A fossil reptile discovered in Brazil may be the oldest known creature that resembles a modern turtle.
The 120-million-year-old find is linked to present-day representatives by its heavily webbed, paddle-shaped foot - an adaptation to life in the sea.
Soft tissue has been preserved on the specimen, allowing scientists to confirm the webbing rather than infer it from the length of the foot bones.
A University of Portsmouth team publish details in the journal Palaeontology.
Representatives of the turtle lineage are known from the fossil record as far back as 200 million years. But these examples look more like tortoises, suggesting they were still very much adapted to life on land.
The latest find has front feet shaped like paddles, much like modern turtles.
"Tortoises have very short foot bones and not very much soft tissue," lead author Sarah Fielding, of the school of Earth and environmental sciences at Portsmouth, told the BBC News website.
"This specimen has slightly longer foot bones and quite a lot of webbing in between.
This juvenile turtle is also being studied
"We're fortunate the deposit it was found in has fine mud which has preserved the webbing - which is quite rare for turtles."
But the hind feet in this specimen seem to be more tortoise-like, suggesting this creature was not fully adapted to a marine lifestyle and spent some significant amount of time on land.
The fossil, which is broken in half, was found by the researchers in a spoil heap of rocks taken out of a small quarry. The rocks belonged to Lower Cretaceous beds of the Crato Formation.
"We have an incomplete slab, so it may once have had a skull as well. It's a real enigma and we don't have the other bits. It's possible somebody found the other half and it is somewhere else being looked at."
However, the researchers have enough of the specimen to determine it belongs to a new species, which they have named Araripemys arturi.
Ms Fielding, Darren Naish and David Martill from Portsmouth found that the specimen differs in some important ways from another fossil turtle from the same formation called Araripemys barretoi.
"The original species - barretoi - has arrow-shaped claws which are quite odd generally in turtles. This one has very simple claws," Ms Fielding explained.
"And the shape of the shell is slightly squared off at the back."
Its shell is also pitted, a characteristic of modern "soft-shell" turtles. These lack a keratin covering on the bony component of their shells.
The researchers have also found a juvenile turtle specimen from the same formation in Brazil, which shows even more preservation of soft tissue. It is currently being described for publication in a scientific journal.