arthropod – Nix Illustration

Cambroraster

Cambroraster falcatus was a radiodont that lived during the Cambrian, about 508-505 million years ago, in what is now western Canada. Part of the hurdiid family of radiodonts, it reached sizes of up to 30cm long (~12″) – making it one of the largest animals known from the Burgess Shale fossil deposits.

Its distinctively-shaped massive head shield led to it being nicknamed “the spaceship” during collection and study of numerous fossil specimens. This was then referenced in its species name of “falcatus“, inspired by its resemblance to the fictional Millennium Falcon from the Star Wars franchise.

Its short frontal appendages bore long curved spiny blades that formed a basket-like structure. Originally these were thought to be used to sift through seafloor sediment, but more recently they’ve been proposed as filter-feeding structures instead.

Pygmaclypeatus

Pygmaclypeatus daziensis was a small early trilobitomorph arthropod that lived during the Cambrian, about 518 million years ago, in what is now southwestern China.

It had a wide flat carapace, about 14mm long (~0.5″), with a single pair of antennae, fourteen pairs of limbs, and a short segmented “tail”.

It also had an unusual arrangement of four eyes — one pair of fixed-in-place trilobite-like eyes on top of its headshield, and a second pair of crustacean-like mobile stalked eyes on the underside. Other four-eyed Cambrian arthropods are known, but Pygmaclypeatus is currently unique for having two completely different compound eye systems.

Its well-developed limbs with paddle-like branches indicate it could swim well, and probably also burrow into soft seafloor sediment. Its upper eyes seem to have been adapted to sensing motion in dim daylight conditions, suggesting they were used to keep a lookout for predators in shallow murky water. Its lower eyes were more sensitive, and may have been used to locate food items such as smaller soft-bodied invertebrates or organic detritus on the seafloor.

Spectember/Spectober 2025 #08: Alphabugs

One last entry for this year!

Dwoll suggested “a family of creatures that have evolved to look like every letter of the Roman/English alphabet”:

A digital sketch of a speculative treehopper bug. It has a large elaborate "helmet" crest on the back of its neck forming a shape that resembles the letter A.

Grafficimex dwolli is a domesticated species of treehopper closely related to the neotropical genus Cladonota.

Its wild ancestor, the now-extinct species Grafficimex ignotus, had an elaborate pronotum “helmet” with a close resemblance to the English letter F. It proved to be surprisingly easy to raise in captivity, being docile around humans and happily using common houseplants such as Monstera as hosts, and it was also quite morphologically variable. Varieties resembling letters such as E, C, and U were quickly developed, and hobbyists began competing to breed more and more new shapes.

Now, after centuries of selective breeding, the English alphabet has been completed, along with a couple of recently-developed breeds with bulbous protrusions that resemble question mark and exclamation mark shapes.

An "alphabet" of speculative Grafficimex treehoppers with crests that resemble the letters from A to Z, along with a pair that resemble a question mark and an exclamation mark.

(Breeds resembling the alphabets of other languages are also in development.)

At about 2cm long (~0.8″), Grafficimex dwolli is rather large for a treehopper, and much like the domestic silk moth it has almost entirely lost the ability to fly.

Along with being kept as novelty pets, often carefully lined up on plant stems to spell out amusing messages, these insects are also quite popular with beekeepers – the honeydew produced by Grafficimex nymphs and adults can be harvested by bees to make dark strong-flavored honeydew honey.

Mosura

Mosura fentoni was a small radiodont living during the mid-Cambrian, about 508 million years ago, in near-equatorial shallow marine waters covering what is now western Canada.

Sixty specimens have been discovered in the Burgess Shale fossil deposits, ranging from 1.5cm long juveniles (~0.6″) to 6cm long adults (~2.4″), giving us a detailed look at Mosura’s anatomy and life history. It had three eyes – two on the sides of its head on short stalks and one in the middle of its face – and a pair of grasping frontal appendages each with six long sickle-shaped spines.

Unusually for a radiodont its body was divided into distinct regions: a four-segment neck, a six-segment mesotrunk with large swimming flaps, and an abdomen-like posterotrunk with up to at least sixteen segments (fewer in juveniles), all bearing gills along their undersides.

Its vaguely moth-like shape led to it being nicknamed “sea-moth” by field collectors, and inspired its genus name – “Mosura” is the Japanese name of the fictional giant kaiju moth-monster Mothra.

With a very high proportion of respiratory surface area for its size, Mosura was probably an active and agile fast-swimming predator, possibly living in low-oxygen waters around the outer continental shelf. Its wide oval central eye may have helped it stay orientated during rapid maneuvers, keeping track of the horizon line similar to the median eyes of modern dragonflies.

Rhyniella

Rhyniella praecursor was an early springtail that lived during the early Devonian, about 410-400 million years ago, in what is now Scotland. Discovered in the exceptionally well-preserved Rhynie chert fossil site, it’s one of the earliest known hexapods.

It was around 2mm long (~0.08″) and closely resembled some of its modern relatives – with distinctive anatomical features like a collophore and a furca – showing that springtails were already well-established in such an early terrestrial ecosystem.

It probably had a similar sort of ecological role to modern springtails, too, being involved in the breaking down of organic matter and the formation of soils.

Greenwaltarachne

Greenwaltarachne pamelae was an orb-weaver spider that lived in what is now Montana, USA, during the mid-Eocene, around 46 million years ago.

Known from a single fossil of an adult female, it had a body length of about 2mm (~0.08″) and a legspan of around twice that. The specimen is even well-preserved enough to show banded markings on the legs resembling those of some modern orb-weaver species.

It would have lived in what was then a rift valley with a tropical climate, along the shoreline of the ancient 160km long (~100 miles) Lake Kishenehn. It was part of a highly diverse ecosystem full of numerous other invertebrates – including miniscule fairyflies, and even mosquitoes with evidence of blood preserved inside their bodies – and a wide variety of mammals ranging from tiny rodents to large brontotheres.

Tardisia

Tardisia broedeae was a small arthropod recently discovered in the 308-million-year-old Late Carboniferous Mazon Creek fossil beds in Illinois, USA.

Its anatomy has distinctive features of vicissicaudatans (close relatives of the trilobites), but its presence in Mazon Creek makes it by far the youngest known member of this group – indicating a previously-unknown ghost lineage of around 100 million years, and inspiring its name based on the time-travelling TARDIS in Doctor Who.

About 1.5cm long (~0.6″), Tardisia had an oval segmented body ending in a pair of pointed “tail” appendages. It also appears to have been eyeless, although some vicissicaudatans had eyes on their undersides so this might just be an artifact of preservation.

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.)