Month: March 2023

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.

Strange Symmetries #10: Shellraiser

Brachiopods (also known as “lamp shells”) superficially look very much like bivalves, but these two groups aren’t very closely related to each other – although they’re both lophotrochozoans, their last common ancestor probably lived sometime in the Ediacaran at least 560 million years ago, and their similarities in appearance are due to convergent evolution.

The two valves of their shells are also arranged differently. Bivalve shells grow on their left and right sides and are usually symmetrical, but brachiopods form their shells from the upper and lower surfaces of their bodies.

As a result brachiopod shells are usually unequal in size and shape but have their own plane of bilateral symmetry down the center – but some of them still managed to become asymmetrical anyway.

Torquirhynchia inconstans lived during the Late Jurassic, about 161-145 million years ago, in the warm shallow seas that covered what is now Europe and Iran. Around 3cm across (~1.2″) it had a strongly ridged shell with an asymmetrical closing edge, positioned high on one side and low on the other.

This unusual uneven arrangment is thought to be an adaptation to living on soft sediments. Asymmetrical brachiopods like Torquirhynchia may have lived with one side of their body mostly buried into the seafloor, and twisted their shell edges so the still-exposed half was raised up to better function for water circulation and filter-feeding.

Strange Symmetries #09: Not Toeday, Satan

The shells of bivalve molluscs are formed on the left and rights sides of their bodies, and so are usually roughly bilaterally symmetric.

But some bivalves break that arrangement, developing asymmetrical valves that can be massively different in size and shape.

Gryphaea arcuata was an oyster that lived during the Early Jurassic, about 200-174 million years ago, in the warm shallow seas that covered what is now Europe and eastern Greenland. Around 6cm long (~2.4″), its left valve was thick and strongly convex and curled, while the right valve was relatively thin and slightly concave forming a “lid”.

The gnarled curled claw-like shape of Gryphaea fossils led to them being colloquially known as “devil’s toenails” in some of the regions where they’re commonly found, with folk beliefs that they had the power to prevent joint pain.

Their shape was actually an adaptation to living on very soft seafloor sediments. The larger curled valve acted sort of like a boat on the soupy mud, supporting the Gryphaea‘s weight and preventing it from sinking.

Strange Symmetries #08: A Different Diplocaulus

With its bizarre boomerang-shaped skull, Diplocaulus is probably the most iconic ancient amphibian. (It even inspired the design of the pokémon Dragapult!) It was part of the lepospondyls, a diverse collection of early tetrapods mainly found in Europe and North America between the Early Carboniferous and the mid-Permian, about 350-270 million years ago.

But one species hung on a bit longer into the late Permian, about 259-254 million years ago, and this late-surviving lepospondyl was perhaps the oddest of them all.

Diplocaulus minimus was the only lepospondyl known from the supercontinent of Gondwana, found in what is now Morocco in northern Africa. About 70cm long, around half of which was its long tail, it had the distinctive elongated skull of a diplocaulid – but in a bizarrely asymmetrical shape.

The left prong of its skull was long and tapering, but the right was shorter and more rounded. This doesn’t seem to have been due to individual deformity or distortion of the fossil material, since more than one skull has been found with the same features, but the reason for such a striking amount of asymmetry in this species is unknown.

Diplocaulids’ head shapes are thought to have acted as hydrofoils, providing lift while they were swimming. Perhaps Diplocaulus minimus‘ much more wonky skull means this species wasn’t relying on that hydrodynamic function as much as its relatives, and something else was going on with its ecology.

…Although, that weird head does bear a surprising resemblance to a proposed asymmetric “flying wing” aircraft design from the 1950s, so it might have worked better for underwater flight than it seems at first glance.

Strange Symmetries #07: Gastropods Do The Twist

Gastropods – snails and slugs – are a group of molluscs that originated sometime in the Cambrian Period, with the earliest definite stem-gastropods known from around 510 million years ago and the first true gastropods turning up in the early Ordovician.

The spiral-coiled shells of snails are their most familiar feature, giving them obvious external asymmetry, but gastropods are also defined by a specific type of internal asymmetry known as torsion.

Torsion is an anatomical process that occurs during larval development, and involves rotating their internal organs, mantle, and shell a full 180° relative to their head and muscular foot. This twists their gut into a U-shape, knots up their nervous system, and brings their respiratory organs and anus up close to their head.

And we still don’t really know why they do it.

One idea (the “rotation hypothesis”) is that it originated as a defensive function after early gastropods began developing their spiral shells. The shell opening may have originally been positioned at early gastropods’ rears, meaning they retracted their bodies back-end-first leaving their heads and sensory structures still vulnerable – but twisting the shell around would allow them to pull their front end in faster instead.

A competing idea (the “asymmetry hypothesis“) instead proposes that the shape of the coiled shell restricted the gills of early gastropods, which may have originally been positioned in mantle cavities on each side of their bodies. In response to this they developed a single larger gill cavity on just one side of their body, and then gradually expanded and rotated this asymmetric feature around to the front for better aeration.

In either case this resulted in some of the rest of their anatomy “coming along for the ride”. And regardless of whatever the original evolutionary advantage of torsion actually was, it made gastropods incredibly successful – they’re a massively diverse group, second only to the insects in terms of sheer number of species, and today they’re found all over the world in almost every habitat from deep sea trenches to high mountain elevations.

A colored line drawing of Spinyplatyceras, an extinct marine snail. It has a low coiling shell covered in very long thin pointed spines, and there are two short tentacles on its head. It's depicted with orange and black striped coloration on its shell, and a purplish body.
Spinyplatyceras arkonense

Spinyplatyceras arkonense lived in what is now Ontario during the mid-Devonian, about 391-385 million years ago. Around 5cm long (2″), it was part of a group of Paleozoic marine snails known as platyceratids, which were probably related to either modern limpets or neritomorphs.

Platyceratids seem to have had a unique parasitic relationship with crinoids, attaching themselves to the top of the host’s body and using their radula to drill into them, either robbing food directly from the crinoid’s gut or feeding on its other internal organs.

The long spines on Spinyplatyceras‘ shell probably helped to deter predators. In an interesting case of coevolution the crinoid hosts of some platyceratids developed their own defensive spines, too – and it seems this wasn’t to prevent the snails from infesting them, but to also discourage the snails’ predators. These crinoids may have been frequently indirectly injured during snail-eating predators’ attacks, and it might have actually “cost” them less to keep enduring an infestation than to deal with the collateral damage of the snails being removed.

Strange Symmetries #06: Trilobite Tridents

The genus Walliserops was one of the weirdest-looking trilobites, covered in numerous pointy spines and sporting a large three-pronged “trident” on the front of its face.

They also had some degree of asymmetry in their bodies. Their tridents often didn’t fork evenly, and their long forehead spines curved off to one side – possibly so they could lift their heads up without stabbing themselves in the back.

Walliserops hammii lived in what is now Morocco during the early-to-mid Devonian, about 403-392 million years ago. Around 5cm long (~2″) It was one of the “short trident” species of Walliserops, and its chunky forehead spine curved particularly strongly to the right.

The function of these trilobites’ elaborate tridents is still poorly understood. But an unusual individual of the long-tridented species Walliserops trifurcatushas been found with a lopsided four-pronged trident, and since it was able to grow to full maturity the shape of the structure probably wasn’t absolutely vital for survival, suggesting it wasn’t used for feeding or sensory purposes.

The tridents may instead have been used for combat with each other similar to the horns of some modern beetles. However, these sorts of features are usually only seen in males, and there’s currently no definite evidence for any significant sexual dimorphism in trilobites.

(Although perhaps like ceratopsid dinosaurs their ornaments were just present in both males and females, being also useful for species recognition, visual display, and defense against predators.)

Strange Symmetries #05: Enigmatic Eldoniids

Fossils of cambroernids were first discovered in the early 1900s, but these Paleozoic animals were so confusing that for a long time their evolutionary relationships were a mystery.

They had coiled bodies and fractal-branching feeding tentacles, and they ranged in shape from worm-like to cup-like to disc-like. Historically various species were classified as sea cucumbers, jellyfish, tunicates, gnathiferans, or lophophorates, but more recently they’ve been recognized as a single united lineage of ambulacrarians, closely related to modern echinoderms and hemichordates.

Discophyllum peltatum lived during the late Ordovician, about 458-448 million years ago, in what is now New York, USA. Up to around 11cm in diameter (~4.3″), it was one of the disc-shaped cambroernids – a lineage known as eldoniids – with a shallow domed body containing a clockwise-coiling sac and delicate feeding tentacles around its mouth.

Its disc would have been tough but flexible, containing numerous supporting radial structures that were probably part of a fluid-filled hydrostatic skeleton, giving it an almost-radially-symmetric body plan superficially resembling a jellyfish.

The lifestyle of eldoniids is still uncertain, but they seem to have mostly sat on the seafloor, possibly extending their tentacles out from under their discs to grab nearby food.

Strange Symmetries #04: Even More Echinoderms

Early echinoderms seem to have gone through an asymmetrical phase before starting to evolving their characteristic radial symmetry. The first truly radial forms had three-way symmetry, but soon a group called the edrioasteroids upped that count to five.

First appearing in the fossil record around 525 million years ago in the early Cambrian, edrioasteroids were mostly shaped like discs or domes, and were immobile filter-feeders that lived permanently attached onto surfaces like the seafloor or the shells of other animals. Unlike most modern echinoderms their pentaradial symmetry was actually created by taking a tri-radial body plan and forking two of their arms near the bases to create a total of five.

A colored line drawing of Thresherodiscus, an extinct early echinoderm. It's a domed disc-shaped creature that looks like it has a starfish merged onto its upper surface – but the "arms" branch many more times than five, and not totally symmetrically, creating an erratic forking pattern. It's depicted with a dark purplish body and brighter orange arms.
Thresherodiscus ramosus

Thresherodiscus ramosus was an unusal edrioasteroid that lived in the shallow seas of what is now central Canada during the late Ordovician, around 460-450 million years ago. Up to about 4cm in diameter (~1.6″), its arms split additional times at irregular intervals, creating a complex asymmetrical branching pattern across its upper surface.

The tips of its arms protruded slightly over the rim of its body, and along with the erratic extra branching this may have been an adaptation to increase its food-gathering surface area.

Another group of early pentaradial echinoderms known as the blastozoans were characterized by erect feeding appendages called brachioles. But some blastozoans abandoned their five-way symmetry in favor of much stranger arrangements, sometimes having as few as two arms – and, in some cases, two mouths.

A colored line drawing of Amygdalocystites, an extinct early echinoderm. It has an oval body with a curving stem coming out from its right side, with the stem ending in a circular holdfast. Two "food grooves" run along its top edge, roughly in line with each other on each side of where its not-visible mouth is located. Each food groove has a single row of long tendril-like feeding appendages growing from its left edge. It's depicted with a red and yellow color scheme, with a striped stem and irregular stripes on its body giving a sort of flame-like pattern, and purple tips on its feeding appendages.
Amygdalocystites radiatus

Known from the same general area and time period as Thresherodiscus, Amygdalocystites radiatus was part of an Ordovician-to-early-Silurian lineage known as paracrinoids, which attached their irregularly-shaped bodies to the seafloor via a stem.

About 5cm long (~2″) Amygdalocystites had just two asymmetric arms forming “food grooves” along its upper edge, each lined with numerous long brachioles along just one of their sides. It probably orientated itself so its body was facing down-current, which would have created eddies that brought suspended food particles within easier reach of its brachioles.