Nix Illustration – Page 24 – Paleontology and science illustration, and feathering ALL the dinosaurs

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.

Styxosaurus

Styxosaurus snowii here was one of the largest known elasmosaurids, named after the mythological river separating the worlds of the living and the dead.

Reaching around 11m long (36′), with half of that being entirely neck, it lived during the late Cretaceous period about 83-80 million years ago in what is now the American Midwest – a region that at the time was submerged under a large inland sea.

With pointy interlocking teeth in its proportionally tiny head, Styxosaurus would have fed on slippery aquatic animals like fish and cephalopods, possibly using its long neck to get up close to its targets while the bulk of its body remained out of sight in dark murky waters. Large numbers of gastroliths found in the stomach regions of some specimens would have been used to grind up the hard parts of prey items after they were swallowed whole.

Phiomicetus

Named after the canine-headed Ancient Egyptian god, Phiomicetus anubis is the first fossil cetacean to discovered, described, and named entirely by a team of Arab paleontologists.

Living during the mid-Eocene, about 43 million years ago, in a shallow sea-covered region that is now part of Egypt‘s Western Desert, Phiomicetus was an early protocetid – an amphibious foot-powered swimmer, at a transitional point in the evolution of whales from deer-like terrestrial animals to fully aquatic screaming torpedoes.

About 3m long (~10′), it had large jaw muscles and sharp teeth with wear patterns that suggest it was a raptorial hunter grabbing and snapping at prey with powerful bites. It would have probably tackled fairly big prey compared to other protocetids, hunting things like large fish, turtles, and even smaller whales in an ecological role similar to that of modern orcas.

Along with the distantly-related long-snouted Rayanistes it’s one of the earliest known whales from Africa, giving us further glimpses at a time period when early cetaceans were first dispersing out from the South Asian subcontinent via the ancient Tethys Sea.

Joermungandr

Named after a legendary Scandinavian serpent, Joermungandr bolti here was a recumbisrostran “microsaur” – part of a group of animals that were traditionally considered to be lepospondyl amphibians, but more recently have been proposed to in fact be a lineage of early reptiles.

Discovered in the Mazon Creek fossil beds in Illinois, USA, this species dates to the late Carboniferous period around 310 million years ago. A single near-complete specimen about 5cm long (~2″) preserves impressions of the body outline and numerous tiny scales, giving us a pretty good idea of what it looked like in life.

Joermungandr had a long streamlined tubular body with small limbs and a short tapering tail, and a stubby snout with fused bones heavily reinforcing its skull. Along with microscopic ridges on its body scales that resemble the dirt-repelling scales of some modern reptiles, this combination of features suggest it was a headfirst burrower that wriggled its way through soil with snakelike motions.