An unusual fossil larva, the ontogeny of achelatan lobsters, and the evolution of metamorphosis

Metamorphosis is a striking feature in the development of an animal. For arthropods, metamorphosis is usually understood as a rapid morphological change, which occurs within one or two moults. We describe here two conspecific fossil larvae of an achelatan lobster from the Solnhofen Lithographic Limestones (Upper Jurassic, southern Germany). These phyllosoma-like larvae represent successive instars. Both have a combination of characters specific for phyllosoma larvae and those typical for post-phyllosoma instars. This find indicates that the developmental pattern of this achelatan species was more gradual or “less metamorphic” than in all other achelatans known, and probably represents the plesiomorphic condition. Obviously, heterochrony played an important role in the evolution of metamorphosis in Achelata. Developmental data from other fossil arthropods, e.g., stomatopod crustaceans or pterygote insects, also point to a more gradual ontogenetic pattern in the ancestors of these lineages, which show a “real” metamorphosis today. The evolution of metamorphosis is linked to selective pressure on early developmental stages, resulting in morphological disparity between pre- and post-metamorphic stages, and a condensation of the ontogenetic pattern, leading to a more rapid morphological change. The influence of both factors can be better evaluated if fossil information is taken into account.