cambrian explosion – Page 4

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.

Nesonektris

Nesonektris aldridgei here was one of the bizarre vetulicolians, a group of Cambrian animals that lived between about 520 and 505 million years ago.

Known from the Emu Bay Shale fossil deposits in Kangaroo Island, South Australia (~514 million years ago), Nesonektris was one of the larger known vetulicolians, growing to at least 17cm long (~6.5″). Like most of its relatives it had a large streamlined forebody with a mouth opening at the front, and no obvious appendages or sensory structures. A groove down each side may have housed gill openings, and a segmented flexible tail provided propulsion for swimming.

Very little is known about the ecology of these animals. They were clearly adapted for active swimming in the water column, and may have filter-fed on plankton – but some other vetulicolians have been found preserved with their guts full of seafloor sediment, suggesting some sort of detritivorous lifestyle instead.

Their evolutionary relationships are also still uncertain, but preservation of what appears to be a notochord in Nesonektris suggests that vetulicolians may have been part of the chordate lineage, possibly close relatives of tunicates.

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

Cambrian Explosion Month #31: Phylum Brachiopoda

While modern brachiopods superficially resemble clams, they’re not actually very closely related to each other. Clams are bivalve molluscs, related to snails and squid, while brachiopods are lophophorates related to bryozoans and horseshoe worms.

Their two shell valves are also arranged very differently – while bivalve shells originate from the left and right sides of their bodies, brachiopods grow theirs on the top and bottom.

They first appear in the fossil record in the early Cambrian, about 530 million years ago, but they may have actually diverged from a tommotiid-like ancestor as far back as the late Ediacaran. Only around 300 species survive today, but during the Paleozoic they were some of the most abundant filter-feeding and reef-building animals with tens of thousands of fossil species known. Different species tended to have strict habitat and temperature preferences, and so their fossils are also useful indicators of how ancient climates changed over time.

Cambrian Explosion Month #30: Phylum(?) Hyolitha

Hyoliths were a group of small shelled animals that first appeared in the fossil record just after the start of the Cambrian, about 536 million years ago. They had conical calcareous shells with a lid-like operculum, and some species also featured long curling spines that made them look like ice-cream cones with mammoth tusks.

They were so odd that for a long time their evolutionary relationships were unknown. They were generally accepted to be lophotrochozoans, but some studies considered them to be part of their own unique phylum while others tended to place them as being closely related to molluscs.

It wasn’t until 2017 that well-preserved soft tissue fossils revealed a tentacled feeding structure that resembled a lophophore – and hyoliths finally found their place in the lophotrochozoan family tree as close relatives of brachiopods and horseshoe worms, possibly even being a stem lineage within the brachiopod phylum.

However, this isn’t universally accepted and some recent studies continue to dispute it. The feeding organ of a different hyolith fossil has been interpreted as not being a lophophore, classifying the group as an early lophotrochozoan stem lineage, while an analysis of shell microstructure has instead suggested realigning them with molluscs. I’m grouping them with brachiopods here, but future discoveries might still make this obsolete.