Flowering Fossil
A UF Paleobotanist And His Chinese Colleague Claim This 145-Million-Year-Old Fossil Shows One of The First Flowering Plants

By Steve Orlando

There are a lot of ways to ship things these days - FedEx, UPS, Airborne Express - but none was good enough for Ge Sun. The Chinese geologist wasn't taking any chances with the contents of the small, silk-covered box he needed delivered to Gainesville.

So he boarded an Air China Eastern plane in Shanghai last March for a 21-hour flight over the top of the planet to hand-deliver his treasure to David Dilcher, a graduate research professor of botany with the Florida Museum of Natural History at the University of Florida.

Nestled on red felt inside the small wooden container was a playing card-sized limestone fossil of a long-extinct plant known as Archaefructus liaoningensis. By the time Sun flew home two weeks later, it would have another name - the world's oldest flowering plant.

Dilcher has been searching for a fossil like this for nearly 30 years, and with good reason. Absent this plant, the world would probably be a very different place today. For instance, without plants' high-energy fruits and nuts, mammals may never have evolved beyond species like armadillo and opossum.

Publication of Sun and Dilcher's paper about the 145-million-year-old fossil as the cover story in the November 27, 1998 issue of the prestigious journal Science sparked a whole new thread of discussion among paleobotanists worldwide.

"This fossil may help us to understand the relationships among existing species - a major problem about which there is considerable controversy. These are major evolutionary questions," William L. Crepet, a former student of Dilcher's and now chairman of the L.H. Bailey Hortorium at Cornell University, wrote in a piece for Science that accompanied Sun and Dilcher's paper.

The fossil also has raised the profiles of two researchers already considered leaders in their respective fields.

"Professor David Dilcher is one of the most outstanding and experienced botanists and paleobotanists in the world," says Sun, deputy director of the Academia Sinica Institute of Geology and Paleontology in Nanjing, China. "His abundant publications on the origin and early evolution of the angiosperms [flowering plants] have served as guidebooks for paleobotanists worldwide, including myself."

Essence Of Flower

The latest milestone in Dilcher's three-decade pursuit of the oldest flower was found in Liaoning Province in northeast China near the town of Beipiao, about 250 miles northeast of Beijing. It was unearthed in a spot known as the Yixian Formation. Some Chinese scientists say the Yixian Formation may date back to the Jurassic Period, more than 142 million years ago.

Described by some as the most important paleontologic site discovered this century, the 46-square-kilometer limestone outcropping has produced fossils that have rewritten evolutionary history. Among the specimens unearthed there: Sinosauropteryx, a feathered dinosaur; Confuciusornis, one of the most primitive birds known; and Zhangheorium, an early missing-link mammal.

What makes the fossils so remarkable is their exceptional degree of preservation. Items as fragile as feathers and dragonfly wings have survived millions of years, stunning scientists with their condition.

In November 1996 Sun was examining several fossils a young colleague had brought in from the Yixian Formation. The colleague suggested one of them - two stems, each about 3 inches long - was a fern.

"To my surprise, one of them was a unique fruiting axis, which was no fern at all although it looks like one at a glance," Sun says. "I found the fruits helically arranged on the axes, each enclosing two to four seeds."

Upon closer examination under a microscope, Sun says: "I confirmed my observation that the fossil was a real angiosperm. I realized that the unique new fossil was just what I had been searching for."

After sharing his find with colleagues, who supported his belief that it was an early member of the legume family, Sun began preparations to publish the discovery. He even gave the fossil a tentative scientific name, Paleolegume liaoningensis, reflecting his suspicion that it was an early legume.

As winter snows covered northeast China's fossil sites and put a temporary halt to his field work, Sun conducted a more thorough examination of the fossil. He began writing manuscripts for publication in Chinese scientific journals, though some doubts about the fossil's true age and identity remained. In December 1997 he decided to consult Dilcher. Along with a Christmas card, Sun sent some photographs of the fossil and asked for advice.

Dilcher was impressed.

"When Sun told me he had gotten a seed out from one of the carpels, then it seemed like it was going to be an early flowering plant," he says. "That's what was so important about this find: It contained the very essence of what a flower is - the seeds that are contained within the fruit."

Looking over the photos, Dilcher noted that the fruits resembled folded leaves. The fossil, Dilcher theorized, was not a legume at all. He sent Sun a fax explaining his thoughts.

"This is a lot like the earliest Magnolioids that we know of, similar to Archaeanthus," he wrote. "I do not mean to say that it is Archaeanthus, but to say that there are similar features that the two share. Thus, to me, it does not look legume-like but much more Magnolioid-like."

In other words, it appeared to be a very early flowering plant.

Dilcher's fax prompted Sun to steer his work in a new direction. He changed the specimen's generic name from Paleolegume to Archaefructus, abandoned the legume theory and made plans to travel to Gainesville with the fossil so Dilcher could examine it in person.

After two weeks of intense study, Sun and Dilcher swept away nearly all doubt about the specimen's identity. They took more detailed measurements and checked for scars of any other organs that might have existed on the living plant. They examined the cellular structure on the axes, and they used epifluorescence to check for pollen.

But the specimen didn't have what would seem to be the most obvious characteristic of a flower: showy petals.

"The big question became, if there's no evidence of petals, what, then, is the flower?" says Dilcher.

The flower, it turns out, consisted mostly of carpels, or leaf-like pods that opened to release seeds. Non-flowering plants have no such structures.

But the stems also bore leaf-like organs, evidence that some other type of structures were attached to the stems just below the carpels. Dilcher says those structures may have been the first primitive version of leaf-like petals and may even have had a faint coloring to attract insects.

That, he says, may well be evidence of where the evolution of plants and insects first intersected. Plants became progressively more dependent upon insects to spread their pollen. In return, insects eventually became equally reliant on flowering plants for food.

The link became stronger still when birds and mammals radiated 45 million to 50 million years ago. Flowering plants began producing fleshy fruits, nuts and seeds that became a food source for the animals, and the animals returned the favor by carrying the seeds in ways the wind could not.

And it all took off with that first flower.

"There's been a profound development of co-evolution between plants and animals, a lock-step change through time," Dilcher says. In the Archaefructus fossil, he says, "we have an early plant that's just starting out in that co-evolution experiment."

Spreading The Word

Through each step of Dilcher and Sun's examination, the angiosperm theory held up, and the two became increasingly confident their Science paper would pass muster.

Still, perhaps the biggest question remained: What did the rest of the plant look like? The flower, Dilcher says, appears to be only the top portion of the plant it came from.

"We don't know if it was a tree or a shrub," Dilcher says, "although common wisdom has it that the early flowering plants probably were woody."

The pair wanted to know if there were more Archaefructus fossils at the Yixian Formation. Last August, Dilcher joined Sun in China for two weeks of searching. Their efforts were unsuccessful, but Sun and his colleagues since have turned up some promising new specimens that are still being studied.

"I take a skeptical point of view, but botanically, I think [the Archaefructus fossil] is going to hold up." Dilcher says.

Some of Dilcher's colleagues have been skeptical, too, specifically about the age of the fossil. A group of Canadian researchers places the age of the Yixian Formation at no more than 122 million years, which would place the Archaefructus fossil in the early Cretaceous instead of the Jurassic.

Dilcher, however, says the Chinese have used radiometric dating to place the age of Archaefructus at between 142 million and 148 million years.

"I defer to my Chinese colleagues. They work in the area and they know the area. I won't get in the middle of it," Dilcher says. "It doesn't matter anyway; if they're 122 million years old [instead of 145 million years old], they're still the oldest."

Lifelong Search

Dilcher began his search for the oldest flower shortly after graduating in 1960 with a master's degree in botany and minors in geology and zoology from the University of Minnesota. He attended the University of Illinois and then transferred to Yale, graduating in 1964 with a doctorate in biology and a minor in geology. He spent a year as a National Science Foundation fellow in Germany and another year as a biology instructor at Yale before moving on to Indiana University, where he spent 24 years on the faculty before coming to the University of Florida in 1990.

When Dilcher entered paleobotany, the study of flower fossils was almost nonexistent. The difficulty of working with such fragile, incomplete specimens had caused many researchers to turn their attention elsewhere.

In fact, in the 1960s, the benchmark oldest flower was a 100-million-year-old specimen discovered before 1910. In terms of major new discoveries or progress in research methods, the field had been stagnant for more than half a century.

Dilcher set out to change that.

In the early 1970s, Dilcher and Crepet, then one of his post-doctoral students, began refining their research techniques on some 45-million-year-old flower fossils Dilcher had collected in Tennessee, lifting out anthers and preparing pollen sacs for examination.

Dilcher and Crepet then began working with another colleague, Howard Reynolds from Hayes State University in Kansas, on some Cretaceous sediments in Kansas. Their fossil collecting continued for several years; eventually, they were able to positively identify, using their new techniques, some magnolia-like specimens that dated back about 100 million years.

"I thought, 'This is good,' but we knew flowers had to go back much further," Dilcher says.

In 1986, Dilcher met Sun at a conference in Nanjing, China. After listening to Dilcher's presentation, Sun "became excited and quite convinced that there must be early angiosperms in China as well," Dilcher says.

Each had found a kindred spirit. Their friendship grew two years later when each had a fellowship at the British Museum of Natural History in London.

Though primarily a geologist, Sun, after meeting Dilcher, focused his research on finding angiosperm fossils in China.

Dilcher, Sun says, "made great contributions to this study so that the paper could be successfully finished and published in Science. If not for him, such a nice and fruitful paper would not have been produced."

Even as media worldwide reported on the Archaefructus discovery, Dilcher and Sun were making plans to further their quest. In addition to wanting to know more about the Archaefructus fossil, they also are certain there are fossils out there that predate it.

"I don't think this was the one and only first flower," Dilcher says.

In August, Sun's Academia Sinica named Dilcher an honorary professor, and Dilcher will be in China in summer 2000 for a conference. He and Sun plan to do some more collaborating during that visit.

"We would expect to continue the search for even older flowering plants in China and even in some other places in East Asia," Sun says. "We have a lot of cooperative work to do in the near future."

David Dilcher
Graduate Research Professor, Department of Natural Sciences
Florida Museum of Natural History
(352) 392-6560, dilcher@flmnh.ufl.edu

Ge Sun
Deputy Director, Academia Sinica Institute of Geology and Paleontology
gsun@jlonline.com

Related web site:
http://www.flmnh.ufl.edu/natsci/paleobotany/paleobotany.htm

Turning Fossils Into Flowers 

UF paleobotanist David Dilcher collaborated with freelance scientific illustrator Susan Trammell to create the original representation of the first flowering plant featured on the cover and in this article.

"I had a concept of what the living plant looked like and wanted to capture the sense of that in the reconstruction," says Dilcher, whose work with other artists is used to illustrate other fossils in this article.

After several consultations with Dilcher, Trammell began to make her initial sketches. Since the fossil was already back in China, she worked from a large color print that was digitally output from a slide of the fossil.

With each successive meeting with Dilcher, Trammell revised the drawing until it represented the scientist's best guess of what this plant looked like when it was bearing fruit 145 million years ago.

"Reconstructing a plant that has been extinct for nearly 145 million years requires a unique balance between adhering to the reality of the fossil fragments and taking poetic license to fill in the unknown," Dilcher says.

Trammell says that once she and Dilcher had settled on a black-and-white line drawing, she made color test sketches of various areas of the plant to confirm that the hues were what Dilcher had in mind.

"I find the process of communication between scientist and artist an interesting and rewarding one," Trammell adds. "The finished illustration hopefully shows the success of that communication, and if it can be a thing of beauty as well as informative, I am very satisfied."

"Working with Susan Trammell allowed the plant that existed only in my mind to come to life on paper for all to see," Dilcher says. "Sharing that vision is the importance of reconstructing these flowers from the ancient past."