ancestors to a mix of teeth and baleen and then to fully toothless with just baleen, but more recent discoveries have begun to cast doubt on that idea. The teeth of ancestral baleen whales weren’t suited to filter-feeding at all, instead still being adapted for predatory piercing and chewing – actions which would have been constantly interfering with and damaging any proto-baleen forming alongside them, and making it seem much more unlikely that there would have ever been a transitional form that had both teeth and baleen at the same time.
But then how did baleen whales get their baleen?
Maiabalaena nesbittae here provides a possible solution. Discovered in Oregon, USA, this early baleen whale dates to the early Oligocene, around 33 million years ago, and compared to most of its modern relatives it was comparatively tiny, only about 4.6m long (15′).
And it had no teeth at all, but possibly also no baleen.
Baleen rarely fossilizes, so it’s unclear whether Maiabalaena actually had any or not, but the shape of its skull suggests it probably didn’t – it lacked the broad thickened upper jaw associated with supporting racks of baleen plates. It instead seems to have been adapted for suction feeding similar to modern belugas and beaked whales, using muscular cheeks and tongue to manipulate water pressure and pull small prey like fish and squid straight into its mouth.
Since it lived at a time when the Antarctic Circumpolar Current was forming and cooling the oceans, changing ecosystems and prey availability, it may represent a previously unknown stage in baleen whale evolution – a point when they’d moved towards specializing for suction feeding and lost their teeth entirely, before transitioning again over to filter-feeding with baleen in a completely separate evolutionary development a few million years later.
The modern sperm whale is already an impressive animal, being by far the largest of the living toothed whales and famous for its ability to dive over 2km down (1.2 miles) to feed on deep-sea animals like giant squid.
But some of its ancient relatives were terrifying.
Livyatan melvillei here has an appropriately monstrous name, inspired by both the Hebrew name for the Leviathan and Herman Melville, the author of Moby-Dick. Known from the Pacific coast of South America during the late Miocene, around 10-9 million years ago, it’s estimated to have measured somewhere between 13.5m and 17.5m long (~44′-57′) – comparable in size to an adult male sperm whale.
Unlike the relatively slender mouth of its modern cousin, however, it instead had thick strong jaws full of enormous teeth.
It was part of a loose grouping of what are known as “macroraptorial sperm whales“, which all had similarly toothy jaws and occupied the same sort of ecological niche as modern orcas, specializing in hunting prey like large fish, squid, seals, and other whales.
A huge fossil tooth found in Australia suggests that Livyatan or a very close relative of it survived at least into the early Pliocene, about 5 million years ago. Around this time a cooling climate and dwindling numbers of its preferred prey would have eventually made a population of such enormous apex predators unsustainable, and driven this “killer sperm whale” into extinction – probably around the same time megalodon disappeared, about 3.6 million years ago.
While most modern toothed whales have jaws full of teeth that are all the same simple pointed shape – an adaptation for better holding onto slippery prey – their ancient ancestors had teeth much more like other mammals, with differentiated incisors, canines, and molars.
At over 3.5m long (11′6″) it was one of the larger known toothed whales around at the time, but it wasn’t the direct ancestor of any living whales. Instead it was more of an evolutionary “cousin” to them, part of an older offshoot lineage that lived alongside the early members of modern toothed whale groups.
Inticetus had a long and unusually wide-based snout, somewhat croc-like in appearance, with sharp pointed teeth at the front and multi-lobed cheek teeth further back. A lack of obvious wear on its back teeth suggests it wasn’t using them to chew up its food, and it may have had a fairly specialized diet – possibly using those back teeth to sieve small prey out of the water in a similar manner to modern lobodontine seals.
Inticetus-like teeth have also been found in Miocene-aged deposits in the eastern USA, the Atlantic coast of France, and southeast Italy, indicating that this ancient whale lineage was quite widespread.
Remingtonocetids were an early branch of the whale evolutionary family tree, known from about 49-41 million years ago and splitting off somewhere between the famous “walking whale” Ambulocetus and the more oceanic protocetids. With otter-like bodies, tiny eyes, and long gharial-like snouts, they lived in near-shore shallow marine habitats and probably swam using a combination of their hind feet and tails.
They were initially found only in Pakistan and India, but then Rayanistes afer here was discovered all the way over in Egypt – suggesting that these early whales were much more widespread than previously thought, dispersing through the Tethys Sea at about the same time as their protocetid cousins.
Dating to the Middle Eocene (~45-41 mya), Rayanistes was probably about 2.5m long (8′2”). It had powerful hindlimb musculature that would have given it a very strong kicking swimming stroke, but it probably couldn’t actually support its own weight on land since its femur wasn’t very well anchored into its pelvis.
Despite commonly being called “killer whales” modern orcas are actually the largest living members of the oceanic dolphin family. Their ancestors are thought to have diverged from other dolphins between 10 and 5 million years ago – and surprisingly their closest relatives are the much smaller snubfin dolphins found in Australasia.
Living during the Pliocene (5-2 mya) in the Mediterranean, Orcinus citoniensis was an early member of the orca lineage, and was probably a transitional form between their early dolphin ancestors and the modern Orcinus orca.
It was half the size of modern orcas, at about 4m long (~13′). While it had a higher tooth count than its living relatives its teeth were also proportionally smaller, suggesting it wasn’t specialized for tackling large prey and probably fed mainly on fish and squid.
Waharoa ruwhenua, a whale from the Late Oligocene of New Zealand (~27-25 mya). Part of an early branch of the baleen whale lineage, it’s known from partial remains of an adult and a couple of juveniles and would have reached a full size of about 6m long (19′8″).
It had an unusually long flattened snout, with its nostrils further forward than modern whales, and only had baleen in the back half of its mouth – an interesting comparison to the intermixed teeth-and-baleen of some other early mysticetes. It’s not clear whether it had any vestigial teeth in the front of its jaws, although a single possible tooth has been found associated with its close relative Tokarahia.
The rather delicate nature of Waharoa’s jawbones suggests it wasn’t capable of rapid lunges at swarms of its small prey, instead probably using slow-cruising surface skim-feeding similar to modern right whales.