halwaxiida – Nix Illustration

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.

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.

Tag: halwaxiida

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