Immature Insect Assemblages from the Early Cretaceous (Purbeck/Wealden) of Southern England.

Robert A Coram,Edmund A Jarzembowski

doi:10.3390/insects12100942

Abstract

Simple SummaryThe non-marine Lower Cretaceous Purbeck and Wealden rocks of southern England provide an important record of insects that lived alongside the dinosaurs. Most fossil remains are those of adult insects from orders alive today, but immature insects and their trace fossils occur in the same deposits. Terrestrial immatures comprise mostly sessile nymphs of true bugs, whereas the aquatic immature fauna is represented by stoneflies and mayflies (rarely), dragonflies (uncommonly), true bugs and true flies (often common in the Purbeck), and the cases of caddisflies (locally common in the Wealden). These fossils help to shed light on the local palaeoenvironment, such as the salinity of water bodies, as well as on the processes that lead to the fossilization of generally fragile insect remains.The record of immature insects from the non-marine Purbeck and Wealden groups (Lower Cretaceous) of southern England is reviewed and expanded. Fossils of adult terrestrial insects are locally common, but terrestrial immature remains are restricted to transported hemipterans, most of which are sessile nymphs or puparia resembling those of extant whiteflies (Aleyrodidae). Remains of immature aquatic insects are more diverse and comprise the extant orders Plecoptera, Ephemeroptera, Odonata, Trichoptera, Hemiptera and Diptera. The Trichoptera are represented by larval cases constructed from a variety of materials corresponding to several ichnogenera. The Wealden immature insects were preserved in predominantly freshwater fluvial settings, whereas the Purbeck ones occur in lagoonal palaeoenvironments, ranging in salinity from brackish to hypersaline. The composition of aquatic immature insect faunas in the latter offers potential for palaeosalinity analysis, although there are complicating factors relating to habitat stability. Uncommon trace fossils such as beetle borings in wood provide evidence of immature insects not represented by body fossils.

Highlights

Introduction and Geological SettingThe Early Cretaceous, somewhat past the mid-point of the Mesozoic Era (‘age of dinosaurs’) and succeeding the Jurassic Period, was a time when the insect fauna was composed virtually entirely of extant orders but still contained archaic elements
The Purbeck terrestrial insect fauna is dominated in both abundance and diversity by Coleoptera, followed by Diptera and Hemiptera
Purbeck and Wealden immature terrestrial insect body fossils are restricted to transported hemipteran remains which contribute little to our knowledge of terrestrial palaeoenvironments and ecosystems

Summary

Introduction

The Early Cretaceous, somewhat past the mid-point of the Mesozoic Era (‘age of dinosaurs’) and succeeding the Jurassic Period, was a time when the insect fauna was composed virtually entirely of extant orders but still contained archaic elements. It lacked groups such as butterflies and bees mostly associated with flowering plants, since angiosperms were yet to dominate terrestrial ecosystems. The corresponding deposits, the Purbeck and Wealden groups, respectively [2] (Figure 1), were laid down in variably inundated lowland settings tthhaatt llaayy aammoonngg aa ccoommpplleexx ooff iissoollaatteedd mmaassssiiffss bboorrddeerreedd bbyy tthhee wwiiddeenniinngg PPrroottooaattllaannttiicc ttoo tthhee wweesstt,, tthhee BBoorreeaall SSeeaa ttoo tthhee nnoorrtthheeaasstt aanndd tthhee TTeetthhyyss OOcceeaann ttoo tthhee ssoouutthh.

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