worm – Nix Illustration

Ursactis

Soft-bodied annelid worms only very rarely fossilize, so the group’s origins during the Cambrian Period are still rather poorly understood. So far about thirteen different species have been found in sites of exceptional preservation, showing that even very early on in their evolution these worms had already diversified into a wide range of ecologies including bottom-feeders, carnivores, swimmers, tube-builders, and even symbiotes sharing living space with early acorn worms.

Ursactis comosa here adds a fourteenth species to the list. Found in a newly-discovered outcrop of the 508-million-year-old Burgess Shale fossil deposits in western Canada, it’s known from nearly 600 specimens clustered together in several large groups, making it the current best-known and most numerous of all Cambrian annelids.

Up to about 1.5cm long (~0.6″), it was a polychaete-like worm bearing bundles of long bristles. There was a pair of large sensory palps on its head, and its body was made up of an unusually small number of segments – just 10, with larger individuals just increasing the size of their segments instead of adding on additional ones like most modern annelids.

Unlike other Cambrian annelids it also shows some evidence of basic tagmatization, differentiating some of the rear segments of its body with much longer bristles.

The large numbers of Ursactis found preserved in one place suggests these worms were exhibiting some sort of swarming behavior. Since ages from juveniles to fully-grown adults are represented together, and their anatomy indicates they were crawling detritivores, they were probably all taking advantage of a particularly nutrient-rich patch of seafloor at the time they were abruptly buried in a mudslide.

Strange Symmetries #02: Oh Worm

Living during the Cambrian Period about 518 million years ago, Wufengella bengtsonii was discovered in the Chinese Chengjiang fossil deposits and was recently named and described in late 2022.

It was a small worm-like animal about 1.6cm long (~0.6″), with bundles of long bristles along its sides and flap-like structures on its underside. Its back was also covered with sclerite armor arranged in a strangely asymmetrical fashion, with larger overlapping plates in the middle and numerous smaller cap-like sclerites distributed unevenly along each side.

Although its bristles and appendages resemble those of annelid worms, the distinctive structure of the sclerites identifies Wufengella as being a member of the tommotiids – early relatives of modern lophophorates (bryozoans, brachiopods, and horseshoe worms).

Its discovery actually confirms an old prediction that lophophorates probably originated from armored worm-like animals, representing an evolutionary link between earlier free-living annelid-like forms and later immobile filter-feeding tommotiids.

It’s not known why the armor on Wufengella‘s back was so unevenly organized – but some of the later tube-like tommotiids also had weird symmetry going on, with forms like Eccentrotheca having irregular sclerites arranged in a spiral around their bodies.

Cambrian Explosion Month #22: Phylum Annelida – Down the Wormhole

Among the various stem-polychaete worms known from the Cambrian, the existence of more modern-style annelids like Pygocirrus hint that the common ancestor of modern forms might have evolved much earlier than previously thought. But this was complicated by the fact that all the known stem-polychaetes seem to have been active crawlers or swimmers, while the oldest modern lineage of polychaetes are burrowers, mostly sedentary, and sometimes tube-dwelling.

So if “crown group” forms that lived buried in the seafloor sediment must have diverged at least as far back as the early Cambrian, where were the fossils of them?

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.