It was over 150 years ago that the first major work began on the fossil insects of the Lower Cretaceous of England. The pioneers were Victorian naturalists, including the Rev Osmond Fisher, John O Westwood and, in particular, the Rev Peter Bellinger Brodie. 1845 saw the publication of Brodie’s A History of the Fossil Insects in the Secondary Rocks of England, the earliest English language book on fossil insects and the first major study of the fossil insects of England. The Victorians collected and described many species from Wiltshire, Dorset and the Weald, and started the ball rolling for British palaeoentomology.
The twentieth century saw little activity in British Cretaceous palaeoentomology. At this time, there was a shift towards the Palaeozoic insects from the Carboniferous, with Herbert Bolton leading the way – Bolton’s major work was published in a monograph on British Carboniferous insects in 1921–1922. A few descriptions were made on British Cretaceous insects in the early twentieth century, most notably Anton Handlirsch’s monograph of fossil insects (1906–1908) included some British Cretaceous insects; but there was no major studies completed. However, in the late twentieth century, interest in the Cretaceous insects of Britain was reawakened by Edmund A Jarzembowski, with his studies on Wealden insects and later the Purbeck insects with Robert A Coram. Into the twenty-first century, Jarzembowski and Coram have remained a driving force for the study of Lower Cretaceous insects of southern England and, through their work and their collaborations with other scientists, hundreds of species have been described and documented from these two important fossil localities.
The Purbeck Limestone Group and Wealden Supergroup are of Lower Cretaceous age (145 to 125mya). When they were deposited, southern England was a very different place from today. It was located at latitude 35°N (Ogg et al., 1994) and had a climate that was much more akin to the present day Mediterranean. Also, during this time, southern England underwent a change in climate, from a semi-arid to a more humid/wet environment (from the lower Purbeck into the Wealden) (Allen, 1998). The landscape was also very different: the Purbeck and Wealden were deposited in two sub-basins – the Wessex and Weald – surrounded by four areas of high land – the Welsh Massif, Armorica Massif, Cornubian Massif and the Londinia Massif (Fig. 1). The latter two were the main influences on the sediment input into the Wessex and Weald Sub-basins.
Fossil insects are found throughout the rocks of the Purbeck and Wealden (Fig. 2). In the Purbeck of Dorset, they are much more common in the Corbula Beds and, in the Wealden, they are most numerous in the Weald Clay Group.
The insects are usually preserved as disarticulated and fragmentary wings. However, occasionally, whole insects can be found, especially in the Purbeck. The fragmented wings can often be too difficult to describe to genus and species level, but they can give us some information about where they lived in relation to the site of deposition. As a general rule, the more complete an insect is, the closer to the depositional site it lived; whereas, the more fragmented the insect is, the further away it probably lived. In the Purbeck and Wealden, the insects have been transported to the site of deposition by, for example, rivers and during transport, the insect can become broken up as it comes into contact with sediment being transported and the riverbed. Also, the longer the transport time, the more chance of decomposition and predation there is. On the other hand, if the insect is living close to the area of deposition, it experiences little if any destruction due to transport. Exceptions to this are when predation occurs. In these circumstances, an insect fragmented by a predator could be mistaken for an insect living far away from the site of deposition and insects being blown in from distances by storms.
In the Purbeck, there is a good mixture of complete and fragmentary insects, whereas, in the Wealden, they are mainly fragmented, in particular, with just wings being preserved. Therefore, it can be surmised that the Purbeck had a mixture of insect habitats, from close to the site of deposition to further away and, in the Wealden, the insect habitats were generally far away from the site of deposition.
The Purbeck Limestone Group outcrops in three main places in England. These are Wiltshire, Dorset and the Weald. The most productive localities for insects are in Dorset, in particular, Durlston Bay, near Swanage (the type area for the Purbeck Limestone Group) and in Wiltshire. The rocks of Durlston Bay were studied in detail by Clements (1993) and his bed number, DB175 (Corbula beds), is the most productive layer for insects. Many of the insects from Wiltshire are known from Brodie’s Insect Limestone Bed at Dinton. The Purbeck rocks are divided into two formations: the older Lulworth Formation and the younger Durlston Formation. They both represent a lagoonal environment and it is this lagoon that was the site of deposition for the insects. A study on the palaeoecology of the Purbeck insects by Robert A Coram (2003) has shown that there were numerous habitats as you move away from the lagoon, such as a beach/sabkah environment, freshwater lake/marsh, damp forest water margin, open glades and, furthest away, a forest inhabited by various different insects. As well as insects, fossils of plants, gastropods, bivalves, ostracods, fish, reptiles, lizards, crocodiles, dinosaurs, pterosaurs and mammals have been found. (See box, Insects of the Purbeck, for more information.)
Insects of the Purbeck
There have been thousands of insect specimens discovered and hundreds of insect species described from the Purbeck, with many more awaiting descriptions. There are 17 orders of insect known (Fig. 3). These are mayflies (Ephemeroptera), dragonflies and damselflies (Odonata), earwigs (Dermaptera), rock crawlers (Grylloblattodea), grasshoppers and crickets (Orthoptera), stick insects (Phasmatodea), cockroaches (Blattaria), book lice and bark lice (Psocoptera), thrips (Thysanoptera), true bugs (Hemiptera), beetles (Coleoptera), lacewings (Neuroptera), snakeflies (Raphidioptera), wasps (Hymenoptera), scorpionflies (Mecoptera), true flies (Diptera) and caddisflies (Trichoptera) (Coram, 2003). The most common insects found within the Purbeck are beetles (over 70 species recorded), true flies (over 30 species recorded), and the true bugs (over 25 species recorded). The beetles are usually represented by isolated elytra, which unfortunately are difficult to identify to genus or species level.