arthropod – Nix Illustration

Falcatacaris

The enigmatic thylacocephalans were a group of bizarre little arthropods, found in marine deposits all over the world from the late Ordovician (~435 million years ago) to the late Cretaceous (~85 million years ago). They had shield-like bivalved carapaces, large compound eyes, three pairs of spiny grasping limbs, and multiple pairs of small paddle-like swimming limbs, but details of their internal anatomy are poorly known and their evolutionary relationships to other arthropods are still very uncertain.

Traditionally they’ve been classified as crustaceans, possibly as close relatives of remipedes or malacostracans – but they’ve also recently been proposed as instead being part of a much more ancient branch of arthropods, potentially related to stem–mandibulates like Acheronauta.

Falcatacaris bastelbergeri was a thylacocephalan living during the late Jurassic, about 150 million years ago, in what is now Germany. Around 2.5cm long (~1″), its carapace had tiny interlocking square “teeth” resembling a zipper along the hinge line between the two valves, a ridge along each side, and a long pointed knife-shaped spine at the front.

Like other thylacocephalans it was probably a swimming predator, likely nocturnal or hunting in murky conditions based on its enlarged eyes, and would have captured smaller aquatic prey using its raptorial limbs.

Tachypleus syriacus

Tachypleus syriacus was a horseshoe crab from the late Cretaceous (~100-95 million years ago) of what is now Lebanon.

Closely related to modern tri-spine horseshoe crabs, it displayed a similar level of sexual dimorphism. Females grew to at least 25cm long (~10″), with rounded front edges to their carapaces and shorter rear spines, while males were around 30% smaller with a scalloped shape to the front of their carapaces.

One recently described female specimen also preserves distinctive nodules around the rim of its carapace, which may represent some sort of sensory structure.

This particular specimen is also unique for preserving a coprolite in the process of being expelled from the horseshoe crab’s body – that’s right, it died while pooping.

Panacanthocaris

Panacanthocaris ketmenica* here was a member of an extinct group of crustaceans known as kazacharthrans – close relatives of modern tadpole shrimp known mainly from Central Asia during the mid-to-late Triassic (but with possible German relatives from both the late Triassic and further back in the late Paleozoic).

Fossils of Panacanthocaris have been found in Kazakhstan and northwest China, dating to about 235-221 million years ago. It was fairly big compared to most of its modern cousins, reaching at least 10cm in length (~4″), and had distinctive spines around the edges of its carapace and its telson.

It’s not clear if it had eyes – there’s a single opening near the front of its carapace that may have housed some, and so I’ve depicted it here with just one naupliar eye similar to the “third eye” of tadpole shrimp.

It probably had a fairly similar lifestyle to its modern relatives, living in shallow freshwater and temporary pools and opportunistically feeding on everything from algae to smaller aquatic animals.

(* Sometimes also called P. ketmenia. May also be the same thing as Iliella spinosa, but until that paper is officially published the current name still stands.)

Triopus

Although Triopus draboviensis here might look like an isopod or a trilobite, this small arthropod was actually part of a rather rare group called cheloniellids.

Known from the early Ordovician to the early Devonian (~480-408 million years ago), only about 7 different species of cheloniellid have been described so far. Their evolutionary relationships were uncertain and controversial for a long time, but currently they’re thought to be distant cousins of trilobites within the Artiopoda.

Living in what is now Czechia during the late Ordovician, about 460-450 million years ago, Triopus is only known from two partial fossils. It was around 4cm long (~1.6″), and like other cheloniellids it had a body made up of wide radiating exoskeleton segments that fully covered its legs, and probably also a pair of whip-like appendages at the rear.

Its body was more domed than those of its relatives, who were generally very flattened, suggesting it was specialized for a slightly different lifestyle or habitat. Without any preserved appendages it’s not clear what its ecological role was, but since other cheloniellids had horseshoe-crab-like feeding structures it may have been a similar sort of generalist, preying on small invertebrates and scavenging carrion on the seafloor.

Spectember/Spectober 2023 #11: A Large Spider

An anonymous submission requested a “spider the size of a coconut crab”:

A shaded sketch of a speculative giant herbivorous descendant of jumping spiders. It's a big stocky spider covered in fuzz, with thick legs ending in cloven-hoof-like claws. It has the characteristic large main pair of eyes of jumping spiders, with the other three pairs more spread out around the front and sides of its wide head, and it also has two pairs of "horns". Its abdomen is wide and round, somewhat flattened on the top, with several spiky structures around the edge.

Ceratohispidus aspectus is a distant descendant of jumping spiders living on an Aotearoa-like landmass, isolated with no mammalian predators.

This particular lineage is notable for both their extreme gigantism (with their larger size and weight causing them to lose the ability to jump) and for having taken up herbivory in a similar manner to one modern species. Most of these big plant-eating spiders are around the size of wētāpunga, and occupy a similar ecological niche, but Ceratohispidus is the largest of them by far – rivalling the modern coconut crab with a body length of up to 40cm (~1’4″) and a legspan of almost 1m (~3’3″).

After reaching sexual maturity at 5-10 years old, adults grow very slowly, molting only once every year or two and taking several decades to actually get anywhere close to their maximum size.

Ceratohispidus’ thick legs end in hoof-like claws, and it selectively browses on vegetation by snipping off pieces with its pincer-like palps. A gizzard-like structure in its digestive system helps to grind up fibrous plant material with small gastroliths, and its wide abdomen houses both large book lungs and a tracheal system with air sacs that can contract and expand to provide a small amount of active ventilation.

While the “horns” and spikes ornamenting its body may provide some defense from the few avian and reptilian predators in its habitat, they’re mainly used as part of highly elaborate visual displays between individuals.

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.

Tuzoia

The tuzoiids were an enigmatic group of Cambrian invertebrates known mostly just from their spiny bivalved carapaces. Although hundreds of fossils of these arthropods were discovered over the last century or so, only vague fragments of the rest of their bodies have been found even in sites usually known for preserving soft tissue impressions.

…Until late 2022, when several new specimens from the Canadian Burgess Shale deposits (~508 million years ago) were described showing tuzoiid anatomy in exceptional detail, finally giving us an idea of what they looked like and where they fit into the early arthropod evolutionary tree.

Tuzoiids like Tuzoia burgessensis here would have grown up to about 23cm long (~9″). They had large eyes on short stalks, a pair of simple antennae, a horizontal fluke-like tail fan, and twelve pairs of appendages along their body – with the front two pairs at the head end being significantly spinier, and most (or all) of these limbs also bearing paddle-like exopods.

The large carapace enclosed most of the body, and was ornamented with protective spines and a net-like surface pattern that probably increased the strength of the relatively thin chitinous structure.

Together all these anatomical features now indicate that tuzoiids were early mandibulates (part of the lineage including modern myriapods, crustaceans, and insects), and were probably very closely related to the hymenocarines.

Tuzoiids seem to have been active swimmers that probably cruised around just above the seafloor, with their stout legs suggesting they could also walk around if they flexed their valves open. The arrangement of their spiny front limbs wasn’t suited to grabbing at fast-swimming prey, but instead may have been used to capture slower seafloor animals or to scavenge from carcasses.

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 #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 #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.)