Category: theme months

Spectember 2023 #03: Raptorial Isopod

Modzilla07 asked for a “eurypterid or anomalocarid-esque isopod”:

A shaded sketch of a speculative swimming predatory isopod. Its shaped similarly to an extinct sea scorpion with a streamlines segmented body that ends in a fin-like horizontal tail fan. It has short antennae, a large pair of spiny grabbing front appendages similar to an anomalocarid, large compound eyes on short thick stalks, and two pairs of its legs are modified into long paddle-like structures.

Agriopterus modzillaseptenorum is descended from scavenger-predator intertidal cirolanids. At about 10cm long (~4″) it’s a giant compared to most other isopods, but not nearly as big as some of the radiodonts and eurypterids it convergently resembles.

Adapted for a free-swimming lifestyle, its second pair of antennae have been modified into spiny raptorial appendages and its first two pairs of legs have become flat swimming paddles. It’s a voracious little predator, usually snatching small fast-moving prey from the water and raking up soft-bodied animals from the seafloor – but groups will sometimes opportunistically swarm on much larger dead, dying, or injured targets.

Spectember 2023 #02: Carnivorous Ungulates

An anonymous submitter asked for a “derived carnivorous, pack-hunting agriochoerid“:

A shaded sketch of a speculative predatory early ungulate. It has a cat-like body, long and low-slung with clawed feet and a long slender tail. Its head looks like a mix between a cat and a camel, with a fleshy cleft lip, forward-facing eyes, leaf-shaped ears, and a mouth full of sharp pointy teeth. It's depicted in two poses: on the left climbing up a tree trunk, and on the right standing and baring its fangs.

Felichoerus ochlos is fairly similar-looking to its herbivorous relatives, but this cat-sized agriochoerid comes from a lineage that initially specialized in eating fleshy fruits – and then shifted towards eating actual flesh.

With its long cat-like body, forward-facing eyes, clawed digits, and flexible limbs, it’s a capable tree climber. Groups of this animal practice cooperative hunting, with one member chasing arboreal prey down to the ground for the rest to mob.

And another anon wanted to see an “obligate carnivore bovine”:

(I see what you did there. A literal carnotaurus!)

A shaded sketch of a speculative predatory bovine. It has a muscular body with humped shoulders and cloven hoofed feet, a thick neck, and a large cow-like head. Two individuals are depicted, a standing long-horned bull and a sitting short-horned cow. The cow has a chunk missing from one ear and has her mouth open to display her sharp teeth.

The bulltcher (Carnovitulus grassator) is a sheep-sized descendant of small buffalo that gradually took up more and more omnivorous diets, eventually becoming somewhat entelodont-like opportunists. This particular species has shifted over into hypercarnivory, occupying a predator niche in an ecosystem lacking other types of carnivorous mammal.

Like their ancestors they still lack upper front teeth, and instead have modified their dental pad into an almost beak-like tough keratinized structure that their sharp lower teeth can slice and self-sharpen against.

These animals live in small matriarchal herds, with bulls usually hanging around on the edges of the group to protect from threats. Bulls have larger backwards-pointing horns, used to compete with each other for mates – but the size of these structures on their skulls results in them having slightly less powerful jaw muscles than cows.

Herds hunt cooperatively, pursuing and harassing larger prey until it can be brought down and torn apart.

Spectember 2023 #01: Kiwi Alvarezsaur

It’s #Spectember time again!

I’m still trying to work through that big pile of speculative evolution concepts from a few years ago, so I’m hoping to make this month sort of a “lightning round” to finally clear out the backlog.

(I’m not going to set a definite posting schedule this year because things are pretty chaotic right now. But I’ll try to fit in as many as I can!)

So let’s start off with a concept from an anonymous submitter, who requested a “kiwi/sengi niche alverezsaur”:

A shaded sketch of a speculative dinosaur. It has a long narrow snout, small eyes, and whiskery facial feathers like a kiwi bird, a round fuzzy body, short chunky arms with large hooked thumb claws, long slender legs, and a long tail with a tufted fan at the tip.

Khamartaia dolabella is similar in size and build to Shuvuuia, about 1m in length (3’3″), with slender legs and stumpy arms with massive thumb claws. Unlike its close relatives, however, it has small eyes and fairly poor vision, relying more on its other senses to forage around during the darkness of night.

It has an acute sense of smell, and its long narrow snout is full of highly touch-sensitive nerves, allowing it to probe around for invertebrate prey in soil, undergrowth, and cracks and crevices. Its chunky thumb claws are used to dig up burrows and to tear through bark to access deeper insect nests.

It mainly relies on its long legs to sprint away from threats, although with its poor eyesight these escapes are often rather ungainly.

Strange Symmetries #23: Convergent Earvolution

Although it’s not visible externally, owls have one of the most striking modern examples of asymmetry. The ears of many species are uneven, with the right ear opening positioned higher up than the left, giving them the ability to pinpoint the sounds of their prey much more accurately.

But surprisingly this isn’t a unique anatomical trait that only ever evolved once in their common ancestor.

Instead, multiple different lineages of owls have actually convergently evolved wonky ears somewhere between four and seven separate times.

The boreal owl (Aegolius funereus), also known as Tengmalm’s owl, is a small 25cm long (~10″) true owl found across much of the northern parts of both Eurasia and North America. While most other owls’ asymmetrical ear openings are formed just by soft tissue, the boreal owl’s lopsided ears are actually visible in the bones of its skull.

But despite how many times owls have convergently evolved asymmetrical ears, and how successful this adaptation has been for them, for a long time it seemed to be something that no other animals have ever mimicked.

In the early 2000s asymmetric ears were reported in the skulls of some troodontid dinosaurs, which seem to have been nocturnal hearing-based hunters similar to owls, but proper details on this feature still haven’t been formally published.

Then, just a couple of weeks ago, another example was finally announced.

The night parrot (Pezoporus occidentalis) is a small ground-dwelling parrot found in Australia, close to the same size as the boreal owl at around 22cm long (~9″). Critically endangered and very elusive, it’s rarely seen and little is known about it – and it was presumed extinct for much of the 20th century, until more recent sightings of living individuals confirmed that the species is still hanging on.

Recent studies of preserved museum specimens have revealed that it seems to have poor night vision but excellent hearing, and that its right ear opening is noticeably asymmetrical, bulging out sideways from its skull. Much like owls the night parrot relies on acute directional hearing to navigate in darkness, but since its diet consists mainly of seeds it’s probably not using this ability to locate food sources. Instead it may be listening out to keep track of the precise locations of other parrots, and for the approach of predators – so its sharp sense of hearing may be the reason this unique bird has so far just barely managed to survive the presence of invasive cats and foxes.

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 #21: Uneven Ungulates

Asymmetry is commonly seen in the headgear of modern even-toed ungulates, with natural genetic variation, developmental stress, and injuries during life sometimes causing very wonky-looking horns or antlers.

No living species have asymmetry as a standard trait, however – but some fossil ungulates did.

Ramoceros osborni was a relative of the modern pronghorn living during the mid-Miocene, about 13 million years ago, in open plain habitats of what is now the Midwest and Mountain states of the USA.

It was smaller than modern pronghorns, around 70cm tall at the shoulder (~2’4″), and males had long antler-like horns with three tines. Bizarrely, one of these horns was always at least twice the size of the other, with “left-horned” and “right-horned” individuals seeming to occur in equal numbers.

It’s not clear how this asymmetry affected combat between males. Could they only properly lock horns with “opposite-sided” rivals, or did this uneven arrangement actually prevent physical fights and restrict them more to just visual displays?

An illustration of the head of Tsaidamotherium, an extinct hoofed mammal distantly related to modern giraffe and okapi. It has a vaguely moose-like head with a bulbous fleshy snout. Its left ossicone "horn" is above its eye and very small, while the right ossicone is much larger and positioned towards the middle of its forehead, forming a wide blunt helmet-like structure like a very stubby fat unicorn horn.
Tsaidamotherium hedini

Meanwhile in China another Miocene ungulate known as Tsaidamotherium hedini also had strange headgear, with an enlarged right “horn” forming a helmet-like dome on top of its head. This species was featured here on the blog just year, so check out that post for more details about it.

Strange Symmetries #20: The 16 Million Year Fiddler Crab Rave

Many decapod crustaceans have slightly asymmetrical pincers, often with one claw being chunkier and specialized for “crushing” while the other is more slender and used for “cutting”.

But fiddler crabs take this sort of asymmetry to the extreme as part of their sexual dimorphism – males have one massively oversized claw, which is used for both visual display to potential mates and for physical fights against rivals.

Some of the earliest fiddler crabs are known from the Miocene of what is now northern Brazil. Although the fossils have been given several different taxonomic names since their discovery in the 1970s (including Uca maracoani antiqua, Uca antiqua, and Uca inaciobritoi) they’re currently considered to be indistinguishable from the modern Brazilian fiddler crab, Uca maracoani, meaning that these crabs have remained externally unchanged for the last 16 million years.

Up to about 4cm in carapace width (~1.6″), modern Uca maracoani are found in coastal mangrove swamps and tidal mudflats around the northern and eastern coasts of South America – and some of these environments have also undergone little change since the Miocene. Males of the species can develop their enlarged pincer on either side of their bodies, with lefties and righties seeming to occur in equal numbers.

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.

Strange Symmetries #18: Flat Fish Friday

Modern flatfish are characterized by their highly asymmetrical skulls, with both eyes positioned on just one side of their head. They aren’t actually born this way, but instead they undergo “eye migration” as juveniles, twisting up their skulls to bring one eye across the top of the head.

Progressive eye migration in a developing Summer Flounder, Paralicthys dentatus.
From Helfman et al (2009). The diversity of fishes. 2nd ed., Wiley-Blackwell.

This bizarre arrangement is the result of flatfish adapting to life laying flat on the seafloor, but instead of slowly widening and flattening themselves out they took an evolutionary “shortcut” by simply tipping their tall narrow bodies over onto one side. Initially this would have left one of their eyes unusable, but random mutations causing slightly asymmetrical skulls would have rapidly become highly advantageous to the earliest members of this lineage – and over time they just got wonkier and wonkier.

We’ve even found fossils of early flatfish in the “halfway there” stage of their evolution!

Amphistium paradoxum lived in what is now northern Italy during the Eocene, around 50-48 million years ago. About 20cm long (~8″), it had one eye partially migrated towards the top of its head, but not all the way around yet, showing a transitional state between its bilaterally symmetric ancestors and its more twisted-skulled modern relatives.

Unlike most modern flatfish Amphistium came in both “right-eyed” and “left-eyed” forms in equal numbers, suggesting that a genetic preference for a specific side also hadn’t developed yet.

Strange Symmetries #17: Spiky Surprise

Styracosaurus albertensis was a ceratopsid dinosaur living during the late Cretaceous about 75 million years ago, in what is now Alberta, Canada. Around 5m long (~16′), it was one of the most elaborately ornamented horned dinosaurs, with a long nose horn and multiple elongated spikes on its frill.

There was actually quite a lot of variation in the frills of Styracosaurus, with varying numbers of long spikes and extra hook-like projections present on some individuals. But one recently-discovered specimen nicknamed “Hannah” is especially surprising – it had a noticeable amount of asymmetry in its skull. The left and right sides show different numbers and arrangements of spikes, so much so that if the two halves had been discovered separately they might have been identified as belonging to two completely different species.

Frill arrangements are often used to define different ceratopsids, so if this level of individual variation and asymmetry existed in other species, too, then we may need to reevaluate some of them.