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An Early Cretaceous anomopod (Crustacea: Branchiopoda) preserved in amber that reveals an unexpected venture during the evolution of the order

ABSTRACT An anomopod branchiopod crustacean that displays many structural similarities to the extant genera Scapholeberis and Megafenestra (family Daphniidae), fortuitously preserved in amber of Early Cretaceous age, is described. Although most of its appendages have been lost, preservation of several structures is vastly better than in most ‘orthodox’ fossils of these delicate animals. The well-preserved carapace suggests that, like extant representatives of these genera, but in no other daphniid, the animal exploited the surface film, beneath which it suspended itself by means of the straight ventral margins of its functionally bivalved carapace, each of which was armed with a row of close-set setae. Unlike the postabdomen of all the many extant anomopods, which is unsegmented, that of the fossil species, which was well preserved, was segmented; its distal half, or rather more, being clearly divided into four segments. Its topographically ventral margin was armed with pairs of very long spines (relatively longer than in any extant species), and the paired, hook-like terminal spines, clearly homologous with the claws of living species, were modified in a way that has no exact parallel in extant forms. The distal segment shows signs of annulations and may have had slight independent mobility. It is suggested that the long ventral spines of the postabdomen facilitated levering the animal forward over soft substrata. Its minute size reflects the fact that it is a first instar. Moreover, it was possibly squeezed out of the brood pouch prematurely when its parent became trapped in resin. The difficulty of assigning a place to the fossil in the phylogeny of the anomopods, all of which, including fossil forms of greater antiquity, have an unsegmented postabdomen, is briefly noted. The possibility that this attribute represents a very ancient atavism is tentatively suggested.