protostome – Page 5 – Nix Illustration

Cambrian Explosion Month #23: Phylum Mollusca – The Stem Weirdos

Molluscs are one of the largest animal phylums, second only to the arthropods, and are also hugely diverse, found in marine, freshwater, and terrestrial environments all over the world. Not only are familiar modern animals like bivalves, slugs and snails, and squid and octopuses included in this huge lineage, but also nautiloids, chitons, tusk shells, monoplacophorans, worm-like aplacophorans, and the extinct ammonites and orthocerids.

Like the annelids they’re lophotrochozoan spiralians, and their exact evolutionary relationships within that group are a bit uncertain. But their fossil history seems to go back at least 558 million years with the “mollusc-like” Ediacaran Kimberella, and the earliest members of most major mollusc lineages had probably already diverged from each other before the start of the Cambrian.

The common ancestor of all molluscs probably had features like an unsegmented body, a muscular foot on their underside, a mantle and mantle cavity, a radula, and possibly a tough but non-mineralized leathery “shell” – and Odontogriphus omalus may represent an early stem lineage retaining that basic body plan into the mid-Cambrian.

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.

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.