The hatching mechanism of 130‐million‐year‐old insects: an association of neonates, egg shells and egg bursters in Lebanese amber

Enrique Peñalver,Andrew Smith,Ricardo Pérez‐De La Fuente,Michael S Engel,Dany Azar

doi:10.1111/pala.12414

Abstract

AbstractHatching is a pivotal moment in the life of most animals. Diverse chemical, behavioural and mechanical methods have evolved in metazoans to break the egg membranes. Among them, many arthropod and vertebrate embryos hatch using ephemeral, frequently convergent structures known as egg bursters. However, the evolutionary processes by which hatching mechanisms and related embryonic structures became established in deep time are poorly understood due to a nearly complete absence from the fossil record. Herein we describe an exceptional c. 130‐million‐year‐old association in Lebanese amber composed of multiple neonate green lacewing larvae, Tragichrysa ovoruptora gen. et sp. nov. (Neuroptera, Chrysopoidea), and conspecific egg remains. Egg bursters with a serrated blade bearing a short process are attached to three longitudinally split egg shells. Embryos of extant green lacewing relatives (Chrysopidae) utilize this egg burster morphotype to open a vertical slit on the egg, after which the burster is moulted and left joined to the empty egg shell. Additionally, the new larval species has extremely elongate dorsal tubercles, an adaptation to carry exogenous debris for protection and camouflage also known from other Cretaceous chrysopoids but absent in modern relatives. The present discovery demonstrates that the hatching mechanism of modern green lacewings was established in the chrysopoid lineage by the Early Cretaceous and proves through direct fossil evidence how some morphological traits related to hatching and linked behaviours, at least in insect embryos, have been subject to a high degree of evolutionary conservatism.

Highlights

Hatching is a pivotal moment in the life of most animals
In a wide diversity of insects, including the Neuroptera, the egg bursters (EBs) is frequently shed with the embryonic cuticle on hatching, leaving it attached to the empty egg shell (Kobayashi & Suzuki 2016)
Regardless of the fact that EBs have been studied for more than a century, their evolution is still far from being understood (Mashimo et al 2014). This lack of knowledge is largely due to the fact that the fossil record of EBs is almost entirely lacking, which is certainly not aided by the small size and relatively ephemeral nature of these structures

Summary

Introduction

Hatching is a pivotal moment in the life of most animals. Diverse chemical, behavioural and mechanical methods have evolved in metazoans to break the egg membranes. The present discovery demonstrates that the hatching mechanism of modern green lacewings was established in the chrysopoid lineage by the Early Cretaceous and proves through direct fossil evidence how some morphological traits related to hatching and linked behaviours, at least in insect embryos, have been subject to a high degree of evolutionary conservatism. Specialized structures for cutting, piercing or tearing the egg membranes present in the embryo tend to disappear soon during post-embryonic life can be used. These can be referred to as egg bursters (EBs) and have received different names depending on the group; for example, egg-. The only previous fossil records of EBs were limited to tetrapods, that is, ‘egg-tooth-like’ structures in several titanosaur embryonic premaxillaries from the Upper Cretaceous of Argentina (Garcıa 2007) and a highly dubious ‘egg-tooth’ reported in a mesosaur embryo from the Permian of Uruguay (Pin~eiro et al 2012)

Methods

Conclusion