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Cambrian Explosion #48: Panchelicerata

Now, at just over halfway through the month, we’ve finally reached the proper euarthropods!

One of the major branches of this group are the chelicerates, which include modern horseshoe crabs, arachnids, and (probably) sea spiders, and a few extinct groups like the sea scorpions. Characterized by having chelicerae mouthparts, they’re a very diverse and successful group of animals, with spider and mite species being especially numerous.

Their lineage is estimated to have diverged from other euarthropods around the start of the Cambrian 540 million years ago, and along with their ancient stem-chelicerate relatives they make up a slightly larger grouping known as panchelicerates. Sometimes these are also combined even further with trilobites and their close relatives to make up the arachnomorphs.

Sea spiders are usually classified as the earliest branch of the chelicerates, but they’ve also been suggested to possibly be a completely separate lineage of the very earliest-diverging euarthropods instead. Late Cambrian microfossils of what may be sea spider larvae have been found in the Swedish Orsten Lagerstätte (~497 million years ago), and if they are actually sea spiders (and sea spiders are actually chelicerates) then these would represent the earliest known true chelicerates in the fossil record.

But the earliest known panchelicerates come from slightly earlier in the Cambrian.

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.

Cambrian Explosion #44: Isoxyida

Isoxyids were a group of small and somewhat enigmatic panarthropods found all around the world’s oceans during the mid-Cambrian, dating to between about 518 and 504 million years ago.

Their most distinctive feature was the large semi-circular carapace covering their bodies, with some species also having long defensive spines at the front and rear ends. They had spherical stalked eyes and two large appendages on their heads – sometimes shaped like sensory antennae, and sometimes like raptorial front appendages – but it’s unclear whether these were anatomically equivalent to the appendages of radiodonts or if they were convergently evolved from a different set of front limbs.

Their body limbs each had two distinct branches (a biramous condition), with a paddle-like structure fringed with gills above, and a slim jointed leg underneath. A small tail fan stuck out from the back of their carapace and was probably used for steering while swimming.

Cambrian Explosion #43: Radiodonta – Splash Of The Titans

The most famous radiodont is the classic charismatic Anomalocaris, but there were plenty of other members of the group who explored very different lifestyles. Instead of big apex predators, some of them became equally large filter feeders – the whales of the Cambrian.

Cambrian Explosion #42: Radiodonta – The Strangest Shrimps

Once the dinocaridids started exploring active swimming lifestyles, one branch of this group quickly became incredibly successful and diverse: the radiodonts. With muscular swimming flaps, head carapaces, stalked compound eyes, disc-like mouths, and large spiny front appendages, they occupied a wide range of ecological roles – and some of them went on to became giants, some of the largest animals of their time.

They were some of the closest relatives to the ancestors of the true arthropods (or “euarthropods”). And while the earliest radiodont fossils are known from about 518 million years ago, much like other panarthropods their actual evolutionary origins have to go back much deeper into the early Cambrian since they already lived alongside representatives of various early euarthropod groups.

Cambrian Explosion #41: Dinocaridida

Probably evolving from Siberion-like lobopodians, the dinocaridids were an “evolutionary grade” of panarthropods that were closely related to the ancestors of true arthropods. These animals were characterized by specialized front appendages on their heads and large swimming lobes along the sides of their segmented bodies, and their group included some of the most famous of the Cambrian “weird wonders”.

The earliest branches of the dinocaridids were the “gilled lobopodians”, which had lobopodian-like legs on their undersides and gills on the upper surfaces of their body lobes. The flap-like structures may have initially evolved just to provide a larger surface area for respiration, but they were quickly co-opted for swimming purposes and opened up a whole new range of ecological opportunities to the ancestral dinocaridids.

Cambrian Explosion #40: Phylum(?) Lobopodia – Then Somehow They Got Weirder

Most lobopodians looked fairly similar to each other, many resembling armored velvet worms, but some of these early panarthropods evolved much more diverse body plans and ecologies during the mid-Cambrian.

Cambrian Explosion #39: Phylum(?) Lobopodia – The Psychedelic Spectacle

The earliest panarthropods were the lobopodians, small soft-bodied segmented worm-like animals with many pairs of legs, usually tipped with claws, and sometimes elaborate ornamentation on their bodies such as spikes, armor plates, and fleshy bumps.

Since they occupy a basal position among panarthropods it’s likely they weren’t really a distinct lineage or phylum, and were probably more of an “evolutionary grade” of various weird early forms, with tardigrades, velvet worms, and potentially all other arthropods descending from somewhere among their ranks.

While little is known about their lifestyles they seem to have had a wide range of ecological roles, including predators, grazers, scavengers, and even filter-feeders. They’re mostly known from around the world in the mid-Cambrian, but fossils from over 200 million years later in the late Carboniferous (~310 million years ago) hint that these panarthropods may actually have been successful elements of Paleozoic marine ecosystems for a very long time.

There’s a whole iceberg of weirdness and mystery surrounding these animals and I can’t possibly do them all justice in only a couple of days. So we’ll just have to take a quick look at a few of them before moving on.