Origins of flowers traced back to fossilized plants from 126 million years ago

NANJING, China — Charles Darwin described it as an “abominable mystery” as to when flowers first appeared on Earth. Now their origins have been traced back to fossilized plants from 126 million years ago.

This is around the time that angiosperms, or flowering plants, began to thrive. They produce flowers and fruits which contain seeds, and evolved from trees and shrubs, or gymnosperms. But how they did so baffled Darwin. He feared it would undermine his theories. The new study published in the journal Nature helps solve the puzzle, 139 years after his death.

Several hundred newly described prehistoric specimens provide a “missing link” in the history of flowers.

“They were extracted from fossilized peat and date to the Early Cretaceous in Inner Mongolia,” says lead author Gongle Shi, a professor at the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences, according to the South West News Service.

Origin of flowering plants
The silicified material was studied using the traditional technique of cellulose acetate peels (C, D) supplemented by X-ray computed micro-tomography (A, B). (Credit: Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences)

The exceptional preservation enabled detailed reconstructions of the green bracts, or flowering leaves, vivid blue stalked cupules and brown seeds. They may have been nibbled on by the spectacular Zhenyuanlong suni — the biggest winged dinosaur ever found — and Yutyrannus huali, the “feathered tyrant.” Primitive mammals including the badger-sized beast repenomamus, which dined on dead dinosaurs, would also have snacked on the plants.

“The reproductive structures display defining angiosperm-like features,” says Shi. They include a backward bending cupule, or the cup-shaped structure that encloses fruits such as the hazelnut and acorn.

The international team used a technique called “phylogenetic analyses,” equivalent to piecing together a plant family tree. They compared the extinct seed plants with ones from 252 to 66 million years ago.

“They are housed in collections around the world and could be related,” notes Shi.

The large dataset confirmed the shared morphological and anatomical similarities. “It suggested a very close relationship to modern flowering plants,” adds Shi.

It’s known that flowering plants arose from an ancestor in the gymnosperms. The fossil record contains many extinct types, and it is not clear which was responsible. It was definitely not one of today’s groups, like conifers, Ginkgo biloba or cycads.

The new plants, which the researchers term “angiophytes,” are very close relatives of modern angiosperms and provide clues to their origin. They show extraordinary diversity in the shape of their reproductive structures.

Fossilized flower plant
The team who found a 125 million-year-old fossilized plant in Mongolia. (Credit: NIGPAS)

“Two hallmarks of angiosperms are the carpel — the female reproductive organ that encloses one or more ovules — and an outer tissue layer called the cupule which surrounds them,” explains Shi. “The outer layer is a characteristic feature of angiosperms that is not seen in other seed plants. Explaining how the these features arose is a vital component for establishing the origins of angiosperms.”

Amazingly, the plant remains were collected almost a century ago and deposited in collections. Shi and colleagues recently unearthed them for a second time from museum drawers. The cupule may have provided extra protection, or aided dispersal, of the seeds which have just one layer, or integument. This is typical of all gymnosperms. Flowering plants have two, and the cupule could be the precursor to the second, outer one.

The “Eureka” moment was realizing that the cupules, with the seeds they surround, are also “bent back” on themselves. It mimics the distinctive ‘recurved’ seeds of today’s flowers. The top is bent back on the stalk that connects it to the plant body.

“Recognition of these relatives provides a partial answer to the question of angiosperm origins,” says Shi. “There are important implications for the emergence of additional characteristic features, such as the carpel and the stamen. They have been hiding in plain sight for almost a century.”

Professor Douglas Soltis, an evolutionary geneticist at the University of Florida who was not involved in the study, describes the fossils as remarkable. “They provide previously missing crucial information about the origin of angiosperms,” he says. “More fossils are badly needed to clarify angiosperm origin further.”

Flowering plants make up about 90 percent of all living plant species, including most food crops. In the distant past, they outpaced plants such as conifers and ferns, which predate them. How they did this, however, has has been a mystery.

Hundreds of millions of years ago, the Earth was dominated by ferns and conifers. But when the first flowers appeared 130 million years ago, they quickly spread to all parts of the world. It changed the landscape from muted green to a riot of vibrant color. There are hundreds of thousands of species, from oaks to wildflowers and water lilies.

Darwin was deeply bothered by how they conquered the world seemingly in the blink of an eye. Mammals and other groups evolved gradually.

In 1879, three years before his death, he wrote in a letter to his closest friend, botanist and explorer Dr Joseph Hooker: “The rapid development as far as we can judge of all the higher plants within recent geological times is an abominable mystery.”

“We are now a vital step closer to getting to the bottom of the mystery,” says Shi.

Report by SWNS writer Mark Waghorn.