#stem-mollusc

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.

Known from the Canadian Burgess Shale fossil deposits (~508 million years ago), this vaguely slug-like animal grew up to 12.5cm long (5") and was probably a grazer, crawling over the seafloor and feeding on microbial mats.

Its classification has been debated for some time, sometimes being placed closer to annelids or brachiopods, but more recent evidence of a radula-like feeding apparatus and a muscular foot suggest it really was a stem-mollusc.

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The affinities of the strange-looking Wiwaxia corrugata have been even more controversial, with its classification flipping back and forth between annelids and molluscs over the last few decades. But in recent years the discovery of a radula-like mouth apparatus and a muscular crawling foot have swung the general opinion more firmly towards stem-mollusc.

Originally discovered in the Burgess Shale, fossils of various wiwaxiid species have now also been found in Cambrian deposits in Utah, Russia, China, Europe and Australia, ranging from about 518 to 504 million years ago – showing that this lineage had a worldwide distribution, and that their body plan barely changed for at least 15 million years.

Wiwaxia had a domed body covered in chitinous plates and spines with a ribbed texture. It could reach around 5cm long (2"), with the longest spines about the same length, and individual sclerites were shed and replaced as it grew.

It seems to have also been a mat-grazer, with its armor serving to protect it against predators like anomalocaridids. Some specimens have small Nisusia brachiopods attached to their plates or spines in what seems to be a commensal relationship – the brachiopods gained a "mobile home" that allowed them to filter-feed higher up in the water column while also being protected from predators (at least until that particular sclerite was shed), and Wiwaxia wouldn't have been harmed or affected by the presence of these "hitchhikers".

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Early grazing molluscs like Odontogriphus and the wiwaxiids seem to disappear from the fossil record in the mid-Cambrian, around the time that the microbial mats they relied on began to become less common. However, some of them may have survived for longer than previously thought, since fossils of isolated spines from the early Ordovician of Morocco (~480 million years ago) and the mid-Ordovician of Portugal (~465 million years ago) may be evidence of late-surviving wiwaxiids, possibly relict populations living in isolated refugia environments.

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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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Halkieria evangelista is the best known species of halkieriid, found in the Sirius Passet fossil deposits in Greenland (~518 million years ago). Up to 8cm long (~3"), it was preserved with all of its armor fully articulated, making it vital in our understanding of the appearance of other halkieriid species that are generally only found as scattered separate pieces.

But those little isolated pieces are incredibly common and widespread, known from Early Cambrian deposits all around the world between about 530 and 513 million years ago – and if halkieriids were aculiferans this suggests their ancestors diverged very early in mollusc evolution, possibly in the late Ediacaran.

While halkieriids largely disappeared after 513 million years ago, fossils from the Canadian Burgess Shale (~508 million years ago) and Australia (~507 million years ago) indicate that the group survived for a little while longer, at least into the mid-Cambrian.

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One of the earliest definite aculiferans is Qaleruaqia sodermanorum from the Aftenstjernesø Formation in Greenland (~516-513 million years ago).

Only known from a few isolated shell plates, its full appearance and size is uncertain. But based on other fossil aculiferans it was probably a worm-like animal around 1cm long (0.4") with a total of eight shells running along its back. Numerous microscopic holes in its shells may represent sensory pores similar to those in modern chitons.

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