Fossilised groundcherries − a type of lantern fruit − have been discovered preserved in amazing detail next to the remains of a volcanic lake in Patagonia, Argentina.

The age of the fossils suggests that lantern fruits evolved much earlier than previously thought, according to a paper published in the journal Science.

Fossil fruit
This fossil groundcherry Physalis infinemundi from Laguna del Hunco in Patagonia, Argentina, is 52 million years old. This specimen displays the characteristic papery, lobed husk and details of the venation Ignacio Escapa/Museo Paleontológico Egidio Feruglio

The fossils belong to the Solanaceae family, which includes potatoes, peppers, tobacco and tomatoes. The family was thought to be about 30 million years old, based on fossil remains of tiny seeds.

Now the fossil remains of two whole intact fruits show that the family is a great deal older than that. The two fossils are from a late-evolving branch of the family, and even that is 52 million years old. This dates them to the period when South America was right next to Antarctica in the supercontinent Gondwana.

Fossil fruit
The new fossil groundcherry Physalis infinemundi from Laguna del Hunco in Patagonia, Argentina, 52 million years old. In this specimen, the former papery and lobed husk is broken at top to reveal the large, fleshy berry underneath that has now turned to coal Peter Wilf/Penn State University

To find such a delicate specimen preserved in great detail is incredibly rare.

"These fossils are unique in the world, they are the only two of the whole nightshade family ever found," study author Peter Wilf, a palaeontologist at Pennsylvania State University, tells IBTimes UK.

"No one has ever seen a fossil lantern fruit before. It's just incredible – you can still see the berry there inside the fruit."

The fruits were found at Laguna del Hunco, the remains of a fossil lake next to a volcano, which is an abundant source of fossils.

"There's something about it that we don't fully understand. We find things preserved there – flowers, fruits, animals – that we just don't see anywhere else in the world," Wilf says. But the fossil fruits are very rare even in the context of this site.

Fossil fruit
Groundcherry fruits on a stream in Pennsylvania. Flotation is the most likely explanation for how groundcherries were transported far enough from shore to eventually sink and become fossilised at the bottom of a deep lake in Patagonia 52 million years ago Peter Wilf, Penn State University

The fruits must have drifted into the lake very gently in order to have maintained their delicate structure, and then have had layers of sediment – a mixture of sand, ash and mud – deposited on top of them very gently. They were then very slowly pressed flat. It's also thought that the lake was very acidic, which would have prevented any biological breakdown of the fruit by microbes.

To find more examples of fruits like this, a focus of research will be Antarctica, Wilf says.

"If they were in southern South America, Antarctica was right next door, so that's the place we should be looking for them," he says. "These fossils are from a late-evolving lineage in the family and it must go back further, at least to the age of the dinosaurs. If we're looking for evidence to back up that idea, then Antarctica is the place we should be looking."

The finding is leading to revisions of our understanding of the origins and evolutionary history of Solanaceae, which contain some of today's most economically important crops.

The fossils are now in the Museo Paleontológico Egidio Feruglio in Argentina, which participated in the research, arranged the expeditions and curated the fossils.

Laguna del Hunco
The Laguna del Hunco fossil site in Chubut, Patagonia, Argentina. The white rocks are fossil lake beds that accumulated 52 million years ago in a sinking caldera, following a series of enormous volcanic eruptions Peter Wilf/Penn State University