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.

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.