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Cambrian Explosion Month #22: Phylum Annelida – Plumes and Tails

Annelids in the Cambrian Period were mainly represented by bristle worm polychaetes, with most known species belonging to early stem lineages. And while more modern-style polychaetes would become abundant during the early Ordovician, the earlier Cambrian forms were still surprisingly diverse.

Cambrian Explosion Month #20: Phylum Annelida – Early Worms

Annelids are a large phylum of segmented worms, found worldwide in marine, freshwater, and terrestrial habitats. Their modern diversity includes bristle worms, tubeworms, earthworms, and leeches, and many burrowing species are hugely ecologically important due to their activities aerating and enriching soil and sediments.

As part of the spiralian lineage they’re considered to be members of a grouping called lophotrochozoans, closely related to animals like molluscs, brachiopods, bryozoans, and ribbon worms. But the exact positions of everything within that evolutionary tree is currently a bit uncertain, with different studies coming up with different answers – annelids might be a basal lineage of lophotrochozoans, or they might be closely related to molluscs, or they might instead be the closest relatives of ribbon worms and flatworms.

Like many other soft-bodied animals annelids have a poor fossil record. The Ediacaran Kimberella and Namacalathus may have been early lophotrochozoans, and the abundant “small shelly fossils” of cloudinids known from the Ediacaran-Cambrian boundary (~541 million years ago) may represent the tubes of early annelids. But otherwise some of the first definite annelid body fossils are bristle worm polychaetes from the early Cambrian.

Cambrian Explosion Month #19: Phylum Chaetognatha

Chaetognaths, commonly known as arrow worms, are a major component of marine planktonic ecosystems all around the world. They’re a fairly small phylum in terms of diversity, with only about 120 known modern species, but in sheer numbers of individuals they’re incredibly abundant – making up as much as 15% of total zooplankton biomass worldwide. They play an important role as predators, feeding on things like copepods, fish larvae, and each other, and can be so voracious that they’re sometimes nicknamed “tigers of the zooplankton”.

And they’ve been doing it for a very long time.

The appearance of protoconodont “teeth” at the start of the Cambrian (~541 million years ago) suggests that arrow-worm-like gnathiferans were some of the first active swimming planktonic predators – taking advantage of ecosystems that were becoming increasingly complex around that time, and laying the early foundations for more modern-style marine food chains.

Unfortunately we don’t know much about their evolutionary origins, with their small fragile soft bodies leaving only a very patchy fossil record. Their relationship to other animals was also rather enigmatic for a long time, and they were only very recently identified as being part of the gnathiferans.

But their ancestors may have been something like Dakorhachis thambus.

Cambrian Explosion Month #18: Stem-Gnathifera

Protostomes are the other major evolutionary branch of bilaterian animals, and by far the most numerous with over a million known modern species (and probably several times more than that still undiscovered). This lineage is distinguished from the deuterostomes based on both embryo development and genetic studies, with the two groups estimated to have shared a common worm-like ancestor sometime back in the Ediacaran Period.

For the rest of this month we’ll be featuring the spiralians, a branch of the protostomes that includes modern annelid worms, molluscs, and brachiopods. Meanwhile their cousins the ecdysozoans will be the focus of the entire second month of this series, later this summer, due to their incredibly rich Cambrian fossil record.

The earliest spiralians must have diverged from other protostomes more than 558 million years ago, if Kimberella and Namacalathus really were early members of the group, but more definite fossils only appear at the start of the Cambrian (~541 million years ago) with protoconodont “teeth” – once thought to be from early vertebrates, but now recognized as probably being jaw elements from a group of spiralians known as gnathiferans.

Cambrian Explosion Month #17: Phylum(?) Vetulicolia & Other Early Deuterostome Weirdos

Vetulicolians were a group of odd Cambrian animals known from between about 520 and 505 million years ago. The front half of their bodies were large and streamlined, with a prominent mouth, no eyes, and five pairs of openings that seem to have been gills, with some species having a rigid exoskeleton-like carapace. Their back half was slender, segmented, and flexible, and functioned as a tail for swimming, giving them an overall appearance like alien tadpoles.

Their evolutionary affinities have been problematic for a long time, but evidence of a notochord in some specimens suggest they were probably related to the chordates in some way. Sometimes they’re considered to represent their own phylum, but they might also be stem-chordates or stem-tunicates.

Cambrian Explosion Month #16: Phylum Chordata – Vertebrata

Vertebrates are by far the most numerous and diverse group of chordates today, with over 65,000 known species including fish, amphibians, reptiles, birds, and mammals. Genetic studies show that they’re closely related to the weird bag-like tunicates, and their shared common ancestor was probably something lancelet-like.

And the earliest true vertebrates would have looked something like Haikouichthys ercaicunensis.

Cambrian Explosion Month #15: Phylum Chordata – Early Forms & Tunicates

Chordates are one of the most diverse animal phyla, ranging from tiny lancelets to sac-like tunicates to all fish and tetrapods. They share a common deuterostome ancestor with echinoderms and hemichordates, probably diverging from them sometime in the Ediacaran Period, and are characterized by having specific anatomical features at some point during their life cycle – a notochord, a dorsal nerve cord, pharyngeal slits, a post-anal tail, and an endostyle.

The earliest chordates were all small soft-bodied animals with no mineralized tissues, so their fossil record is poor aside from rare locations with exceptional preservation. But one of the best known examples is Pikaia gracilens from the Canadian Burgess Shale fossil deposits (~508 million years ago).

Cambrian Explosion Month #14: Phylum(?) Cambroernida

Modern hemichordates and echinoderms are the closest living relatives of each other, part of a larger lineage of deuterostome animals known as ambulacrarians – but they also seem to have had some other strange cousins during the Cambrian.

Cambroernids were a bizarre group with branching feeding tentacles and a gut enclosed in a coiled sac. They came in a range of forms from worm-like to cup-like to disc-shaped, and despite their fossils being known since the early 1900s their evolutionary affinities were a longstanding problem. Various species had been interpreted in the past as sea cucumbers, jellyfish, tunicates, gnathiferans, or lophophorates, but in recent years they’ve been recognized as all being related, and linked to the ambulacrarians.

And it’s still not entirely clear where in that group they actually belong. They were probably a weird early stem lineage, but they might also be early stem-hemichordates or stem-echinoderms.

Cambrian Explosion Month #13: Phylum Echinodermata – Sticking Around

It seems like echinoderms became five-way symmetric incredibly quickly following the group’s first appearances in the early Cambrian. We don’t really know why this secondary radial symmetry evolved in the group – but we do know that the common ancestors of all modern pentaradial echinoderms were suspension-feeding animals that lived attached to the sea floor.

And those ancestors were probably a group called the edrioasteroids.

Cambrian Explosion Month #12: Phylum Echinodermata – Radial Revolution

While many of the earliest echinoderms had bizarre asymmetrical forms, at some point members of their lineage adopted radial symmetry instead – a development that would eventually lead to the familiar five-way symmetry of most modern species.

And they may have transitioned to that via three-way symmetry.