Earliest Cretaceous cocoons or plant seed structures from the Wealden Group, Hastings, UK

Complete metamorphosis evolved in insects towards the end of the Palaeozoic Era. A wide range of pupation strategies existed, and numerous biosedimentary structures associated with these have been described. The fossil record of endogenous materials associated with pupation, e.g. cocoons, however is more limited. Here we report six amber-coloured specimens from the earliest Cretaceous of southern England tentatively identified on collection as insect cocoons. These were analysed by FTIR, stereomicroscopy and X-ray microtomography to elucidate their origin. Interpretation of FTIR data was inconclusive as the spectra showed some differences from those of amber. A seed pod origin seems likely for at least two of the objects, based on their size, shape and lineations on their surfaces. Three specimens are more cocoon-like, based on overall morphology and a fibrous surface texture. Whilst plant megaspore membranes have features analogous with these specimens, and cannot be ruled out, the similarity to and variability found within insect cocoons, coupled with the range of potential insect architects present at the time of origin, make an insectan origin more likely. We review a number of hymenopteran clades whose extant members construct comparable cocoons. It is possible the possible cocoons were resin-coated to protect the larva inside from predation, or passively came into contact with resin prior to burial.