#euarthropoda

Cambrian Explosion #57: Tuzoiida

What were tuzoiids?

We don't know.*

Despite hundreds of specimens having been found, and around 20 different species being described, these arthropods are an ongoing puzzle.

They're known from between about 518 and 505 million years ago, in deposits associated with tropical and subtropical regions all around the world. They had large spiny bivalved carapaces up to 18cm long (7"), shaped like an upside-down domed taco shell, with a distinctive reticulated net-like surface ornamentation – but the rest of their ecology and anatomy is very unclear.

Most fossils are just empty carapaces, which appear to have been made of unmineralized chitin. Rare examples of soft-part preservation show they had a pair of stalked eyes sticking out the front, and a pair of short simple antennae, but impressions of the rest of their bodies are fragmentary and indistinct enough to not be particularly helpful.

They may have been related to the similar-looking isoxyids, or they might have closer affinities to the pancrustaceans or possibly even the equally enigmatic thylacocephalans. 

Unless we find some better fossils with clearer anatomy, they're going to remain a mystery.

Tuzoia canadensis was one of the pointier-looking tuzoiids, known from the Canadian Burgess Shale deposits (~508 million years ago). About 10cm long (4"), its carapace was lined with many long spines, with a more prominent spiky side frill than most other species.

Its reticulated ornamentation had fairly large polygonal facets, and like in other tuzoiids this is thought to have had a strengthening function – allowing it to create a large but very thin carapace with enough structural integrity to hold its shape.

———

* Edit: As of 2022, tuzoiid soft tissues have been found! Turns out they're early mandibulates and probably closely related to hymenocarines.

———

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".

———

Fortiforceps foliosa was a fairly rare megacheiran known from the Chinese Chengjiang fossil deposits (~518 million years ago).

About 4cm long (~1.5"), it had stalked eyes, 20 body segments, and a fluke-like tail. There was a pair of club-tipped antenna-like sensory structures on its head, and its stout great appendages featured three long scissor-like spines.

A recent analysis of its anatomy supports the idea that megacheirans were stem-euarthropods, and that their great appendages were derived from the front appendages of radiodont-like ancestors.

———

Leanchoilia superlata was a rather common megachairan in the Canadian Burgess Shale deposits (~508 million years ago), known from over a thousand specimens.

It was about 5cm long (2") and had a hook-like "horn" at the front of its head, along with a fairly slender body with wide flaring lobes on each segment, and relatively long limbs. It had one or two pairs of downwards-facing eyes (the preservation of them is unclear) and its great appendages featured long whip-like feelers extending from the spines.

A recent study of a closely related species from China, Leanchoilia illecebrosa, shows it possessed a reduced labrum in front of its mouth, and supports the idea that megacheirans were very close relatives of chelicerates with their great appendages convergently evolved from a different set of front limbs.

———

Cambrian Explosion #54: Trilobita – Transform and Roll Up

Most trilobites were able to roll themselves up into a protective ball – a behavior known as enrollment or volvation – exposing just their heavily armored backs to attackers. They're often found fossilized curled up like this, and rare preservation of soft tissues shows that they had a complex system of muscles to help them quickly achieve this pose while simultaneously tucking their antennae and all their limbs safely inside their enrolled shells.

Some species also developed sharp defensive spines and spikes that jutted out when they enrolled, making themselves even more daunting to potential predators in one of the earliest known examples of an evolutionary "arms race".

———

The agnostids were a group that first appeared in the early Cambrian very soon after the redlichiids and ptychopariids, and lasted until the end of the Ordovician about 444 million years ago. They probably evolved from some of the first ptychopariids*, and were highly specialized compared to other early trilobites, with large head and tail shields that usually looked almost identical, sometimes lacking eyes entirely, and only having two or three thorax segments – showing some convergent similarities to their nektaspid cousins.

( * There's been some debate about whether agnostids are actually trilobites, or whether they're closely related "trilobitomorph" artiopodans or even stem-crustaceans that just happened to somewhat resemble trilobites. This is due to soft-tissue preservation revealing limb anatomy that was very different from trilobites and looked more like the legs of crustaceans. But despite some other oddities, like having six pairs of head appendages instead of trilobites' usual four, the rest of their anatomy still places them firmly as artiopods, and as either trilobites or almost-trilobites.)

Found all around the world mainly in deeper-water environments, agnostids probably spent most of their time crawling around on the sea floor, but they may also have been able to swim in a semi-enrolled posture. They're also sometimes found preserved sheltering inside the empty hard parts of other animals, such as hyolith shells or Selkirkia tubes.

Some agnostid fossils have been found clustered in large numbers around the remains of other animals, suggesting they may have been opportunistic predators and scavengers, sometimes even cannibalising each other.

Agnostus pisiformis was a typical member of this group, known from the mid-to-late Cambrian (~506-492 million years ago) from sites including Sweden, Siberia, England, Wales, and Canada – widespread and common enough that it can be used as an index fossil to help date rock layers.

It grew to about 1cm long (0.4"), and while its front and back ends looked very similar the tail shield can be identified by a small pair of spines on its outer edge. Despite having only a couple of articulated body segments to work with it was capable of rolling up so its head and tail shield met and fully enclosed its body, forming a pea-sized protective capsule.

———

The corynexochids were another major group of trilobites to appear in the early Cambrian, descending from early redlichiids. They were one of the longest-lived trilobite lineages, lasting until the late Devonian about 376 million years ago, and often had large tail shields and spines adorning their bodies.

Olenoides nevadensis was a fairly rare species of corynexochid, known from the Wheeler Shale (~507 million years ago) and Marjum Formation (~505 million years ago) in Utah, USA 

Up to about 5cm long (2"), it was very spiky for a Cambrian trilobite, with long "cheek" spines, pointy side lobes on its body segments, a row of spines along its back, and four pairs of spines on its tail shield. Like some other early trilobites it wouldn't have quite been able to roll into a tight ball, instead forming more of a "cylinder" – with its bristling spiny segments protecting its partially-exposed sides, and its tail shield lining up with its head to present an intimidating row of spikes.

———

Cambrian Explosion #60: Crustacea – Larvae Larvae Everywhere

One of the characteristic features of the crustacean lineage are their larval forms, passing through various tiny larval stages. They often look nothing like their eventual adult forms and historically weren't even recognized as being the same species, with their complex lifecycles not being properly recognized until the late 1800s.

A lot of Cambrian crustaceans are only known from their larvae, preserved in exquisite microscopic detail in sites of "Orsten-type preservation". Only disarticulated fragments of larger-bodied forms have been found in a few places, and it isn't until much later in the Paleozoic that fossil crustaceans actually seem to become abundant in marine ecosystems.

It's not clear why there's such a bias in their early fossil record compared to most other arthropods, but possibly they were just very very rare animals early on. Adult forms may have mostly lived in places where they just didn't fossilize, while their tiny larvae sometimes dispersed into different environments with a better chance of preservation.

One of the earliest known of these crustacean larvae, and one of the oldest known true crustaceans, is Wujicaris muelleri.

Found in the Chinese Chengjiang fossil deposits (~518 million years ago), this microscopic larvae was just 270μm long (0.01"). It had a wide shallow head shield with a backwards-pointing spine, a pair of eyes, and another long spine projecting from its underside at the front of its body.

Like similarly-shaped modern larvae it probably lived on or just above the seafloor as meiofauna, using its developed head appendages for both locomotion and catching food particles.

For such an early example of a crustacean it's surprisingly similar to modern forms, resembling the "metanauplius" stages of some copepods and barnacles. Along with Yicaris, another larva from the same deposits, it was a probably a basal member of the major crustacean lineage that both those groups are part of: the altocrustaceans.

It suggests that some groups of crustaceans established their specific larval forms very early on in their evolution, and hit on a something that worked so well for them that they've barely needed to change it in over half a billion years.

———

The pentastomids, or "tongue worms", are a very unusual group. Small worm-like animals that almost exclusive parasitise the respiratory tracts of vertebrate hosts, outwardly they don't even look like they're arthropods – but their true affinities are revealed by their chitinous "skin" and arthropod-like nervous system.

Their evolutionary affinities have been controversial, but they're now generally considered to be highly specialized and modified crustaceans and very closely related to the parasitic fish lice. An alternate hypothesis proposes them instead as being surviving early panarthropods, related to tardigrades or lobopodians – but this is based purely on morphology, while the crustacean placement is also supported by genetic evidence.

While their fossil record is very poor, there are several different larvae known from the late Cambrian, including Heymonsicambria scandica.

Found in the Swedish Orsten Lagerstätte (~497 million years ago), this larva was about 0.5mm long (0.02"). Like other tongue worms it had four limbs on its head ending in hooked grasping claws, but unlike modern forms it also had two pairs of vestigial legs further down on its body.

It's unclear if these very early pentastomids were actually parasitic, or what their hosts would have been at the time if they were. Conodonts have been proposed as potential hosts, but they also may have initially externally parasitized other types of arthropods – a Silurian-aged fossil shows an ancient tongue worm "caught in the act" of clinging on to an ostracod.

———

Cambrian Explosion #61: Crustacea – Little Wigglers

We're finally at the end of this series, and to finish off let's look at one of the few types of Cambrian true crustaceans that are known only from fully mature adults: the skaracarids.

These tiny soft-bodied meiofaunal animals are known from late Cambrian areas of "Orsten-type preservation" in Sweden and South China, with a possible additional fragmentary occurrence in Poland – suggesting that they had a global distribution.

Three different species have been described, with Skara anulata from the Swedish Orsten Lagerstätte (~497 million years ago) being the largest of them.

It was still minuscule even as an adult, just 1.5mm long (0.06"), and had an eyeless head with a small protrusion at the front tip that may have been some sort of sensory organ. It had five pairs of appendages on its head and one pair at the front of its body, with most of them bearing long dense spines along their edges, then the rest of its long body was limbless and ended in a forked "tail".

With its spiny limbs it would have been a filter-feeder, probably swimming just above the seafloor and either sifting out suspended particles from the water or brushing along the surface of the sediment to stir up organic matter and microbial biofilms from the mud.

The skaracarids exact evolutionary relationships are uncertain, although they've been proposed to be related to either the copepods or the oligostracan crustaceans, particularly the mystacocarids and the fish lice.

———

There's still a lot of murkiness in the crustaceans' Cambrian fossil record, and disputes over the arthropod evolutionary tree in general, so there's plenty of room for future discoveries to give us some interesting surprises.

And one day, probably not even that far in the the future, everything I've written and illustrated for this series will be just as ridiculously outdated as the bizarre old reconstructions of Hallucigenia and the hypotheses about the Burgess Shale "weird wonders" all representing near-alien "failed" branches of life with no living relatives.

I can't wait to see what weird things we discover next.

———