Musacaris is an extinct genus of Cambrian arthropod from the Orsten Lagerstätte, formerly synonymous with Martinssonia. It contains only a single species, Musacaris gerdgeyeri.
Musacaris is a small (roughly 0.8 mm long), unsegmented arthropod, with four pairs of appendages. It has a pair of antennae, and three pairs of biramous appendages[1] On its head, it also has an unusual projection, possibly the future position of the mouth. The unsegmented body suggests all current specimens of Musacaris may be so-called "head larvae". The five developmental stages are quite similar, with the same amount and positioning of the limbs. Currently, Musacaris is thought to be a basal mandibulate or stem-crustacean, possibly sister to labrophorans (Crustacea and Phosphatocopina).
Musacaris is known from thirteen specimens of different larval forms, all from the Orsten Lagerstätte in Sweden, alongside twenty-one other specimens referred to Martinssonia elongata.[2]