November 2021 – Page 3 – Nix Illustration

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.

Cambrian Explosion #38: Phylum Onychophora

Onychophorans, or “velvet worms”, are a small phylum of terrestrial worm-like panarthropods found in the tropics and temperate regions of the Southern Hemisphere, with around 200 known modern species. Resembling caterpillars, they have tubular segmented bodies, stubby clawed legs, and the unique ability to shoot jets of immobilizing glue-like slime at their prey.

It’s not clear when exactly in their evolutionary history velvet worms shifted from marine to living fully on land, but the oldest terrestrial onychophoran fossils come from the Carboniferous (~300 million years ago). And much like their cousins the tardigrades the ancestry of this group seems to trace all the way back to somewhere in the “evolutionary grade” of Cambrian lobopodians.

Cambrian Explosion #37: Panarthropoda & Phylum Tardigrada

Now that the other ecdysozoans are out of the way, we’re actually getting to the main stars of this series: the panarthropods.

Containing all the modern arthropods, tardigrades, and velvet worms, along with the extinct lobopodians and dinocaridids, the earliest panarthropods are thought to have originated somewhere towards the end of the Ediacaran Period and then diversified incredibly quickly in the early Cambrian. Trace fossils and “small shelly fossils” hint at an arthropod presence only a few million years after the start of the Cambrian (~537 million years ago) and by 521 million years ago there were already trilobites all around the world – which means by that point the ancestors of all the other major lineages of true arthropods (chelicerates, myriapods, and pancrustaceans) must have also already diverged from each other.

Unfortunately due to the patchy nature of the fossil record and the rarity of good preservation conditions we only really have a few snapshots of this group’s diversity from points in the Cambrian after the main burst of their evolutionary explosion had already happened. But as more and more fossils are discovered we’re gradually piecing together a fairly decent idea of early panarthropod relationships, and some of the most famous “weird wonders” that were once thought to be entire separate “failed experiment” animal phyla are now properly reunited with their relatives.

There’s still disagreement on a lot of the details of their evolutionary relationships, but at least we’re pretty sure nobody’s reconstructing these poor things upside-down and back-to-front anymore.

…Probably.

Cambrian Explosion #36: Phylum Loricifera

Even more obscure and poorly-understood than the mud dragons, loriciferans weren’t even discovered until the 1970s. Over 40 living species of these tiny meiofaunal animals are currently known, but much like the kinorhynchs there are probably many more still to be described.

Less than 1mm long (0.04″), their most distinctive feature is the “lorica”, a stiff corset-like casing surrounding their body. They’re also the first multicellular organisms discovered able to live completely without oxygen in a deep basin in the Mediterranean Sea.

Cambrian Explosion #35: Phylum Kinorhyncha

After the slightly unfortunately-shaped priapulids, let’s move on to something much safer-for-work: dragons!

More accurately, kinorhynchs, tiny spiky scalidophoran worms with the delightful common name of “mud dragons“. These animals weren’t even discovered until the mid-1800s and are so small – less than 1mm (0.04”) in size – that they’re considered to be “meiofauna“, wriggling around between grains of sediment using the spines on their heads to pull themselves along.

Cambrian Explosion #34: Phylum Priapulida

Named for their resemblance to human penises, priapulids (or “penis worms”) are marine scalidophoran worms that live on or in muddy seafloor sediment, with some species having a surprisingly high tolerance for oxygen-poor environments and toxic levels of hydrogen sulfide. Despite being a rather low-diversity phylum with only around 20 living species, they’re widespread and sometimes very numerous, with over 80 adult individuals per square meter (~10ft²) recorded in some locations.

The earliest definite modern-style priapulid in the fossil record comes from the late Carboniferous (~308 million years ago), but their ancestry was probably somewhere in the early Cambrian among the taxonomic mess of palaeoscoloecids and archaeopriapulids.

Cambrian Explosion #33: Early Scalidophora

The earliest branches of the ecdysozoan evolutionary tree are made up of the scalidophorans – animals with spiny retractable proboscises, represented today by the worm-like priapulids and kinorhynchs, and (sometimes*) the weird little loriciferans.

Cambrian Explosion #32: Rise of the Arthropods

The world of the Cambrian Period was a strange combination of both familiar and alien. The land would have seemed rather barren, populated mainly by microbes and algae, yet the oceans teemed with creatures already identifiable as sponges, comb jellies, jellyfish, acorn worms, vertebrates, echinoderms, arrow worms, annelids, molluscs, and brachiopods – small and primitive-looking in some cases but still recognizable enough.

But at the same time there were “weird wonders” everywhere, things much harder to identify, with shapes so bizarre that their initial discovery was met with laughter.

Animal life was exploring so many different possibilities for body plans and ecologies, and one lineage in particular dominated this explosion of evolutionary experimentation: the arthropods.

Arthropods are represented today by the chelicerates (sea spiders, horseshoe crabs, and arachnids), myriapods (millipedes and centipedes), crustaceans, and insects, and together these groups make up over 80% of all known living animal species and are vital parts of almost every ecosystem on the planet.