Windows into Early Cretaceous Britain: the plant debris beds of the Wessex Formation

Some parts of Lower Cretaceous Britain was subject to regular, short-lived wildfires caused by lightning strikes after long dry seasons, phenomena which played an integral role in forming the fossil-rich plant debris beds of the Wessex Formation. Here, the early tyrannosauroid Eotyrannus lengi stalks the edge of such a wildfire. Note that this Eotyrannus is based on new skeletal reconstructions presented in recent papers (e.g. Naish 2011), not the better known, original reconstruction presented by Hutt et al. (2001).

If you're into Mesozoic reptiles, you could find yourself in much worse places than southern England. Much of the exposed geology in the southern part of the UK belongs to a unit known as the Wealden Supergroup, a series of Lower Cretaceous rocks representing ancient alluvial fans, river channels and floodplains. Many of Britain's Cretaceous dinosaurs and pterosaurs stem from Wealden deposits, along with numerous other types of fossils including armoured dinosaurs, plesiosaurs, famous sauropods and weird, burrowing amphibians.

A slumped plant debris bed in the Wessex Formation, Brighstone Bay, Isle of Wight. Image borrowed from the UK Fossil Network forums, by one only known as 'Alan'.

Fossils occur found throughout Wealden rocks but, as is often the case in palaeontology, the majority are concentrated into narrow horizons. One type of Wealden fossil bed deserves special praise and attention: the plant debris beds of the Wessex Formation. Plant debris beds are narrow, green-grey bands of pebbles, mud and plant debris which comprise only a fraction of the Wessex strata, but represent a tremendous source of its fossils. Indeed, these beds provide the majority of Britain’s Cretaceous dinosaur species as well as many other fossil species, including many rare microvertebrates. Debris bed fossils range from small, badly preserved portions of plant and isolated, broken bones, teeth and scales, to substantial chunks of very large organisms - partial or near-complete animal skeletons and 3 m long logs (below). With continental deposits relatively rare in the Lower Cretaceous, the plant debris beds represent an important window into European faunas of this time, and studies into their palaeontology are ongoing (see below).

Enormous, pyrite-riddled chunks of fossil tree trunks, like these bits of the conifer Pseudofrenelopsis, litter the beaches beneath the Wessex Formation after weathering out of plant debris horizons. The ruler in this image is 150 mm long.

The story behind the plant debris beds has intrigued scientists for decades, leading to detailed research into their formation. Because the Wessex Formation represents a complex environment - an arid floodplain dominated by enormous, meandering rivers which were bordered by wooded highlands, and subjected to long summer months with temperatures well over 30°C but short, cool and rather wet winters - several different ideas about debris bed genesis have been proposed (best summarised and explored in Sweetman and Insole 2010). Plant debris bed sediments bear characteristics of debris flows; powerful, water-saturated sediment surges which ooze across landscapes to create poorly organised pools of mud and detritus. Such flows were clearly not regular events in the Wessex palaeoenvironment. Although many plant debris beds are known, they are relatively minor components of the Wessex Formation and are randomly distributed within Wessex strata. They were not, therefore, seasonal events and must reflect particularly unusual or extreme environmental conditions. Some have suggested that intense river flooding events and bank breaches account for these deposits, but plant debris beds are not associated with river sediments in a manner predicted for breached riverbanks deposits. Because the plant remains they contain are similar to leaf-litter found in modern forests, it is likely that they originated external to the Wessex floodplain, perhaps starting on nearby upland, wooded areas, not within the river channels. Indeed, debris flows generally start on slopes when water saturated soils and sediments become too heavy and unstable to resist gravity. The slopes required to begin plastic sediment flows are not large, and the relatively low upland areas surrounding the Wessex floodplain were likely sufficiently inclined to catalyse debris flows. Topographic highs on the floodplain itself may also have done the job. Presumably, the heavy rainfalls incurred during winter seasons was the water source which saturated Wessex soils to a critically unstable level.

The secret ingredient
This is only half the story, however. Sediment flows do not start after most heavy rainfalls because precipitation is mostly absorbed by leaf litter, intercepted by plant canopies, and soils are bound by vegetation. We know that the Wessex palaeoenvironment was fairly well-vegetated, and it is likely that its plants prevented Wessex slopes from collapsing. The secret ingredient required to make a debris flow, it seems, was fire (above). A common component of all plant debris beds is the abundance (about 50%) of burnt plant material, suggesting they were only formed after fires - likely caused by lightning strikes after long, dry summers- had swept through surrounding areas. An absence of burnt tree trunks suggests Wessex wildfires were not particularly intense, their main effect being removal of canopy cover, low-level vegetation and leaf-litter. This left the environment denuded enough for rainwaters to directly interact with soils and underlying sediments. Modern wildfires raise soil temperatures to hundreds of degrees and alter their physical properties, reducing water capacity and increasing erodibility. The result is a perfect recipe for debris flows: unprotected, easily transportable soils and sediments are left exposed to heavy precipitation, which likely arrived in earnest during winter storms.

Model of plant debris bed deposition on the Wessex Formation floodplain. Based on Sweetman and Insole (2010).

The range of sediment and fossil sizes within the plant debris beds indicate that they did not travel far, maybe a few kilometres at most, but they hoovered up any organic and sedimentary material they encountered. Large sediment flows can travel relatively quickly – up to 16 kph – and carry objects weighing many tonnes. Large dinosaur carcasses and tree trunks would be carried without hesitation by flowing oozes of debris moving across the Wessex floodplain. The surges finally lost momentum when they reached depressions such as ponds, oxbow lakes, abandoned river channels or simply topographic lows, creating the thin bands of sediment we can see today in Wessex Formation cliffs. The rarity of complete animal remains suggests that few animals were killed in the transportation process, and most vertebrate fossils probably represent bones or carcasses collected en route by the debris flow. This model for plant debris bed formation is, of course, rather generalised and may not apply to all beds. Each plant debris horizon is unique and, although this model likely accounts for at least some aspects of each, each has its own characteristic depositional history. Interestingly, no other fossil horizons match the sedimentological properties of the plant debris beds, making them important to not only palaeontologists, but also sedimentologists.

It is, of course, palaeontology which benefits most from these deposits however. Ongoing examination of the debris beds fossils, largely by renowned Wealden expert Steve Sweetman, continues to reveal new discoveries. Scientists now recognise the plant debris beds as key sources of Cretaceous microfossils as well as larger, macro-scale remains. These are extracted by sieving large quantities (literally tonnes) of plant debris bed sediment, followed by many hours hunched over microscopes to analyse and identify the new finds. This hard work has certainly paid off, adding significant detail to our understanding of the Wealden palaeobiota (below). We now know that dinosaurs were only a fraction of the tetrapod fauna in these environments, with lizards, amphibians and other small animals comprising the bulk of Wessex diversity. New discoveries are still being made, and it's an exciting time to work on Wealden fossils.

How plant debris beds changed the world. A, Wessex Formation tetrapod assemblage prior to bulk sampling and detailed study of plant debris bed fossils; B, the same assemblage after. Data from Sweetman and Insole (2010).

Plant debris beds conservation
The exciting fossil content and accessible nature of many plant debris beds has made them a favourite source of fossils to hobbyists, private collectors and professionals for centuries. This interest has undoubtedly contributed to our detailed understanding of the Wealden fossil assemblage and will continue to do so in future. It is essential, however, that plant debris beds and other Wealden exposures are treated with care and responsibility. All too often, a walk along Wessex Formation exposures reveals depressing signs of geological vandalism: holes bulldozed into slumped cliffs in vain efforts to seek fossil-bearing horizons; messages carved into soft sandstones; dinosaur footprint casts with smashed toes, and even trackways with individual prints removed using power tools. Plant debris beds are often more conspicuous by the smashed rocks surrounding them than their lithological features. While some geological vandalism clearly reflects activities of bored, idle individuals, other types - and particularly that associated with debris beds – reflects the desires of eager individuals to discover and excavate fossil remains. We have to keep this in check. Over-enthusiasm not only risks damaging important specimens but also the surrounding sediments and other, less desirable fossils, both of which offer essential details on the depositional context of a fossil specimen. Remember that hammer blows do not only remove overburden, but also smash whatever lies beneath the surface.

The point here is not, of course, that Wealden fossils should be the sole remit of trained collectors, but that we should all be conscientious about our geological heritage. It is often far wiser, for instance, to alert local museum or university staff about an exciting find before collecting it, rather than risking damaging the specimen and it’s geological context by taking it immediately. If nothing else, contacting local professionals can provide sound advice on an appropriate manner to collect and preserve fossils. As with any fossil discoveries, accurate records must be made about the location and horizon of a new find and, if the specimen looks like it may be important, collectors should strongly consider accessioning their finds to a museum. Collectors who work with museums and scientists are frequently involved in the science that can follow a new discovery, helping to analyse and document the find in scientific papers and books. I can vouch from personal experience that this can happen relatively quickly. A new Wealden fossil accessioned to Dinosaur Isle (the Museum of Isle of Wight Geology under any other name) or the Natural History Museum seems to always get local palaeontologists buzzing, and several Wealden experts are well known for analysing new specimens within weeks of their arrival. If they are important, they end up being written up into technical papers, may be further featured in other palaeontological books and media, and may even end up on public view in museums.

What you'll want to understand fossils from plant debris beds, or any other part of the Wealden, for that matter.

How do you know if a fossil is 'important' enough to bring it to the attention of expert? Fossil identification guides, such as the excellent and highly comprehensive English Wealden fossils (Batten 2011) and Dinosaurs of the Isle of Wight (Martill and Naish 2001) are a useful means to gauge not only the identification of a Wealden fossil find, but also how ‘significant’ it may be. Many Wealden vertebrates are especially poorly known and new data on them is highly sought after, so it may be worth getting any well-preserved vertebrate material checked out. Doing so ensures that the window into Lower Cretaceous Britain offered by these remarkable beds remains widely open to all, which seems only right considering the importance of of these beds to British palaeontology.