By David Arnold
If you visit Trafalgar Square today, you will undoubtedly be surrounded by tourists, grinning down at unnatural angles at their smartphones as they try to get a decent selfie with Nelson. However, if smartphones and Nelson’s column had existed in London 120,000 years ago, then they’d probably want to look up from their phones and watch out for a stampede of elephants or a hungry lion waiting to pounce!
The environment of London back during the Last interglacial (about 120,000 years ago) was very different to the 21st century bustling capital as we know it…..or was it? Today we are in an interglacial; just one of many interglacials that has occurred in the past 2.6 million years. But today we can take twenty selfies with Nelson without having to look out for a single elephant. So where are the elephants today? Why were they here in the past and not now?
Geologists, zoologists and naturalists have been asking themselves these types of questions for centuries. Charles Darwin himself started out by asking questions like this. His Victorian compatriots were fascinated by fossils; from ancient fossil dinosaurs to more recent specimens nearer the surface. What fascinated them were the strange animals they were discovering in the fossil assemblages; many were recognized by Victorian excavators as extinct beings, and taken as evidence of the biblical deluge reported in Genesis. However, advances in our understanding of evolution changed this view.
As well as extinct animals being found in fossil assemblages, there was strong evidence that the past distributions of extant (still living) species were very different than their modern ranges. From the fossil evidence in central London we can see that hippos were right at home with the elephants and lions, far from their modern range in Africa.
The reason we find these fossils in the sediments is that vertebrate fossils are often quite well preserved, particularly if the bones were buried quickly, as this reduces the chances of weathering, and destruction of the bones by the environment or other animals. The excellent condition of the fossils at Trafalgar Square indicates a quick burial in the sediments.
Deposits of rivers and lakes can yield many vertebrate fossils, as water is a key resource for survival, and whole skeletons of large mammals have been found in lake sediments. Larger mammals such as mammoths may get stuck in the lake muds when drinking or feeding, and any lucky carnivore in the area would take advantage of a stranded mammoth, killing the individual or scavenging from an easily available carcass. The Last Interglacial fossils at Trafalgar Square are interpreted by scientists as part of a river deposit of the Thames1. It is likely that the large mammals perished in the river and were buried by river muds, particularly the large hippo skull seen above. The skull is unlikely to have been moved by the river flow or carried around by carnivores due to its weight.
River deposits may also contain remains of the fish and amphibians that lived in the river, and can be very valuable in providing information about what the environment was like in the past. In the case of Trafalgar Square, we can estimate the minimum temperatures at the time of deposition because of the presence of European Pond Terrapin (Emys orbicularis) among the fossils. This species is found in the warmer climate of the Mediterranean today because it needs a minimum temperature of 18°C for its eggs to hatch. This tells scientists that at Trafalgar Square summer temperatures must have been at least 18°C.
The presence of Hippos can also tell scientists about past temperature. The winter temperature at this time cannot have been below freezing because hippos cannot survive when water bodies freeze. This, combined with the information from the pond terrapin and other species, indicates that the temperatures at Trafalgar Square in the past were warmer in summer, and winters were much milder than the present day. Scientists have tested plant, pollen and invertebrate remains, and found the same temperatures recorded2,3,4, adding further evidence to the story that central London was a more Mediterranean-like in the past. The use of these multiple lines of evidence is known as a “multi-proxy reconstruction” of past environments.
So try to imagine a River Thames with hippos and pond terrapins basking in the warm summer heat, surrounded by lions on the floodplain and straight-tusked elephants emerging out of the nearby woodlands. It may have been Mediterranean in Last Interglacial London but you’d have had a hard time finding for space for your sun lounger amongst the herds of beasts roaming around!
1. Stuart, A.J. (1976) The History of the Mammal Fauna During the Ipswichian/Last Interglacial in England. Philosophical Transactions of the Royal Society: B, 276: 945, 221-251
2. Phillips, L. (1974) Vegetational history of the Ipswichian/Eemian Interglacial in Britain and Continental Europe. New Phytologist, 73, 3, 589-604.
3. Coope, G.R. (2001) Biostratigraphical distinction of interglacial coleopteran assemblages from southern Britain attributed to Oxygen Isotope Stages 5e and 7. Quaternary Science Reviews’, 16-17, 1717-1722.
4. Preece, R.C. (1999) Differentiation of the British late Middle Pleistocene interglacials: the evidence from mammalian biostratigraphy. Quaternary Science Reviews, 20, 16-17, 1693-1705.