Cambrian Explosion #46: Megacheira

The megacheirans were a grouping of Cambrian stemeuarthropods that had a distinctive pair of “short great appendages” on their heads with long finger-like spines. Their bodies were fully arthrodized into multiple hard-shelled segments, and their double-branched biramous limbs featured both legs and gill-fringed paddle-like flaps.

As small predators and scavengers they were important elements of some Cambrian ecosystems, and probably swam around just above the sea floor using their arm-like great appendages to grab and tear apart food items.

They weren’t really a distinct lineage, more of an “evolutionary grade“, usually considered to be in the “higer stem” close to the common ancestor of all the major euarthropod lineages, or possibly the very earliest actual euarthropods – but some studies instead place them as more “advanced” euarthropods closely related to chelicerates.

Part of this classification disagreement comes down to whether their great appendages were anatomically equivalent to the front appendages of radiodonts or whether they were convergently evolved, and whether one or both of those structures are also related to the development of antennae and chelicerae in later euarthropods or if they became highly modified into the labrum instead. It’s a subject of longstanding and ongoing debate so complicated that it’s known as the “arthropod head problem” – or sometimes “the endless dispute”.

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