Abstract

Cladistic analysis of the Middle Jurassic ammonite radiation shows how the Aalenian-age ancestral stock, known variously as the Hammatocerataceae or Hammatoceratinae, gave rise to numerous new groups in Bajocian times. Cladistic analysis shows this ancestral stock is not a single lineage but two: the ‘Erycites group’, which gave rise to the Stephanocerataceae, and the ‘Hammatoceras group’, from which all other Bajocian groups derived.

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