The fossilised bones of two ancient hippos have been found in Norfolk.
Artist's impression: Hippos survey an ancient Norfolk
They are said to be more than 450,000 years old and were recovered from a quarry along with horse, hyena, fish and a variety of rodent remains.
Researchers believe the fossils open a new window on the UK's past in the early Middle Pleistocene when average temperatures were about 2C higher.
The discovery was made by scientists from the Natural History Museum and Queen Mary, University of London.
"The site was first visited by a local amateur geologist who came across these deposits and realised they contained something that was really very exciting," palaeontologist Simon Parfitt, from the NHM, told BBC News Online.
The quarry is some 25km (15 miles) in from the coast. The researchers are not revealing the precise location because it is unsafe for sightseers.
The fossils are about 10-15m (30-50ft) under the surface. They were covered by glacial deposits that are known to be just under half-a-million years old.
The nature of the finds indicates they may be 50,000 to 200,000 years older still.
The hippos would have weighed in at something like six to seven tonnes - about half as big again as today's descendants.
They had very prominent eyes which served as periscopes when submerged in the water.
It is likely the hippos died through natural causes and their bones show evidence of having being gnawed by hyenas.
The animals would have lived at a time when Norfolk had a landscape populated by an unusual mixture of familiar plants and animals and more exotic species now found only in the African savannah.
"It would have been about two degrees warmer than it is now," Mr Parfitt said.
"The landscape would have looked quite familiar - a big river and a very broad flood plain. But some of the animals would have been very different to what we are used to."
He continued: "The importance of the new site is that we think it records an unrecognised warm stage. We know from the study of the ice caps and the deep ocean that the climate was fluctuating very quickly.
"Every 40,000 years or so you had a cold phase and then it got warmer again, and this is possibly a 10,000-year slot in that jigsaw we didn't know about before."
Some of the fossils collected to date go on show from Thursday at the NHM as part of its Festival of Fossils.
The researchers are in a race to excavate the quarry site. The dig location is to be redeveloped in the next few months.