explosive fruit found under ancient Indian lava flows

An international team of researchers from Rashtrasant Tukadoji Maharaj University in Nagpur, India, and the Florida Museum of Natural History describes a new fossil species of fruit from specimens discovered in Indian chert.

The fruit of Euphorbiotheca deccanensis. Image credits Florida Museum / Kristen Grace.

India’s Deccan Traps form one of the largest volcanic features on Earth today. They were formed by a series of giant volcanic eruptions between 66 and 65 million years ago. Although precise dating of these eruptions is difficult due to several factors, they are believed to have occurred over a period of 30,000 years during the time period indicated. These eruptions covered an area of ​​almost 500,000 square kilometers in several layers of basalt rock, piling up in some places more than 2 kilometers deep. The “trap” in the name is a geologic term that refers to igneous, dark-coloured, fine-grained, non-granitic rock – in this case, layers upon layers of basalt flows.

Despite the violent conditions in which this structure arose, the Deccan Traps are today one of the richest fossiliferous areas in the world. A new species of extinct plant, described from fossilized fruit specimens discovered in the region, adds to the rich list of discoveries made in the traps of the Deccan.

Explosive fruit

Bodies of shale, chert, limestone and clay are contained in alternating bands between basalt flows in the region, and they are rich in fossilized plants and animals. These layers capture a snapshot of the quiet periods and ecosystems that formed between eruptions.

The newly described species was most likely a shrub or small tree growing around the area’s hot springs (an ecosystem that resembles those of present-day Yellowstone National Park). At the time the current specimen was fossilized, India was crossing the equatorial zone; the humid and warm conditions of the time allowed several tropical species, including bananas, water ferns, mallows, and relatives of modern crepe myrtles, to thrive on its land. A puzzling discovery from this time is that, although petrified wood is common in Deccan traps, most examples found had small diameters, indicating a lack of large trees – a surprising fact for a tropical region.

“India was located at a low latitude, so we expected to find large forest giants. But that’s not what we see,” says lead author Steven Manchester, a paleobotanist at the Florida Museum of Natural History.

Manchester suspects that the basalt layers covering the area prevented the development of deep roots, which in turn limited maximum tree growth in the area. Another possible explanation he considers is that because the region was very volcanically active, the forests were regularly wiped out by the lava flows, so the trees here simply didn’t have enough time to grow. . Although bad for the trees, these events created a lot of volcanic ash, which helped fossilize plant material in the area.

The fossilized fruit described in this study was found preserved in a body of chert, silica-rich rocks, by co-author Dashrath Kapgate. The lava is rich in silica, so it is possible that active volcanism in the area at that time helped create the conditions for its fossilization.

The researchers explain that the fruit “didn’t really match any known plant group.” They took a large amount of CT scans of the fossil as well as fruits of extant species to allow direct comparison. Ultimately, similarities to fruit specimens provided by the Smithsonian Institution led the team to conclude that the fossil belonged to a member of the spurge family (Euphorbiaceae). However, because it was so different from any existing plant, they classified it as an entirely new species, deccanensisof the fossil type Euphorbiotheque.

Based on the orientation of the fibers inside the fruit, the team is convinced that they were meant to disperse their seeds by exploding at maturity – a common mechanism in other plants of their genus. After the explosive ripening of the fruits, they dry out considerably – losing up to 64% of their original weight – which creates a lot of tension in the rigid outer layers. At some point this leads to the fruit breaking, the stored tension sending seeds flying everywhere.

“You hear this loud pop, and the seeds and fruit pieces are flying everywhere,” Manchester said, describing the process in the rubber trees. “We think this is also the case for these two fossil species, because we see the same anatomy, where the fibers of the inner and outer layers of the fruit wall are oriented in opposite directions, which helps to create a couple .”

The team hopes their discovery will help us piece together the distribution of species at a critical time in Earth’s history. By the time this fruit fossilized, India and Madagascar had already broken away from the supercontinent Gondwana and were heading north into modern Eurasia. Life here evolved in isolation throughout the Cretaceous, leading to unique species including (it is thought) modern grapes and the ancestors of whales. When India finally collided with Asia, these plant species had the opportunity to spread to new lands, populating today’s Europe and Asia.

The article “Fruits of Euphorbiaceae from the Late Cretaceous Deccan Intertrappean Beds of India” was published in the International Journal of Plant Science.