Earliest record of fossil insect oothecae confirms the presence of crown-dictyopteran taxa in the Late Triassic

Abstract

Although dissimilar in their overall appearance and life habits, the praying mantises (Mantodea) and cockroaches (Blattodea, including their eusocial relatives, the termites [Isoptera]) are grouped within the clade Dictyoptera, based on – among other significant characteristics – the laying of eggs in a compound structure called an ootheca. The origin of the Dictyoptera and the currently recognized taxa within is, however, a controversial topic among entomologists. This has resulted from disparities in the divergence age estimates obtained from phylogenetic analyses based on molecular data together with the limited and controversial fossil evidence attributable to these groups. Here, we report two new oothecae ichnospecies found in a Carnian (237 to 227 mya. lowermost Upper Triassic) deposit from Argentina. Morphological comparisons and Scanning Electron Microscope and X-ray Energy Dispersive Spectroscopy analyses of fossil and extant oothecae of mantises and cockroaches were performed in an attempt to solve their systematic placement within Dictyoptera and fossil allies, such as †Alienoptera. In addition to being the earliest known record of oothecae, this discovery moves the origin of this specialized reproductive strategy back by 100 million years. As direct fossil evidence, these specimens provide an important calibration and reference point that can inform future research on the origins and timing of diversification of the Dictyoptera.