What nymphal morphology can tell us about parental investment – A group of cockroach hatchlings in Baltic amber documented by a multi-method approach

Abstract

We describe a piece of Baltic amber, about 50 million years old, which contains a group of 13 small cockroach nymphs. These specimens were documented with different methods to explore the advantages and limitations of certain imaging techniques: (1) light-based methods, such as stereo-macro photography, composite imaging under polarised light, combined with virtual surface reconstruction, and (2) X-ray micro-computed tomography, processed as volume renderings and surface reconstructions. All nymphs within the amber piece are of the same size and do not exhibit any noticeable morphological variance. Their developmental state and the way in which they are arranged indicate that these nymphs represent hatchlings. Dictyopterans (including Mantodea and Blattodea with Isoptera as ingroup) exhibit a wide range of different types of social and brood care behaviour. The evolution of this complex set of characters has been addressed repeatedly in extant-based approaches, yet deep-time aspects of this evolutionary process have rarely been addressed. The specimens described here could represent a case of a group of blattodean nymphs hatching from an ootheca, which would represent the first fossil record of such a process, or even possibly provide the first indirect evidence of social behaviour in fossil non-termite dictyopterans, indicating that it was already developed 50 million years ago.