Flowerless plants make fancy amber

Everything eventually comes back in style. A type of amber thought to be invented by flowering plants may have been en vogue millions of years even before these plants evolved, suggests an analysis of newly discovered amber droplets. What kind of plant produced the droplets remains a mystery, but researchers say in the Oct. 2 Science that it could have been a predecessor of the ancient conifers or some strange extinct fern.

Ambers are fossilized plant resins known for their golden luster and almost mineral-like qualities. Scientists found the new droplets in a 320-million-year-old coal deposit in Illinois. Their age was a surprise in itself, says Ken Anderson of Southern Illinois University in Carbondale. They formed in the period known as the Carboniferous, when swampy forests of ferns and giant lycopsid trees dominated the Earth. Early conifers developed toward the end of the period. Fossilized resins from this era are typically waxy, unlike those from flowering plants, which wouldn’t evolve until nearly 200 million years later.

“We thought, ‘It’s got to be from an early conifer,’” says Anderson, who coauthored the new study with P. Sargent Bray of Macquarie University in Sydney, Australia.

Some early conifers — the relatives of today’s pines, spruces and firs — produced amber that would be familiar to gem collectors today. But such a find in a coal bed would still have been surprising, since conifers were barely on the scene.

When the researchers analyzed the amber, though, they discovered a chemical signature known only from the amber of flowering plants. That find suggests that some very old plants figured out how to make a particular type of resin that its modern relatives get credit for, an idea that has stopped paleontologists in their tracks.

“It’s a sobering story for people who study amber,” says David Grimaldi of the American Museum of Natural History in New York City, who wrote a commentary on the research in the same issue of Science.

Many plant resins have a solid fraction that hardens upon exposure to air, and another fraction that evaporates (giving off the scent of pine, frankincense and myrrh).

In life, the solid stuff acts as a Band-Aid, sealing off plant wounds to prevent infection by fungi or other microbes. It also notoriously traps wayward insects and other creatures that stumble in.

In death, many of these resins fossilize into ambers, which are typically categorized into classes based on the chemistry of their compounds, especially terpenoids. Terpenoids of ambers that come from conifers look different from terpenoids in ambers that derive from flowering plants.

When they peered at the symmetry and shape of the terpenoids in the Carboniferous amber droplets, Bray and Anderson expected them to look like conifer compounds. But instead the molecules resembled those in ambers from flowering plants.

The find suggests that chemical composition alone isn’t enough to definitively pinpoint an amber’s origins, Grimaldi says. If multiple plant groups evolved similar strategies for making resin, then chemistry can deceive.

Fossilized plant parts that accompany the amber are the key to clinching its pedigree, he says. Unfortunately, there was no such plant matter near the Carboniferous droplets. So even though the amber droplets are definitely not from a flowering plant, their origin remains unknown.

“This is a new wrinkle in the story of amber, a story that is much longer than we thought it was,” Anderson says.

That wrinkle, though, may come as no surprise to chemists, says David Gang of the Institute of Biological Chemistry at Washington State University in Pullman.

There’s a huge diversity of terpenoids in plants, which are crucial for making important hormones, including cholesterol. Since plants of the Carboniferous already made terpenoids, they wouldn’t have to do much to make the version of the compounds found in the Illinois amber, Gang says. A new version of a single enzyme could do the trick and could be accomplished with just one or two mutations.

“This is interesting and neat, but not startling,” Gang says. If anything, it highlights the dearth of plant chemistry information that scientists have. Researchers tend to focus on plants that are interesting to humans, Gang says. Things like citrus or pine smell nice, but there are 400,000-odd plants species out there. 

Nonetheless, the discovery probably has entomologists drooling. Amber has entombed spectacular specimens from days long ago, including termites, bees, algae, gnats and even a small frog and lizard. The Carboniferous boasted plenty of winged insects, including dragonflies as big as hawks, Anderson notes, so this opens the possibility of finding some trapped in amber.

And the find will probably be exciting for those who value amber for its beauty alone. There’s evidence that Neolithic people gathered amber on the shores of the Baltic Sea about 13,000 years ago (amber floats, which can sometimes confound its origins), and it may have been used in jewelry some 30,000 years ago, notes Jean Langenheim of the University of California, Santa Cruz. “This is an important discovery,” she says, one that promises to stimulate discussion in the plant resin community. “As we say, once you get interested in resins, you get stuck.”