Extreme lower jaw elongation in a placoderm reflects high disparity and modularity in early vertebrate evolution

Jaws are a key vertebrate feature that arose early in our evolution. Placoderms are among the first jawed vertebrates; their fossilsyield essential knowledge about the early diversification of gnathostome feeding strategies, diets and modularity. Modularity can be expressed through disproportional lengths of lower and upper jaws as in swordfish or halfbeaks. Alienacanthus malkowskii is an arthrodire from the Famennian of Morocco and Poland, whose most remarkable feature is its lower jaw, which is twice as long as the skull. This is theoldest record of such extreme jaw elongation and modularity in vertebrates. The gnathal plates of Alienacanthus posses ssharp, posteriorly recurved teeth that continue anterior oft he occlusion in the inferognathals. The dentition suggests a catching and trapping live prey function, and the jaw occlusion is unique among placoderms. This armoured ‘fish’ expands themorphological and ecological diversity during one of the first radiations of jawed vertebrates with a combination of features so far unrecorded for arthrodires.

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