The fossil record of cockroach oothecae

Today, the insect group Dictyoptera includes cockroaches, termites and praying mantises with an almost worldwide distribution. The evolutionary origin of the ingroups is still debated, but there is no doubt that cockroach-like representatives of Dictyoptera (sensu lato) or ‘roachoids’ were already abundant in the Carboniferous more than 300 million years ago.

A special feature of extant species within Dictyoptera is their ability to form a special egg case, the so-called ootheca. According to current knowledge, this characteristic was probably not shown by Carboniferous early roachoids due to the morphology of their egg-laying structures. The first direct evidence of oothecae can be found in fossils from the Cretaceous in amber and limestone. Since this time, cockroaches have developed various strategies with regard to oothecae: Some groups simply drop their oothecae after producing them or carrying them for a while (oviparity), other groups keep the oothecae in a brood pouch just before the nymphs hatch (ovoviviparity), or even longer (viviparity). However, the evolution of these strategies is largely unknown, and oothecae are surprisingly rare in the fossil record.

We present different cases of fossil oothecae in amber and limestone from the Cretaceous, Eocene and Miocene. We discuss different types of oothecae and indications for their mode of oviposition. With these new finds we can provide further information for the evolutionary reconstruction of the reproductive strategies of Dictyoptera in the deep time.