arthropod – Page 4 – Nix Illustration

Cambrian Explosion #47: Bradoriida

The tiny bradoriids first turn up in the fossil record just before the earliest known trilobites, about 521 million years ago, and very quickly became some of the most abundant euarthropods in the mid-Cambrian. Found all around the world, they were clearly important components of many Cambrian food webs and probably had varying lifestyles from species to species, ranging from living on the seafloor to actively swimming around in the water column.

Less than 2cm long (0.8″), they’re mostly known just from fossils of their bivalved carapaces, but some specimens preserve evidence of a pair of antenna and varying arrangements of biramous and uniramous limbs.

They were traditionally thought to be crustaceans closely related to ostracods, but some studies have instead shifted them towards being considered stem-crustaceans or stem-mandibulates instead. And more recently rare high-detail preservation of the soft anatomy of a few species have suggested they actually belong even further down the arthropod evolutionary tree, as “higher stem” euarthropods positioned between the megacheirans and the earliest actual euarthropods.

Cambrian Explosion #46: Megacheira

The megacheirans were a grouping of Cambrian stem–euarthropods that had a distinctive pair of “short great appendages” on their heads with long finger-like spines. Their bodies were fully arthrodized into multiple hard-shelled segments, and their double-branched biramous limbs featured both legs and gill-fringed paddle-like flaps.

As small predators and scavengers they were important elements of some Cambrian ecosystems, and probably swam around just above the sea floor using their arm-like great appendages to grab and tear apart food items.

They weren’t really a distinct lineage, more of an “evolutionary grade“, usually considered to be in the “higer stem” close to the common ancestor of all the major euarthropod lineages, or possibly the very earliest actual euarthropods – but some studies instead place them as more “advanced” euarthropods closely related to chelicerates.

Part of this classification disagreement comes down to whether their great appendages were anatomically equivalent to the front appendages of radiodonts or whether they were convergently evolved, and whether one or both of those structures are also related to the development of antennae and chelicerae in later euarthropods or if they became highly modified into the labrum instead. It’s a subject of longstanding and ongoing debate so complicated that it’s known as the “arthropod head problem” – or sometimes “the endless dispute”.

Cambrian Explosion #45: Stem-Euarthropoda

The true arthropods – or euarthropods – make up the majority of panarthropods, and include the modern chelicerates (sea spiders, horseshoe crabs, and arachnids), the myriapods (millipedes and centipedes), and the pancrustaceans (crustaceans and insects), along with various completely extinct groups like the trilobites.

The earliest fossil evidence associated with some sort of euarthropod presence are trace fossils dating to only a few million years after the start of the Cambrian (~537 million years ago), and the group’s common ancestor is estimated to have split off from the radiodont lineage no more than 550 million years ago in the end of the Ediacaran.

…And this is where things get more complicated.

Austrolimulus

Horseshoe crabs are famous examples of “living fossils“, having changed their external appearance very little over hundreds of millions of years. But some fossil species were much more varied in shape than their morphologically conservative modern relatives, such as Austrolimulus fletcheri here.

Living in freshwater environments in what is now New South Wales, Australia, during the Middle Triassic (~247-242 million years ago), Austrolimulus had incredibly long spines on each side of its head, reaching a span of around 18cm (7″) – wider than its total body length!

The function of these spines is unclear, but they may have acted like a hydrofoil in fast-moving currents, or they may have served a defensive purpose by making Austrolimulus‘ carapace too wide and unwieldy for some predators to deal with.

Eons Roundup 10

Time for some more PBS Eons commission work!

An illustration of an extinct marine invertebrate. It has large spiny appdanges at the front of its head, stalked eyes, multiple pairs of fin-like swimming flaps along the sides of its body, and a pair of long streamer-like "tails". — Lyrarapax unguispinus

An illustration of an extinct marine invertebrate. It has large densely-spined appdanges at the front of its head, small eyes on short stalks, multiple pairs of fin-like swimming flaps along the sides of its body, and three pairs of fan-like tail fins. — Lyrarapax unguispinus

The radiodonts Lyrarapax and Tamisiocaris, from “How Plankton Created A Bizarre Giant of the Seas”
https://www.youtube.com/watch?v=G0oKBPZODhM

An illustration of an extinct lizard-like reptile. It has a row of short spines down its back, and its mouth is open showing "buck teeth" at the front of its jaws and multiple rows of teeth further back. — Sphenotitan leyesi

An illustration of an extinct lizard-like reptile. It has a row of short spines down its back, and a proportionally big head with large eyes. — Sphenotitan leyesi

The rhynchocephalians Sphenotitan, Clevosaurus, and Kawasphenodon, from “When Lizards Took Over the World”
https://www.youtube.com/watch?v=peeX3PKOE_w

April Fools 2021: Time for T. rex

Did you know that in all the time I’ve been posting paleoart here, I’ve never actually done a proper full illustration of the most popular name in all of paleontology?

I think it’s finally time to fix this glaring oversight.

It’s time for T. rex.

Thylacares

Thylacocephalans were a bizarre group of extinct marine arthropods that often looked like tiny alien creatures – and whose evolutionary relationships are still uncertain. Despite existing in oceans around the world for at least 350 million years, their fossil record is rather spotty and their internal anatomy is often poorly preserved, making it difficult to figure out anything more specific than “probably some sort of crustacean“.

Possible thylacocephalans are known from as far back as the Cambrian, but one of the earliest definite members of the group was Thylacares brandonensis.

Living during the early Silurian, around 436 million years ago, in the region that today is part of Wisconsin, USA (found in the same fossil formation as last week’s Venustulus), this species measured up to about 7.5cm long (3″). Its body was enclosed by a large bivalved carapace, with protruding stalked eyes and what may have been a pair of antennae, along with smaller raptorial limbs than its later relatives.

While it was less specialized than other thylacocephalans it was probably a similar sort of swimming predator, catching prey with its spiny limbs.

Venustulus

The synziphosurines were ancient marine chelicerate arthropods that were traditionally thought to be early representatives of the horseshoe crab lineage. But more recent studies have shown them to occupy a slightly more basal position on the chelicerate evolutionary tree, instead being related to the common ancestor of sea scorpions, horseshoe crabs, and arachnids.

(Also horseshoe crabs might actually be part of the arachnids, closely related to ricinuleids!)

Venustulus waukeshaensis was one of the earliest known synziphosurines, living in what is now Wisconsin, USA during the early Silurian, about 436 million years ago. It grew to around 8cm long (~3.2″) and had six pairs of appendages on the underside of its body, with the first pair modified into chelicerae and the rest being walking legs.

It also appears to have been blind, lacking any evidence of eyes despite its fossils being fairly well-preserved – suggesting it lived in conditions where vision wasn’t much use, such as dark murky water or burrowing around in seafloor sediment.

Weird Heads Month #17: Trapjaw Ants From Hell

Ants first evolved sometime in the Late Jurassic or Early Cretaceous, but only really began to diversify about 100 million years ago in the Late Cretaceous after the rise of flowering plants.

One of their evolutionary experiments around that time was a group called the haidomyrmecinae – also known as the “hell ants”.

Known from Asia, Europe, and North America, hell ants had bizarre-looking heads, possessing huge upward-curving scythe-shaped mandibles and a horn-like projection between their antennae.

They were fast-moving arboreal predators that would have fed mainly on other invertebrates such as soft-bodied beetle larvae, and unlike most modern ants their workers were probably solitary hunters. They were capable of gaping their mandibles by almost 180°, and when they got close enough to their targets the long sensory hairs around their faces triggered their jaws to snap vertically upwards, impaling their prey against their horn in a unique trap-jaw mechanism.

Some species also reinforced the exoskeleton of their horns with metal particles, strengthening them against impacts from both struggling prey and their own powerful jaws.

Ceratomyrmex ellenbergeri was one of the oddest-looking of all known hell ant species. Known from a few specimens preserved in amber, with adult workers up to 6mm long (~0.25″), it lived during the Late Cretaceous of Myanmar about 100-94 million years ago.

It had an especially pronounced horn and very long mandibles, which may have been adaptations for tackling significantly larger prey items than other hell ants.

And due to this being a species known from Burmese amber, sadly we also have to address the controversy surrounding these sorts of specimens. This amber is currently mined in incredibly dangerous conditions, often using child labor, with sales of both jewellery and paleontological specimens directly funding the ongoing violent conflict in the region.

It’s the fossil equivalent of blood diamonds, and a huge ethical dilemma for the paleontology community.

Weird Heads Month #06: Trilobite Trains

Trilobites were one of the most successful groups of early animals, existing for over 300 million years – and during that time they developed a huge diversity of weird heads, with various arrangements of spines, horns, eyestalks, and even long snouts and tridents.

But perhaps one of the oddest was the genus Odontocephalus, known mainly from the early-to-mid Devonian and represented here by Odontocephalus aegeria.

Living about 390 million years ago in northeast North America, this trilobite grew up to around 9cm long (3.5″). And although it wasn’t overall very elaborately ornamented, the front margin of its head had a row of extensions that flared out to meet at their tips, forming something resembling the cowcatchers used on trains.

The actual function of this structure is unknown. It might have been purely used for visual display since trilobites had excellent vision – but Odontocephalus was also a fast-moving bottom-dweller, and its “cowcatcher” may have served the same sort of purpose as its modern equivalent, deflecting small obstacles in its path as it trundled along the seabed.