Scientists have discovered a 52-million-year-old fossil of a berry that may help understand the evolution of the economically important plant family that includes potatoes, peppers and tomatoes.
Tomatillos, ground cherries and husk tomatoes – members of the physalis genus – are unusual because they have papery, lantern-like husks, known to botanists as inflated calyces that grow after fertilisation to extend around their fleshy, often edible berries.
They are a small portion of the nightshade family, which includes many commercially, scientifically and culturally valuable plants among its more than 2,400 living species.
This entire family has had a notably poor fossil record, limited to tiny seeds and wood with little diagnostic value that drastically limited understanding of when and where it evolved.
The researchers from Pennsylvania State University in the US examined two fossil lantern fruit collected in an area in Argentina that was temperate rainforest when the plants grew, 52 million years ago.
These are the only physalis fossils found among more than 6,000 fossils collected from this remote area and they preserve very delicate features such as the papery husk and the berry itself.
The fossil site was part of terminal Gondwana, the ancient supercontinent comprised of the adjacent landmasses of South America, Antarctica and Australia during a warm period of Earth history, just before their final separation.
"These astonishing, extremely rare specimens of physalis fruits are the only two fossils known of the entire nightshade family that preserve enough information to be assigned to a genus within the family," said Peter Wilf, professor at Pennsylvania State University in the US.
"We exhaustively analysed every detail of these fossils in comparison with all potential living relatives and there is no question that they represent the world's first physalis fossils and the first fossil fruits of the nightshade family," said Wilf.
"Physalis sits near the tips of the nightshade family's evolutionary tree, meaning that the nightshades as a whole, contrary to what was thought, are far older than 52 million years," he said.
Typically, researchers look for fossilised fruits or flowers as their first choice in identifying ancient plants.
Since the fruits of the nightshade family are very delicate and largely come from herbaceous plants with low biomass, they have little potential to fossilise.
The leaves and flowers are also unknown from the fossil record. This presents a problem for understanding when and where the group evolved and limits the use of fossils to calibrate molecular divergence dating of these plants.
Molecular dates calibrated with previous fossils had placed the entire nightshade family at 35 to 51 million year ago and the tomatillo group, to which the 52 million year old fossils belong, at only 9 to 11 million years ago.
The research was published in the journal Science.