Archaeostracan (Phyllocarida: Archaeostraca) antennulae and antennae: Sexual dimorphism in early malacostracans and Ceratiocaris M'Coy, 1849 as a possible stem eumalacostracan

Although there is a relatively robust fossil record of archaeostracan phyllocarids, preserved antennulae and antennae are rare. Few examples have been described. A review of archaeostracans with preserved antennulae and antennae is provided, as well as a description of a specimen of Ceratiocaris cf. macrouraCollette and Rudkin, 2010, with preserved antennae, and a detailed description of a specimen of Ceratiocaris papilio Salter in Murchison, 1859, from the Silurian of Scotland. The presence of antennulae with two subequal length rami in Rhinocaridina and Echinocaridina supports previous assertions that possessing biramous antennulae is a malacostracan synapomorphy. An antennal scale in Ceratiocaris, in contrast to those of Rhinocaridina and Echinocaridina, but consistent with eumalacostracans, suggests that ceratiocarids could represent stem eumalacostracans. Hooked antennae in C. papilio, similar to copulatory clasping antennae of Nebaliopsis typica G. O. Sars, 1887, are interpreted to represent the earliest evidence of sexual dimorphism in malacostracans.