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.

Strange Symmetries #16: Go Home Heteromorphs You’re Drunk

Most ammonites had spiral-coiling shells, but during the Cretaceous a group known as the heteromorphs evolved a much weirder range of forms. Some were straightened, some were hooked, some had helical snail-like shapes, and some even ended up bearing a strange resemblance to paperclips. 

But one of the most bizarre of all was the genus Nipponites, whose ribbed shell looked like a bundle of tangled asymmetrical coils.

Nipponites bacchus lived in what is now Hokkaido, Japan, during the late Cretaceous about 90 million years ago. Around 10cm long (~4″), its shell was less tightly coiled up than its better-known relative Nipponites mirabilis, but these looser whorls were formed in the same way via a series of U-bends in different directions during its growth.

Despite their irregular and ungainly appearance, the unique shape of these ammonites seems to have actually been very hydrodynamically stable. They weren’t fast-moving, but they didn’t need to be, probably spending most of their time floating suspended in the water column catching small planktonic prey from around themselves.

Strange Symmetries #15: Serrated Saw-Snoots

Long flattened snouts lined with pointy tooth-like denticles have convergently evolved at least three separate times in cartilaginous fish: in modern sawsharks and sawfish, and in the extinct sawskates.

This repeated “pristification” suggests that saws are just incredibly useful and relatively “easy to evolve” structures for these types of fish, being both highly sensitive to bioelectric fields and able to physically slash and stab to kill prey.

Onchopristis numida was a sawskate known from what is now Northern and Western Africa during the mid-Cretaceous, about 95 million years ago. Up to about 3m long (~10′), it lived in both saltwater and freshwater, and was probably a bottom-dwelling ambush predator similar to modern angelsharks.

Whenever a denticle was lost from its saw, a larger one would grow to replace it, and over the life of an Onchopristis this resulted in an increasingly extreme amount of saw asymmetry.

Modern pristified fish also have rather asymmetrical saws. Sawfish are commonly born with a different number of denticles on each side, while sawsharks add extra denticles of varying sizes as they age, with the ongoing replacement of lost denticles resulting in more uneven arrangements over their course of their lives.

It’s not clear if the asymmetry gives any sort of advantage to these fish – but if nothing else it probably doesn’t cause them any disadvantage, so there’s no evolutionary pressure to stay more symmetrical.

Strange Symmetries #14: The Tooth About Baryonyx

Almost all toothed theropod dinosaurs had exactly four teeth on each of their premaxillary bones, the paired bones at the very tip of the upper snout.

A diagram of the various bones in the skull of Spinosaurus.
Spinosaurus skull by AS | Public domain

The semi-aquatic spinosaurids were an unusual exception to this with six or seven teeth per premaxilla – and one particular member of this lineage seems to have been just a little bit weirder.

Baryonyx walkeri lived during the early Cretaceous, around 130-125 million years ago, in what is now southeast England. About 9m long (~30′), it had distinctive enlarged curving claws on the first fingers of its hands, along with a long narrow snout with a “rosette” at the tip followed by a notch (a shape convergent with the jaws of modern pike conger eels).

And that premaxillary rosette had a strangely asymmetrical arrangement of teeth.

A closer view of the lineart for Baryonyx's premaxillary rosette. The six left teeth are indicated in pink, and the seven right teeth in dark green.

The left side had six teeth, and the right side had seven.

Why? We don’t know!

Baryonyx skull material is rare and fragmentary, so it’s unclear if this was actually a characteristic feature of the species or if the known asymmetric rosette just represents an unusual individual.

Strange Symmetries #13: The Hermit Crab Cycle

Hermit crabs are crustaceans that first appeared at the start of the Jurassic, about 201 million years ago. Despite their common name they aren’t actually true crabs, instead being a classic example of convergently evolving a crab-like body plan via carcinization.

They also have noticeably asymmetric bodies, with abdomens that coil to one side and differently-sized front claws.

A photograph of the modern hermit crab Pagurus bernhardus, without any shell. Its soft abdomen curls to the right in a tight spiral.
Pagurus bernhardus by Arnstein Rønning | CC BY 3.0

And while modern hermit crabs are famous for inhabiting scavenged snail shells, their fossil record suggests this wasn’t always the case.

Originally, they seem to have lived in ammonite shells.

Palaeopagurus vandenengeli lived in what is now northern England during the Early Cretaceous, about 130 million years ago. Around 4-5cm long (~1.6-2″), it was found preserved inside the shell of the ammonite species Simbirskites gottschei.

Its left claw was much larger than its right, and together they would have been used to block the shell opening when it was hiding away inside. And while the exact shape of its abdomen isn’t known, it probably asymmetrically coiled to the side to accomodate the spiralling shape of the host shell.

Hermit crabs seem to have switched over to using gastropod shells by the Late Cretaceous, around 90-80 million years ago, possibly due to marine snails developing much stronger sturdier shells during this period in response to the increasing prevalence of specialized shell-crushing predators. The more upright snail shells would also have been much easier to drag around the seafloor than ammonite shells – and meant that they were ultimately less affected by the total disappearance of ammonites during end-Cretaceous mass extinction.

Strange Symmetries #12: Pterosaur Crossing

Rhamphorhynchus muensteri was one of the first pterosaurs known to science, and its snaggletoothed snout and long vaned tail have become classic features of many fictional “pterodactyls”. But despite its prevalence in pop culture depictions, it actually seems to have been quite a highly specialized pterosaur compared to its closest relatives – and a few specimens also seem to have an unusual little bit of asymmetry going on.

Living during the Late Jurassic, about 150-145 million years ago, around the warm shallow seas of what is now southeast Germany, Rhamphorynchus had a a wingspan of up to at least 1.8m (~6′), with larger fragmentary fossils suggesting a maximum of around 3m (~9’10”).

It had proportionally long wings, splaying intermeshing needle-like teeth, and a toothless beak at the tip of its jaws. The lower beak hooked strongly upwards, while the upper seems to have varied from upwards-curving to straight to downward-curving in different individuals – and some of these arrangements mean the keratinous beak tips must have crossed when the jaws closed, twisting to each side to asymmetrically pass each other similarly to modern crossbill birds.

Several specimens have been found with fish and cephalopod remains preserved in their guts, and along with the pointy intermeshing teeth this indicates Rhamphorhynchus was probably mainly piscivorous, occupying a similar ecological role to modern seabirds.

The different shapes of the toothless jaw tips may suggest there were several distinct populations of this pterosaur species exploiting slightly different food sources to each other, and the crossing beaks may have been an adaptation to pry the soft parts out of hard-shelled prey.

Strange Symmetries #11: Step Up To The Plate

Stegosaurs are some of the most popular and recognizable dinosaurs thanks to their unique appearances, with small heads, elaborate back plates, and spiky thagomizer tails.

Closely related to the ankylosaurs, they first appeared in the mid-Jurassic about 170 million years ago. While they lasted until at least the mid-Cretaceous (~100 milion years ago), their heyday was in the latter half of the Jurassic, ranging all across Asia, Europe, Africa, and North America – and the North American species like the eponymous Stegosaurus developed especially elaborate plates in a distinctive asymmetrical pattern, not arranged in pairs like most other stegosaurs but in alternating rows along each side of the midline of their backs.

Hesperosaurus mjosi lived around 156 million years ago during the late Jurassic, in what is now Wyoming and Montana in the Western United States. It was closely related to Stegosaurus but was a little older and a little smaller, about 5-6m long (~16-20′).

Much like its more famous relative its plates seem to have alternated along its back, which may have been an adaptation to maximize visible surface area while minimizing the number of plates, saving on the energy needed to grow such large elaborate ornamentation.

Hesperosaurus might also represent a rare case of possible sexual dimorphism in non-avian dinosaurs, with wider more rounded plates potentially interpreted as belonging to males and taller pointed plates belonging to females.