Explosive fruit fossils found entombed in volcanic rock

Distinctive fossil fruit capsules have revealed a new plant species present among plant material embedded in volcanic rock layers in India.

Just before the final scenes of the Cretaceous Period, India it was a rogue subcontinent on a collision course with Asia. However, before the two land masses merged, India passed over a “hot spot” within the Earth’s crust, triggering one of the largest volcanic eruptions in Earth’s history, likely contributing to the extinction of the dinosaurs.

The fossils – which likely exploded to disperse their seeds – may be the oldest fruit discovered to date from the Spurge family (Euphorbiaceae), a group of plants with more than 7,000 species, with well-known representatives including poinsettia, castor , rubber trees and crotons.

The fossilized fruits were discovered near the village of Mohgaon Kalan in central India, where remnants of once-widespread volcanic rock lie just below the surface in a complex mosaic.

“You can walk up these hills and find bits of flint that have just eroded through the topsoil,” lead author Steven Manchester, curator of paleobotany at the Florida Museum of Natural History, said in a statement. “Some of the best places to pick are where farmers have plowed the fields and moved the bits to the side. For a paleobotanist, it’s like finding little Christmas presents along the edge of the fields.”

Although there is some uncertainty, the volcanic eruptions are thought to have lasted up to a million years, occurring in prolonged pulses that blanketed the surrounding landscape with thick layers of lava up to 1 mile deep. Today, the basalt rocks left over from the eruptions, known as Deccan traps, cover an area larger than the state of California.

The most violent of the volcanic events, which occurred at the end of the Cretaceous, may have been triggered by an asteroid impact half a world away.

“The impact in Yucatan may have caused seismic disturbances that actually disturbed the regime on the other side of the planet, causing lava to erupt,” Manchester said.

Sandwiched among the basalt, paleontologists have found slates, flint, limestone, and clays stacked in a giant layer of alternating bands, most of which are rich in fossilized remains of plants and animals. These fossils give insight into what appear to have been relatively uneventful periods of stability between massive lava flows.

The newly described species were likely shrubs or small trees that grew near hot springs created by the interaction of groundwater with naturally heated rocks below the surface, similar to present-day environments in Yellowstone National Park.

At the time of its conservation, India was inching across the Earth’s equatorial zone, creating hot, humid conditions that supported a number of tropical species, including bananas, water ferns, hollyhocks, and relatives of modern crepe myrtles. .

Petrified wood is a common find in Deccan traps, but most of them are small in diameter, suggesting a lack of large trees whose conspicuous absence has stumped scientists trying to piece together the ecological history of the region.

“India was located at a low latitude, so we would expect to find large forest giants. But that’s not what we’re seeing.” Manchester said.

It’s not clear why the trees couldn’t get taller, but Manchester suspects the underlying basalt may have restricted root growth. Alternatively, he said, the plants may have been part of young forests that grew in volcanically active regions, which would have wiped out surrounding vegetation before it had a chance to mature.

“Fossils are more likely to be preserved when there have been recent eruptions, which creates a large amount of volcanic ash that can bury and preserve plants.” he pointed.

Co-author Dashrath Kapgate found fruits of the new species pristinely preserved in a matrix of flint. But with only the fruits to go on, determining which plants they belonged to required a significant amount of research.

“It didn’t really fit well into any known plant group,” said lead author Rachel Reback, who studied the fossils while working as a university researcher at the Florida Museum. “We ended up having to take a large number of CT scans of not only the fossils we had, but also the fruits of living species so we could directly compare them.”

The researchers ultimately determined that the fossils belonged to the Euphorbia family by studying similar fruit samples provided by the Smithsonian Institution. However, one of the fossils was so unlike anything they had seen that they determined it represented an entirely new species belonging to the fossil genus Euphorbia.

The orientation of the fibers within the fruit indicated that they were likely explosives, a common means of seed dispersal in other euphorbias, such as cassava, rubber trees, crown of thorns, and castor oil plant.