Planet Earth

Golden Window on a Lost World

Tens of millions of years ago the residents of Carribean forest were trapped in glowing ribbons of resin. Today they're telling scientists what their world was like.

By Virginia MorellAug 1, 1993 5:00 AM


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One day in the late Eocene, a jaguar prowled through a bamboo thicket on the Caribbean island of Hispaniola. As the cat padded by, several seeds snagged onto its fur with their tiny hooks, a dispersal trick that often carried the seeds to fertile ground. This time, however, their free ride ended in a pool of resin, and immortality of a sort. Irritated by the seeds’ spiky hooks, the cat rubbed against the trunk of a Hymenaea tree, a great resin producer of the American and African tropics. A wound in the tree’s bark oozed a puddle of the sticky stuff, and by chance the cat left a tuft of fur and one of the annoying grass seeds in the goo. Later, another flow of resin spilled over the fur and seed, sealing both in what would prove to be one of nature’s ultimate time capsules.

Tens of millions of years later the resin, still encapsulating the fur and seed, would surface again, now hardened into the fossil called amber. A miner tunneling into one of the Dominican Republic’s amber mines recovered this nodule, and the specimen eventually found its way into the hands of paleoentomologist George Poinar at the University of California at Berkeley.

Poinar’s passion for amber has nearly outgrown his office. His shelves are jammed with plastic bags of amber waiting to be sorted; photos of favorite amber specimens decorate his office walls; and a map of the Caribbean is centered squarely over his desk. It’s not difficult to understand why Poinar is so enamored: each piece of amber forms an exquisite crystalline world, often filled with fragile insects, flowers, and bits of moss.

To display his discoveries, Poinar prefers the detail of photomicrographs, and in one such blowup the cat hair and seed are clearly visible. It’s a rare find, Poinar says. Through the resin’s golden window, the seed’s tenacious hooks look burnished and surreal, like a kind of giant, mutant Velcro grasping the spiky tuft of hair. We can’t say absolutely that the animal was a jaguar, Poinar says, but we analyzed the hair with a high-resolution microscope, and it came out as a carnivore, not a rodent--the first evidence of carnivores in that forest. And it very well could have been a jaguar, since there are reports of grass spikelets exactly like this one being found on jaguars in South America today.

These particular species of carnivore and bamboo are now extinct on Hispaniola, the large Caribbean island that encompasses both the Dominican Republic and Haiti. Therefore this amber-embalmed specimen offers a unique snapshot of a lost world. Together with similar specimens, it presents a rare opportunity to piece together--in great detail--what this small corner of Earth looked like 40 million to 25 million years ago. It was then a forest of tall, broad-leaved evergreens whose trunks glistened with shiny ribbons of resin. Beetles and termites scuttled under the bark, seeking the rich food of decaying wood; small reptiles crawled up and down the trunks, looking for food; ants paraded overhead, carrying bits of dead insects or leaves; nearby swarmed bees, flies, and midges. From time to time these unwary denizens--along with everything from feathers to flowers to frogs--would become trapped in the resin that served as the trees’ first line of defense against destructive insects. Because of the resin’s remarkable embalming powers, nearly all of these specimens--so many of which now adorn Poinar’s office--are displayed in their full three- dimensional glory.

Three thousand miles from Poinar, David Grimaldi, a paleoentomologist at the American Museum of Natural History in New York, places a piece of amber containing a termite beneath his microscope. Amber is not like any other fossil substance, he says, adjusting the focus. It used to be thought that the inclusions were simply husks of the original animals, but often there is actual tissue inside.

In September 1992 Grimaldi’s research team made headlines by extracting viable 30-million-year-old DNA from just such a termite--a demonstration of how actual the amber-preserved insects’ tissue can be. Poinar too has found astonishingly ancient DNA, in six amber-embalmed stingless bees, which he thinks might be as old as 40 million years. And this past June--just in time for the release of Jurassic Park--Poinar announced that he’d extracted DNA from an extinct weevil entombed in Lebanese amber believed to be 135 million years old, squarely in the age of dinosaurs.

Grimaldi’s and Poinar’s teams are far from being able to re- create ancient creatures, as Michael Crichton’s fictional scientists do; each of the teams has extracted only a fragment of a single gene. Nevertheless, their research has sparked a resurgence in the study of amber fossils as key resources for the study of evolutionary biology. For example, Grimaldi and his team have already used the termite’s DNA to resolve a long-standing dispute about the family tree of termites and cockroaches; and Grimaldi has plans for similar studies for other species. Poinar, for his part, is investigating the relationship between the stingless bees and today’s honeybee. Grimaldi’s team and another group in Japan are also investigating the possibility of extracting a developmental gene from a well-known preserved insect, such as a fruit fly, and then inserting it into a modern-day relative to see if the gene affects the growth patterns of the living fly.

Moreover, by re-creating the ancient amber forest of the Dominican Republic, Grimaldi, Poinar, and their colleagues hope to resolve greater evolutionary issues--for the carnivore and bamboo are not the only species extinct on Hispaniola today. Nearly 40 other species that were fossilized in the amber have vanished also. Even the tree that produced the resin is now gone. We’d like to know why these particular species went extinct while others survived, says Grimaldi. By looking through the window that the amber provides, we may find an answer.

Deposits of amber are found throughout the world and range in age from 300 million to 1.5 million years old. For many years amber from the Baltic regions (which is between 55 million and 35 million years old) was the best known and studied, because of both its abundance and its inclusions of ancient organisms. But today the Dominican Republic’s amber (40 million to 25 million years old) is the scientific darling, largely because it contains more specimens. Approximately one out of every 100 pieces of Dominican Republic amber holds some remnant of the past, compared with one in 500 pieces for Baltic amber.

Scientists first became aware of the Dominican Republic’s amber riches in the 1960s. But even as they collected the island’s amber- preserved fossils, they had not mastered the mystery of the amber itself. They were not sure what tree had produced the resin, or why and how it had produced so much. At the time, paleontologists assumed that most amber deposits, including those in the Baltic area and the Dominican Republic, originated in coniferous forests. It’s what most scientists coming from temperate climes were used to--pine trees that produced pine resin, explains Jean Langenheim, a plant ecologist at the University of California at Santa Cruz. In 1964, however, Langenheim proved them wrong. She identified the tree that produced amber deposits in Chiapas, Mexico, not as a pine but as a flowering member of the legume family. Her innovative study used infrared spectroscopy to compare the ancient amber with resin produced by modern plants.

The resin compounds absorb different light wavelengths, she explains, and these spectra are sufficiently different that they can be used to produce a chemical fingerprint for each plant. Combining these findings with evidence gained from studying the bits of leaves and pollen preserved in the amber, Langenheim deduced that the ancient tree in Chiapas was closely related to the modern Hymenaea courbaril. Today H. courbaril grows in forests from Mexico to South America to the Caribbean, where it is known as a great resin producer.

Langenheim’s success in identifying the source of the Chiapas amber turned her into a sort of resin junkie, pursuing resin-producing trees and plants around the world. To further understand the trees’ chemical defenses, she now grows many of these tropical resin producers in her greenhouse. So it wasn’t surprising that when paleobotanist Francis Hueber discovered what he thought might be a clue to the identity of the Dominican Republic’s amber tree, he contacted Langenheim. Hueber curates the Smithsonian Institution’s collection of amber-preserved botanical specimens. I first visited the Dominican Republic in the 1970s, he recalls, and the number of amber shops and the volume of material was mind-boggling even then. It’s more so now. And, oh, you get hungry for some of the things you see.

On that first trip Hueber purchased one of the Smithsonian’s first pieces of Dominican Republic amber: a two-inch polished lump containing what looked like a leaf. But as I studied it, I realized that it was actually a large petal of a flower, says Hueber. The fan-shaped petal was dotted with hundreds of tiny resin pockets--structures containing a chemical resin concoction often lethal to caterpillars and beetles. Hueber knew that Langenheim had identified such pockets on the petals and leaves of Hymenaea trees, and he suggested that they join in a collaborative study of this specimen.

Together they discovered that the petal also came from a species of Hymenaea, although not the one found in Mexico. Instead, the petal most closely resembled the petals of H. verrucosa--a tree found today only in East Africa, and one Langenheim had suggested was the ancestor of the 13 Hymenaea species found in the Americas today. Langenheim speculates that H. verrucosa was once widely distributed in Africa and that ocean currents carried its seedpods to the Americas 65 million years ago. From those few parent plants evolved all of the New World species, including the now extinct Dominican Republic variety, which Poinar named H. protera.

With the island’s amber-producing tree identified, scientists could finally tie the insects and other organisms they were finding in the amber to a particular forest environment--one dating to at least 25 million years ago. At that time, Hispaniola was an island as it is today--although 20 million years before that, when sea levels were dramatically lower, it had been part of one huge, ring-shaped island incorporating Puerto Rico and Cuba. This super-island may even have been attached at times to Central America. Grimaldi, who has made six collecting trips to the Dominican Republic to gather modern insects for comparison with those in the amber, believes that during this phase many plants and animals migrated to Hispaniola from the mainland. Then when the sea rose again, Hispaniola was left a separate island. The species isolated on its shores evolved over time to fill nearly every ecological niche available.

We can infer a number of things about the environment from these specimens, says Grimaldi. For example, this is the diving beetle dytiscid, and if you look closely, you can see the fringes on the hind legs. It’s odd that a diving beetle would be caught in amber, but they do make dispersal flights [away from their hatch sites to a new home], and this one may have mistaken the resin for water. The beetle is dark brown, small, and snub-nosed, with long hind legs adapted for diving. Its presence and that of other water-loving insects indicates that the ancient amber forest was wet, at least for part of the year. There probably were not standing pools of water, says Grimaldi, because we know that Hymenaea trees today don’t tolerate brackish water very well. They prefer to live along streams. So I picture a forest with small streams feeding through it and emptying into a lagoon.

Rather than being grouped in stands, the Hymenaea trees themselves were probably scattered throughout the forest, as are their living relatives. Towering and broad-leaved, modern Hymenaea trees form part of the forest canopy--and very likely Hymenaea protera did as well. We know that fig trees and palms shared the forest with Hymenaea, since fig wasps and palm-feeding beetles were entombed in the resin. Orchids, bromeliads (pineapple-like plants), and vines probably festooned the smooth-barked trunks and branches, while ferns and grasses sprouted below. We have several different species of mosquitoes, and one species is specialized to breed in the water that collects in bromeliads, notes Grimaldi. Other pieces of amber contain the roots of aerial plants, such as orchids, while specimens of land snails suggest a rich covering of leaf litter on the forest floor. But there had to be sandy stretches of earth as well because sand flies also landed in the resin.

Creamy white metalmark butterflies hovered in the canopy overhead, perhaps seeking the nectar of Hymenaea’s flowers, while tiny moths laid their eggs on shelf fungi growing on the tree’s trunk. Later the moths’ caterpillars would find nourishment by burrowing into the fungi-- which have not been found in the amber but which Grimaldi knows must have been there because of the presence of these moths. We have a few caterpillars of other species in the amber, Grimaldi adds. They were probably feeding on the Hymenaea leaves and somehow fell into the amber.

So defensively armed are modern H. courbaril trees that every part--bark, twigs, green seedpods--oozes resin at the slightest injury. In the Amazon, Langenheim found several species of Hymenaea dripping with resin stalactites, some four inches wide and four feet long. Often resin covered the tree’s trunk, while big lumps of it weighing several pounds were buried in the soil beside the roots. Langenheim correlates this copious amount of resin weeping with the great variety of wood-boring, leaf-chewing insects that trees in the tropics must ward off.

The vast Dominican Republic deposits indicate that the ancient Hymenaea protera was similarly under attack and equally well armed--and like some giant form of flypaper, the resin trapped anything and everything that touched it. Thus there are amber specimens containing downy feathers, perhaps plucked by a woodpecker when preening on a branch overhead, while others hold tufts of rodent fur, maybe left behind when a rat or squirrel brushed too close to a sticky limb. Rodents scurrying up the tree trunks may also have knocked off the bits of mosses and liverworts now found in the resin.

There are hundreds of flies and stingless bees encased in chunks of golden amber. The flies, often groups of tiny male midges, swarm next to trees, waiting for a female to fly by. Those in Grimaldi’s collection had apparently flown too close to a resin-covered trunk, leaving them stuck forever in the insect equivalent of a singles bar. The stingless bees, too, hovered close to the trees, particularly where fresh resin was present, since they used it to build their nests. But the job was fraught with hazards. Besides the danger of being trapped, the bees faced the threat of lurking predators--tiny, fat-bodied assassin bugs that also harvested the resin, fixing little droplets to their front legs. They then lay in wait next to a resin pool, and when one of the bees came within reach they lunged forward to catch it on their sticky forelimbs.

In some cases, Hymenaea’s chemical defenses worked against it by attracting the tree’s enemies. In particular, the wood-boring platypotid beetles--one of the most common specimens preserved in the amber--were apparently enticed by the scent of the resin. As their relatives do today, they burrowed into the bark, where they released ambrosia fungi from pockets on their thorax. The fungi further damaged the tree and assisted the beetles by acting as a plug against the resin. Sometimes, though, the trees evidently managed to snare their attackers: many large pieces of amber contain the beetles, their tunnel galleries, their droppings and bits of sawdust, and the fungi.

Some creatures that found themselves stuck in the resin appear to have tried to escape. One grasshopper in Grimaldi’s collection was entombed while squirting a defensive spray from its abdomen, which left a tiny chain of chemical globules in the amber. It reacted as if the resin was its enemy, Grimaldi explains. But there was no escape, and a second flow of resin ended the grasshopper’s life. Some insects, such as pseudoscorpions, were unwittingly pulled into the resin by other creatures. Some pseudoscorpions are phoretic, meaning they hitch rides on other insects to move, and this one’s attached to a platypotid beetle that dragged it right into the resin, says Grimaldi.

More than 40 genera of ants have been found in the amber, along with some of their probable tormentors. This is a male twisted-wing parasite, Grimaldi says, focusing his microscope on a particularly exquisite specimen. The creature is shaped somewhat like a fly and is equipped with large, floppy, diaphanous wings. The females are completely different, Grimaldi says. They look like tiny worms and live in the abdomens of fire ants. When they mature and emerge from the ant, it’s very ugly.

Many of these species are now extinct, yet they all have living relatives, often widely dispersed--some in Central and South America, others as far afield as Australia, India, and the Indo-Pacific. Even Hymenaea protera, the tree that produced the resin, has been replaced on Hispaniola by Hymenaea courbaril--the tree Langenheim studied in Mexico. What became of the inhabitants of the ancient amber forest? We have a lot of work to do before we can answer that question, says Grimaldi. We don’t know when any of these species vanished; it could have been as recently as a million years ago. But I don’t think the insect extinctions are related to the disappearance of H. protera, which may have been completely cut down in historical times. Certainly the extinct stone flies and termites didn’t require H. protera to live.

Grimaldi suspects instead that rising and falling sea levels-- along with certain peculiarities of island ecologies--caused the extinctions. You have to remember that we’re looking at island populations, island faunas, and ecologists are very much aware of how fragile they are. Species on islands evolve in a completely different population context than do those in large continental populations. Because island species may originate with only one or two founding parents, they are often more closely related genetically than those on continents. They also tend to be specialists, concentrating on making a living in one region of the environment. Thus, if for some reason a competing species is introduced from the mainland--one that is much more of a generalist and not so fussy about what it eats or where it nests--the island species often goes extinct.

Grimaldi thinks that’s what happened on Hispaniola. It’s possible that what we’re looking at in the amber is a remnant fauna that had evolved in isolation for 20 million to 30 million years, since the islands drifted away from Central America. Then, perhaps 30 million years ago, in the late Oligocene, the sea level dropped and continental species were able to cross onto the islands again. They would have wreaked havoc, outbreeding and outcompeting any similar island species. That could explain these extinctions.

By documenting the species that did go extinct and comparing them with their living relatives, Grimaldi hopes to further biologists’ understanding of fundamental evolutionary mechanisms: how new species occur, and how and why others vanish. The amber-preserved insect fossils are particularly suited for this type of research; unlike the megaextinction of the dinosaurs, for example, the extinctions on Hispaniola did not wipe out every species but only select ones--a pattern that is more typical of extinctions in general (if less dramatic). This means scientists can look for common ecological threads that link the extinct species but not the survivors, and eventually find a pattern.

There are other mysteries, the chief one being: Why did so much resin accumulate in one part of the Dominican Republic 30 million years ago? Although some researchers believe the amber accrued over tens of thousands of years, simply as a response to wood-boring insects, Grimaldi prefers to explain the vast deposits with a natural disaster.

From his collection he produces bark bugs, bark lice, more wood- boring beetles and termites, and many species of flies, midges, and ants-- all associated in some way with dead, decaying, and dying wood. Were the Hymenaea trees unhealthy, or were these insects simply part of the forest’s natural cycle? We’ll probably never know for certain, says Grimaldi, but I think that some catastrophe, probably a tropical storm, destroyed the forest, similar to what we saw in the Everglades after Hurricane Andrew. Branches are broken, trees are snapped off at the trunk, and this causes massive amounts of resin to bleed. The beetles and termites move in, causing more damage, and they flourish because of all the dead wood. Maybe then there’s a high tide, or another storm sweeps through, and the trees and resin are buried in the lagoon’s sediments.

Grimaldi’s and Poinar’s success in extracting ancient DNA from insects may also shed some light on the extinctions, enabling the researchers to study genetic differences between these organisms and their living relatives. Furthermore, researchers will be able to see how fast a particular species has evolved over time. For example, Grimaldi’s team discovered that one extinct Dominican Republic termite is relatively unchanged from its living Australian descendant. There were only eight base-pair changes over 30 million years, says Grimaldi, which means that this insect is highly conservative.

That there is any DNA-containing tissue left in these organisms after so many millions of years is due, the researchers believe, to the sugars and alcohols in the resin. These inhibit microbial growth and stop the natural decay process. We know that the preservation happens quickly, and we know that the resin acts essentially as an embalming agent, says Grimaldi. It really is mummific; it dehydrates the specimens.

Resin that turns to amber begins to harden almost immediately, simply from exposure to light and air. Small molecules called monomers link up in chains and webs to form polymers. Over time, the resin continues to polymerize, making it highly resistant to environmental influences, and if other conditions are right--if the resin is buried, if it is covered by seawater--in several million years it will become amber. When it finally comes into the hands of a scientist, the amber is like a natural form of glass and will fracture just as easily. (It’s the very hardness of amber that makes it both gemlike and impervious to traditional chemical analysis; it’s largely insoluble and not easily broken down into its basic elements. To this day, scientists have not succeeded in re-creating amber in the lab.)

In the Dominican Republic, the buried amber deposits were brought back to the surface when, some 20 million years ago, two tectonic plates collided, lifting shoreline sediments into a newly formed mountain range. Rains then eroded the mountains, and piece by piece the amber tumbled again into view.

The amber deposits themselves are often spotted after a rainstorm causes a mud slide, exposing a fresh vein of lignite, or fossilized plant matter. The local people see that dark vein and say, ‘Ahhhh, amber!’ says Grimaldi. They follow the lignite, digging by hand with pickaxes, shovels, even tin cans. But how they spot the amber is beyond me. The tunnels are narrow, dark, and wet, and they often collapse, killing people. Grimaldi climbed down one once, compelled, he says, by science. But so claustrophobic did he find the tunnel that he has never gone back inside. Instead the miners bring out the specimens--dark, opaque lumps that they crack with a machete to see if there are any inclusions. Those pieces with organisms are funneled to the shops in Santo Domingo and Santiago, and the shopkeepers then send word out over an informal amber network to scientists like Grimaldi, Hueber, and Poinar--who are always hoping for the call that there is something new, something different, something that will help them add yet another piece to the puzzle of Hispaniola’s past.

Meanwhile, the amber cycle in the Dominican Republic continues. It may no longer begin with Hymenaea protera, but H. courbaril is equally armed with resin, dripping it over its enemies in great icicles of gold-- some of which surely end up buried at the edge of the sea, slowly turning into amber.

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