Dr James E Jepson (UK)
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|
|The insects represent two broad environments: terrestrial and aquatic. The terrestrial fauna is represented predominantly by woodland insects, such as wood-feeding beetles, sap-sucking bugs, predatory dragonflies, damselflies, lacewings and snakeflies, and scavenging cockroaches. The aquatic fauna has water beetles and bugs, and nymphs of dragonflies, damselflies, rare mayflies and also caddisfly larvae. In the more brackish water environments, hardy water bugs and dipteran larvae are the only insects able to tolerate the salinity.|
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.
The Wealden Supergroup outcrops in two sub-basins: the Weald Sub-basin and the Wessex Sub-basin. It is the Weald Sub-basin that yields the most fossil insects, whereas the Wessex has yielded only a few. Amber has been found in both.
The Wealden of the Weald Sub-basin outcrops in southeast England. Here, the rocks are divided into the Hastings Group and the Weald Clay Group. The general environment at the time of deposition was a river floodplain with short lived, sand-filled channels. The localities where insects have been found are mainly in brick and tile works. Only one insect locality is known from the Hastings Group (Wadhurst Clay), which is Quarry Hill Brickworks in Kent, but only a few species have been described from here. However, it has yielded an impressive lacewing (Kalligrammatidae) with an eyespot preserved on its wing (similar to the ones on butterflies today).
That few species have been described most likely represents the fact that little work has been carried out on this locality. Most of the insects from the Wealden are known from the Weald Clay, with the main localities being Clockhouse Brickworks, Auclaye Brickworks, Rudgwick Brickworks, Smokejacks Brickworks and Keymer Tileworks. Clockhouse Brickworks has yielded the most insect fossils, with thousands of specimens collected. Other fossils found from the Weald Sub-basin include plants, isopods, conchostracans, ostracods, bivalves, gastropods, fish, reptiles, dinosaurs and pterosaurs.
The insect preservation differs at each locality. For example, at Clockhouse, they are mainly preserved in lens-shaped layers and well-cemented, cross-bedded basin casts of calcareous sandstone. At Auclaye, the preservation shows an exceptional amount of detail, with the insects being preserved in phosphatic concretions. Another example of preservation is at Smokejacks were they are preserved in sideritic concretions (Jarzembowski and Palmer, 2010). (See box, Insects of the Wealden – Southeast England, for more information.)
|Insects of the Wealden – Southeast England|
|As with the Purbeck, there have been thousands of specimens collected from the Wealden’s Weald Sub-basin and hundreds of species described, with 14 orders being recorded (Fig. 8). These are dragonflies and damselflies (Odonata), grasshoppers and crickets (Orthoptera), cockroaches (Blattaria), book and bark lice (Psocoptera), thrips (Thysanoptera), true bugs (Hemiptera), beetles (Coleoptera), lacewings (Neuroptera), snakeflies (Raphidioptera), wasps (Hymenoptera), scorpionflies and hangingflies (Mecoptera), true flies (Diptera), termites (Isoptera) and caddisflies (Trichoptera) (Jarzembowski, 1984). The two most abundant groups in the Wealden are the beetles and cockroaches, with over 1,000 specimens of each collected (Jarzembowski, 1995).|
The insects again represent two distinct habitats: terrestrial and aquatic. The terrestrial fauna represents a woodland environment, away from the site of deposition. The wood and vegetation feeders include beetles, true bugs (including the Wealden aphid, Penaphis woolardi), termites, grasshoppers and crickets, scavenging cockroaches, pollinators (such as true flies), wasps, predators (including dragonflies and damselflies), lacewings, snakeflies and wasps. The true flies suggest that it may have been damp woodland. The aquatic fauna is represented by freshwater insects including dragonflies and damselflies, caddisflies, aquatic true flies and true bugs. The water body in the Weald Sub-basin was saline (Allen, 1998) and, therefore, these freshwater insects would have lived in fresher water away from the site of deposition.
The climate was wetter and more humid than the Purbeck and fossils such as the termite, and along with other evidence (for example, the sedimentology), this suggests a warm, humid environment, such as the Mediterranean today. Burnt insect remains are also often encountered, which tell of times when there were widespread fires.
Amber found within the Hastings Group has so far only yielded a spider’s web and microbial inclusions (Brasier et al. 2009). No insects have, as yet, been discovered.
The Wealden of the Wessex Sub-basin outcrops on the Isle of Wight and in Dorset, and it has yielded few insects, compared to the Weald Sub-basin. The Wessex Sub-basin Wealden is divided into two formations: the Wessex and Vectis Formation. The Wealden on the Isle of Wight is the best place to find fossil insects and it is here where most of the insect remains have been discovered, including in amber (but some have been found in Dorset). The majority of insects come from the Shepherd’s Chine Member, with some known from the Cowleaze Chine Member (Vectis Formation) (Twitchett, 1994). The amber is from the older Wessex Formation (Nicholas et al., 1993). The insects here are of a more fragmentary nature than those from the Purbeck and Weald Sub-basin, indicating a greater distance and/or period of travel (see box, Insects of the Wealden – The Isle of Wight and Dorset, for more information).
|Insects of the Wealden – The Isle of Wight and Dorset|
|The insects recorded are beetles (Coleoptera), cockroaches (Blattaria), true bugs (Hemiptera), true flies (Diptera) and caddisflies (Trichoptera) (Twitchett, 1994; Heads, 2006, 2008). Trace fossils of insects are known from this sub-basin, with borings in wood being attributed to termites (Francis and Harland, 2006). The amber found in the Wessex Sub-basin has arthropod inclusions – one spider and one species of true fly have been described (Selden, 2002; Jarzembowski et al., 2008). More specimens are awaiting description.|
The Purbeck and Wealden are very important fossil insect sites, not only in Britain but in the world, as there are very few diverse fossil insect sites of this age. They give us an insight into insect faunas before the major radiation of the angiosperms, which took place five to ten million years after the Wealden times, and before the evolution of the major pollinating insects of today, such as butterflies and bees. Although much work has been done, and we are developing a good understanding of the diversity and ecology of these insect deposits, there is still a vast amount more to do.
For example, only a handful of major studies of insect groups have been completed, such as wasps (Rasnitsyn et al., 1998), grasshoppers and crickets (Gorochov et al., 2006), dragonflies and damselflies (for example, Jarzembowski, 1988, 1994; Jarzembowski and Nel, 1996; Nel and Jarzembowski, 1996, 1998), lacewings (Jepson and Penney, 2007; Jepson et al. 2009b; Jepson et al. in review) and snakeflies (Jepson and Jarzembowski, 2008; Jepson et al. 2009a; Jepson et al. 2011).
There are still thousands of specimens that need studying and orders of insects, which are waiting to be studied in detail, as well as the potential to collect more specimens.The climate change recorded in the rocks from the arid lower Purbeck to the wetter, more humid upper Purbeck and Wealden, had an effect on the insects. In the arid climate, there were fewer moisture loving insects and some of the insects present were much smaller (for example, snakeflies). When the climate became wetter and more humid, there was an increase in insects, such as dance flies and caddisflies, which are found in wetter environments, and the small snakeflies became much larger.
The author is currently an honorary researcher at the School of Earth Atmospheric and Environmental Science at the University of Manchester.
Thanks to Rob Coram (British Fossils), Ed Jarzembowski and Fred Clouter (Maidstone Museum) for images of fossil insects and Lisa D Abbott (University of Manchester) for useful comments.
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