Cambrian Explosion Month #25: Phylum Mollusca – Shelling Out

The exact evolutionary relationships of the main groups of modern molluscs are rather debated, with several different proposed family trees. But one of the main possibilities is that there are two major lineages: the aculiferans and the conchiferans.

Modern conchiferans include slugs and snails, cephalopods, bivalves, tusk shells, and monoplacophorans – all groups that ancestrally have either a single-part shell or a two-part bivalved shell, with some lineages later becoming secondarily shell-less.

The ancestral conchiferans are thought to have been monoplacophoran-like molluscs, limpet-like with a cap-shaped shell, and likely diverged from a common ancestor with the aculiferans around the end of the Ediacaran. (But modern monoplacophorans probably aren’t “living fossil” descendants of early Cambrian conchiferans, and may instead be close relatives of cephalopods that have convergently become similar in appearance to their ancestors.)

Some of the earliest conchiferans were the helcionelloids, a lineage of superficially snail-like molluscs with coiled cone-shaped mineralized shells. They appeared in the fossil record at the start of the Cambrian (~540-530 million years ago) and lasted until the early Ordovician (~480 million years ago), and have been found all around the world as components of the “small shelly fauna“.

And while they’re usually tiny, only a couple of millimeters in size, they may actually represent juveniles or larvae – there’s evidence that at least some species grew up into much larger 2cm (0.8″) limpet-like adult forms.

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Cambrian Explosion Month #24: Phylum Mollusca – Coats of Mail

Much like Odontogriphus and Wiwaxia, the evolutionary relationships of a group called the halkieriids have been debated for a long time. These animals looked like “slugs in chain mail“, covered in thousands of tiny overlapping mineralized armor plates along with a larger shell plate at each end.

In the past they’ve been assigned to different parts of the lophotrochozoan family tree, sometimes being placed closer to annelids or brachiopods, but at this point they’re generally accepted to be molluscs. The spiny species Orthrozanclus may link halkieriids with wiwaxiids in a larger “halwaxiid” lineage of early molluscs – or they might instead be early members of a group called aculiferans

Aculiferans are represented in modern times by chitons and aplacophorans, and they’re distinguished from all other molluscs by having either eight shell valves (chitons) or no shell at all and a worm-like body covered with tiny calcareous spines (aplacophorans).

(Also chitons are especially weird, with magnetite teeth and thousands of eyes in their armor plates.)

A related fossil species called Calvapilosa kroegeri from the early Ordovician of Morocco (~480 million years ago) seems to link halkieriids with aculiferans, placing the chain-mail-slugs as a stem lineage close to the common ancestor of modern forms.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Cambrian Explosion Month #11: Phylum Echinodermata – Increasing Asymmetry

During their early evolution, echinoderms started developing unusual asymmetric body plans – and some of them were so strange-looking that for a while it wasn’t clear if they even were echinoderms.

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