Spectember/Spectober 2023 #08: Various Filter-Feeders

Admantus asked for a “freshwater baleen whale”:

A shaded sketch of a speculative freshwater baleen whale. It has a very wide duck-like snout with whisker-like bristles, short baleen inside its mouth, very small reduced eyes, and broad paddle-like flippers.

Rostrorutellum admantusi is descended from small cetotheres that became isolated in a large inland body of water (similar to the modern Caspian Sea), eventually becoming landlocked and gradually reducing in salinity towards fully freshwater.

Highly dwarfed in size, just 2-3m long (~6’6″-9’10”), they’re slow swimmers with broad duck-like snouts that are used to scoop up mouthfuls of sediment and strain out their invertebrate prey in a similar feeding style to gray whales.

Due to the murkiness of the water, and the lack of large predators in their environment, they have poor eyesight and instead use sensory bristles and electroreceptors around their snouts to navigate and detect prey.

And an anonymous submission requested a “whale-like filter-feeding marine crocodile”:

A shaded sketch of a speculative filter-feeding crocodile. It has spatula-like jaws lined with many delicate closely-spaced needle-like teeth, flipper-like limbs, and a long paddle-like tail.

Sestrosuchus aigialus is a 6m long (~20′) crocodilian closely related to the modern American crocodile, living in warm shallow coastal waters.

It’s adapted for an almost fully aquatic lifestyle convergently similar to the ancient thalattosuchians, swimming with undulations of its long tail and steering with flipper-like limbs. But unlike other crocs it’s specialized for filter-feeding, with numerous delicate needle-like teeth in its jaws that interlock to sieve out small fish and planktonic invertebrates from the water.

A couple more suggestions also asked for “fully aquatic pinnipeds” and “future crabeater seal evolution”:

A shaded sketch of a speculative filter-feeding fully aquatic crabeater seal. It has four wing-like flippers, a streamlined body, and elongated jaws with many lobed teeth used to sieve krill.

Euphausiolethrus volucer is a fully aquatic descendant of the crabeater seal. About 5m long (~16’4″), it occupies the ecological niche of a small baleen whale in the krill-abundant Antarctic waters that lack most actual baleen whales.

Its jaws contain numerous finely-lobed teeth that are used to strain krill from the water, and it utilizes all four of its wing-like flippers to swim in an “underwater flight” motion similar to that of plesiosaurs.

Highly social, it tends to congregate in pods that cooperate to herd swarms of krill for easier feeding.


While last week’s Tutcetus was the smallest known basilosaurid whale, another recently-announced member of that group was almost the complete opposite.

Perucetus colossus was a chonker. An absolute unit of a whale.

Living during the Eocene (~38 million years ago) in shallow marine waters covering what is now the coast of Peru, this ancient whale is known from several vertebrae, ribs, and parts of its pelvis. As a result its full size is uncertain, but based on the proportions of other basilosaurids it was probably somewhere around 17-20m long (~56′-66′) – similar in length to the larger specimens of Basilosaurus.

However, it had much thicker denser bones, even more so than those of its close relative Antaecetus, suggesting that its full body mass was much higher than the rather slender Basilosaurus – and possibly heavier than even modern blue whales despite being shorter in overall length.

Perucetus’ thickened vertebrae were also fairly inflexible in most directions, indicating it was a sirenian-like slow swimmer with limited maneuverability – but it did have a surprisingly good ability to bend its body downwards. Without skull material it’s unclear what its diet was like, but it may have been a suction-feeder hoovering up seafloor prey like modern grey whales or walruses.

I’ve reconstructed it here with a speculative bristly fleshy downturned snout inspired by the weird skull of Makaracetus, an earlier whale that may have also been a walrus-like bottom-feeder.


Tutcetus rayanensis was a whale that lived in warm shallow tropical seas covering what is now Egypt during the Eocene, about 41 million years ago.

It was an early member of the “basilosaurids“, a grouping made up of multiple early cetacean lineages (an “evolutionary grade“) representing some of the first fully aquatic forms. Like other members of this group it probably would have had a rather long and slender body shape – but unlike most of its relatives Tutcetus was comparatively tiny, estimated to only have been around 2.5m long (~8’2″).

The fusion of the skull bones in the one known fossil specimen indicate it was almost fully grown at the time of its death, and the pattern of tooth replacement suggests this small basilosaurid species matured very rapidly – a sort of “live fast, die young” life strategy.

Tutcetus’ small size and early demise also inspired its genus name, with “Tut” referencing the teenage Egyptian Pharaoh Tutankhamun.


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.

Eons Roundup 13

I haven’t posted any PBS Eons commissions here for quite a while, so let’s catch up a bit of the backlog:

The Cretaceous mammals Repenomamus robustus and Repenomamus giganticus, from “When Mammals Only Went Out At Night”

A carcass of the whale Borealodon, from “How Ancient Whales May Have Changed the Deep Ocean”

And the early vertebrates Haikouichthys and Myllokunmingia, from “Why Sour May Be The Oldest Taste”

Strange Symmetries #22: The Whalerus And The Twisted Tusks

Mammalian tusks usually grow in symmetrical pairs with only minor developmental asymmetry, but a few species have evolved much more uneven arrangements.

A colored line drawing of the extinct toothed whale Odobenocetops. Its body is beluga-like but it has a face more like a walrus than a whale, with a big fleshy bristly upper lip and a pair of protruding tusks. The right side tusk is much longer than the left. It's depicted with a mottled brown and white color scheme.
Odobenocetops peruvianus

Odobenocetops peruvianus was a small toothed whale that lived during the Miocene, about 7-3 million years ago, in shallow coastal waters around what is now Peru. Around 3m long (~10′), it was a highly unusual cetacean with binocular vision, a vestigial melon, muscular lips, and a pair of tusks – features convergent with walruses that suggest it had a similar lifestyle suction-feeding on seafloor molluscs and crustaceans.

In males the right tusk was much more elongated than the left, measuring around 50cm long (~1’8″) in this species and up to 1.35m (4’5″) in the closely related Odobenocetops leptodon. Since these teeth were quite fragile they probably weren’t used for any sort of combat, and they may have instead served more of a visual display function.

And despite being closer related to modern narwhals and belugas than to other toothed whales, Odobenocetops’ long right-sided asymmetric tusks actually seem to have evolved completely independently from the iconic left-sided asymmetric spiral tusks of narwhals.

An edited meme image using screenshots of Dr. Doofenshmirtz from "Phineas and Ferb". The text reads: "If I had a nickel for every time whales evolved asymmetric tusks, I'd have two nickels. Which isn't a lot, but it's weird that it happened twice."

A colored line drawing of an extinct woolly mammoth. It's an elephant-like animal covered in a thick coat of brownish hair, with a high domed forehead, small ears, and long curving tusks. The tusks are noticeably asymmetrical, one curving more downwards than the other.
Woolly Mammoth (Mammuthus primigenius)

The woolly mammoth (Mammuthus primigenius) lived across Eurasia and North America during the last ice age, mostly from the Pleistocene about 400,000 years ago to the early Holocene about 10,000 years ago – altohugh a few relict populations survived until around 4,000 years ago in isolated areas of Alaska, Siberia, and eastern Russia.

Around 3m tall at the shoulder (~10ft), these hairy proboscideans had very long curving tusks that were used for digging out vegetation from under snow and ice, scraping bark from trees, and for fighting.

The tusks showed a lot of variation in their curvature, and were often rather asymmetrical, a condition also seen in the closely related Columbian mammoth. Like modern elephants mammoths may have also favored using one side over the other for certain tasks, which over their lifetimes could result in uneven wear exaggerating the natural asymmetry even more.

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.

A sketch showing Ensidelphis' bizarre side-curving snout. A hypothetical straight snout is shown outlined in blue, while the actual curvature is overlaid in red.
Expectation vs reality

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.


Antaecetus aithai was an early whale that lived during the late Eocene (~40 million years ago) in what is now Morocco, at a time when northern Africa was covered by a warm shallow sea.

It was part of the “basilosaurids“, some of the first fully aquatic cetaceans – traditionally considered to be a single defined group, but more recently found to be more of an “evolutionary grade” of multiple early whale lineages – and much like Basilosaurus it had elongated back vertebrae that would have given it a very long slender body shape.

Antaecetus also had a proportionally smaller head and smaller teeth than other basilosaurids, along with much denser bones and a stiffer spine that would have made it a rather slow swimmer with reduced maneuverability. It was also fairly small overall compared to most of its relatives, probably around 6m long (~20′).

It was probably a slow-moving coastal water animal somewhat like modern sirenians – except unlike manatees and dugongs it was carnivorous. Its relatively delicate teeth suggest it was feeding on soft-bodied prey like cephalodpods, and with its lack of speed it must have been some sort of ambush predator, waiting around for potential prey to come within striking range.

It Came From The Wastebasket #11: A Cetothere Change

Cetotheres were a group of small baleen whales, one of three major lineages of these cetaceans alongside the rorquals and the right whales. They first appeared in the fossil record in the mid-Miocene, about 14 million years ago (but are estimated to have actually originated 10-15 million years earlier), and disappeared during the Pleistocene about 2 million years ago.

First recognized in the mid-19th century, for a long time the cetotheres were used as a wastebasket for all fossil baleen whales that didn’t clearly fit into any modern whale families. By the start of the 21st century nearly 30 different genera representing numerous different species were all lumped into the group – and the genus Cetotherium was another wastebasket in itself with at least 12 assigned species, many of which were based on fragmentary or dubious remains.

An illustration of Ciuciulea, an extinct cetothere baleen whale. It looks similar to modern rorquals, with a streamlined and relatively slender body, but lacking the typical throat grooves of that type of whale. Its jaws are long and narrow, with a paired blowhole at the top of its head and small eyes set near the corners of its mouth. It also has fairly small flippers, a dorsal fin set about two-thirds of the way down its back, and a horizontal tail fluke. It's coloration is countershaded, mostly grey on top and white underneath.
Ciuciulea davidi

This was finally cleaned up in the 2000s, when a revision of the cetotheres cut the group down to just 6 genera. Since then a handful of additional new genera and species have been named, and while a few polyphyletic Cetotherium species may still need tidying up the cetotheres have overall gone from being a total taxonomic mess to actually being one of the best studied groups of fossil baleen whales.

Their exact evolutionary relationships with each other are still in flux, but the most surprising discovery from the improved understanding of these ancient whales is that they might not be extinct after all.

A set of three screenshots from the animated movie "Ice Age: Dawn of the Dinosaurs". The first image shows the two opossum characters Crash and Eddie asking, "Were you killed?!" The second image shows the weasel buck looking away and replying, "Sadly, yes…" The third image shows Buck leaning closer and adding "but I lived!" Buck's face has been photoshopped into that of a Cetotherium whale in both shots.

A study in the early 2010s suggested that the pygmy right whale may actually be a living cetothere. This classification was initially controversial, but further discoveries of fossil relatives of this enigmatic modern whale and comparisons of their distinctive inner ear anatomy have provided stronger evidence for an evolutionary link.

At this point it seems fairly likely that the pygmy right whale really is either the last surviving representative of the cetothere lineage, or at least is a very close evolutionary “cousin” (a “cetotherioid”) closer related to them than to any other modern baleen whales.


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.

Casatia thermophila lived about 5 million years ago during the early Pliocene, in the Mediterranean Sea around Tuscany, Italy. Although known only from a couple of partial skulls and a few vertebrae it was probably similar in size to its modern relatives, around 5m long (16’4″).

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.