Sexual dimorphism of the mineralised part (shell, carapace) of invertebrate animals is recognised in many fossil groups; for example, in ammonoids, trilobites and especially in ostracod crustaceans, arguably the most species-rich and specimen-abundant group of arthropods in the fossil record. Shell dimorphism in ostracods is most stark in several major Palaeozoic groups. Beyrichioidean ostracods are known abundantly worldwide from hundreds of genera in Ordovician to Carboniferous deposits and are characterised by a distinctive well-defined shell dimorphism in which the presumed female of the species develops a so-called brood pouch (crumina) on each valve. However, Ametrobeyrichia schizopyge, a Silurian ostracod species from the UK, challenges the definition of the group: it is, ostensibly, a non-dimorphic beyrichioidean. Reasons for its seemingly non-dimorphic nature include heterochronic mechanisms. Apparently not all beyrichioideans had cruminal brood care strategy.
