platanistoidea – Nix Illustration

Aureia

Aureia rerehua was a toothed whale that lived during the late Oligocene and early Miocene, around 23-22 million years ago, in shallow coastal waters covering most of what is now Aotearoa New Zealand.

It was closely related to the waipatiids – a group traditionally classified as platanistoids (the South Asian river dolphin lineage), but more recently proposed as instead representing a separate earlier branch of the toothed whale evolutionary tree.

About 2m long (~6’6″), Aureia had distinctive tusk-like teeth that splayed outwards from its snout, interlocking when its mouth was closed. Along with a flexible neck and its fairly delicately-built skull and jaws, this suggests it was specialized for catching small prey in a “fish trap” of teeth, a unique feeding strategy for a toothed whale.

Along with different feeding specializations in other close relatives like Nihohae, Aureia shows us how multiple species of these ancient Aotearoan cetaceans were able to coexist in the same place and time by diversifying into different novel ecological niches.

Nihohae

Nihohae matakoi was a dolphin that lived in the coastal waters around what is now Aotearoa New Zealand during the late Oligocene, about 25 million years ago. Part of a group known as waipatiids, it was much closer related to modern South Asian river dolphins than to modern oceanic dolphins.

Around 2m long (6’6″), it had unusually long tusk-like teeth at the front of its jaws, splaying out almost horizontally forwards and to the sides.

These teeth lay too flat to effectively interlock as a “fish trap”, and their fairly delicate structure and lack of wear marks suggests they also weren’t used for piercing large prey, sifting through gritty sediment, defending against predators, or for fighting each other. But Nihohae did have a highly flexible neck and the ability to quickly snap its jaws from side to side – although with a relatively weak bite force, suggesting it was primarily tackling small soft-bodied prey that could be easily swallowed whole.

Overall its feeding ecology seems to have been similar to modern sawfish, stunning prey such as squid with rapid slashing swipes of its jaws.

Strange Symmetries #19: Wonky Whales

Toothed whales – the branch of cetaceans that includes modern dolphins, porpoises, beaked whales, and sperm whales – have surprisingly asymmetrical skulls, with some of the bones skewed to one side and just the left nostril forming their blowhole.

Some of the most obvious external manifestation of this lopsidedness can be seen in sperm whales, which have their blowhole at the front left side of their head, and in male narwhals, which usually have a single left-side tusk.

This sort of asymmetry first appeared in the skulls of early toothed whales around 30 million years ago. And since the highest amounts of wonkiness have gone on to develop in lineages that hunt in dark, cluttered, or murky waters, this suggests that the trait is somehow linked to the evolution of complex echolocation.

Some ancient members of the river dolphin lineage also had some additional unusual asymmetry, sometimes having slightly sideways-bending snouts.

Ensidelphis riveroi was one of the weirdest of these, living around the coasts of what is now Peru during the Miocene, about 19 million years ago. Around 3m long (~10′), it had a very long narrow toothy snout that curved distinctly off to the right along its length.

It’s not clear what the function of this bend was, or even if the only known skull actually represents the normal condition for this species. But Ensidelphis’ bendy snoot might have been used to probe around in muddy seafloor sediment or to extract prey from crevices, possibly like an underwater version of the modern wrybill.

Ensidelphis

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.

Furcacetus

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.

A closeup view of the jaws of Furcacetus.

Much like slender-snouted crocodilians and spinosaurids, this arrangement would have allowed Furcacetus to make quick bites at small-fast-moving prey like fish and crustaceans.