Introduction: From Where and Whence
Step back in time and explore with us a primeval forest that flourished some 15-45 million years ago and then disappeared, leaving testimony of its existence in amber from the Dominican Republic. The description of this extinct forest will be extrapolated from select fossils recovered over the past 20 years. Before our exploration of the ancient forest begins, some important facts are needed to gain a perspective of the events that occurred so long ago. These facts are analogous to the knowledge an explorer of today would gather before embarking on a tripa bird's-eye view of the history and environment of a foreign country. Of course the time frame is quite different.
When asked "Where are we going?" the answer is more complex than just to the Dominican Republic or the Caribbean island of Hispaniola. We are traveling back to witness events that commenced during the age of the dinosaurs upon a mutable, shifting earth surface conveying an island across the sea on a journey that took millions of years. So we begin with the question of when Hispaniola first appeared in the earth's history. There are almost as many theories about the origins of the Greater Antilles and past earth movements in the Caribbean as there are scientists investigating this question. Although the following scenario agrees in principle with most of these theories, some of the dates presented here were obtained from information based in part from amber fossils.
What are now the islands of the Greater Antilles (Cuba, Puerto Rico, Jamaica, and Hispaniola) first emerged as molten magma spewing from volcanic activity along a marine shelf between North and South America. This probably occurred some 100 million years ago in the Cretaceous period when the dinosaurs ruled the earth. For millennia, this shelf remained beneath the sea but moved slowly east toward the Americas. It lacked the mass to breach the waves until, many millions of years later, subsequent lava flows pushed the rock above the sea and created a separate span of land roughly across where Central America is located today. This event, which connected North and South America, occurred near the end of the Cretaceous period, during which the earth sustained a cycle of extensive earthquakes and volcanic eruptions.
The newly formed terrain, known as the Proto-Greater Antilles, emerged barren and sterile about 65 million years ago. As the lava cooled, organisms began to arrive. First, the algae came as spores that were fortunate enough to land in moist crevices on the lava and germinatethus starting a succession of life that in time would culminate in a mature forest. Soon after the algae were established, other spore-forming plants such as moss, ferns, and lichens found favorable niches for growth. These plants, together with the elements, modified the rocky surface just enough for small pockets of soil to accumulate, forming a foothold for higher plants. Next appeared seed plants in the form of herbs and shrubs. It didn't take long for dispersing insects to discover the plants, followed by spiders and parasites to prey on the insects and so on until a complete ecosystem with reptiles, birds, and mammals appeared. Eventually, the Proto-Greater Antilles was covered with a forest full of life garnered from the adjacent land masses. Animals and plants, thriving in what is now southern Mexico, spread onto the young land from the north, and from the south came floral and faunal elements from South America. Included among the new plants were algarrobo trees, producing the resin that would eventually form the basis of this work. This ancient forest represented an early assimilation of biota from North and South America.
The Proto-Greater Antilles land mass containing the foundation of the amber forest rested on the Caribbean continental plate that broke away from its position between North and South America and started moving slowly eastward about 60 million years ago. During this ponderous journey, other life forms would periodically arrive in the forest, some from the air, such as insects, and some by over-water dispersal, such as reptiles. At times of low ocean levels, when the land mass butted up against shallow ridges, animals might have been able to board the shifting ark. Some of these animals undoubtedly were successful in establishing themselves, thus adding new elements to the amber forest. The tectonic history of this terrain was quite complex as it smashed into and slid around land masses and other drifting plates along its way. Despite an eventful journey, it arrived in the Caribbean about 25 million years ago, with the area of the Dominican Republic remaining emergent since its appearance some 65 million years ago.
Some questions remain regarding the age of the amber. A number of mines are scattered throughout portions of the northern mountain range of the Dominican Republic known as the Cordillera Septentrional as well as in the eastern part of the country. Deposits from these diverse locations may be from different time periods, but just how different is not known. Dating has been attempted by chemical analyses of the amber as well as by an examination of marine microfossils that occur in the bedrock. The youngest times suggested are 15-20 million years, based on foraminifera fossils, while the oldest proposed are 30-45 million years, based on coccolith fossils. Could this forest have survived for a much longer period, like 60 million years? If algarroba trees were producing resin during that entire period, what happened to the amber formed during the intervals spanning 4-15 million years and 45-60 million years ago? If a continual supply of amber did exist, was it destroyed by natural forces or does it still remain somewhere, locked away deep in the bowels of the earth? Or was the resin production limited to a relatively short period during the lifetime of a unique algarroba tree that produced copious amounts? In spite of the uncertainties regarding the exact dates, the vast array of fossils allows us to complete our paleo-reconstruction of the ancient forest.
Most of the amber lies buried in the fog-shrouded peaks of the north. The ranges cradling these treasures are thought to have formed in recent times, perhaps between 25,000 and 10,000 years ago. During this period of mountain genesis, the sedimentary beds of limestone and shale containing the amber were uplifted. Emergence is a very tortuous process involving the bending and breaking of rock layers. Thus the fossilized resin was subjected to tremendous shear forces, which undoubtedly destroyed vast amounts of it and left extensive fractures in the remaining material. No wonder it is rare to find a complete piece larger than a golf ball without any internal fractures.
The Taino Indians may have been the first people to note the presence of this gem and appreciate its beauty, although other indigenous cultures were present on the island of Hispaniola when the Tainos arrived. When Christopher Columbus came in the 15th century, the Tainos presented treasured pieces to him, but the Spaniards were more interested in gold, and amber fell into oblivion. It was "rediscovered" at the beginning of the twentieth century, first as jewelry and then for its scientifically valuable fossils, and now it provides a source of income to many.
Amber mines are little more than small, tortuous tunnels carved into the sides of the mountains, or sometimes pits sunk deep into the ground. Certainly there is no mine in the Dominican Republic as elaborate as the one shown in the movie Jurassic Park, where miners wearing hard hats and carrying pneumatic drills were depicted pushing car loads of fossilized resin along rails from deep within a huge mine. In reality, it is rare to be able to stand fully upright in an amber mine. The narrow tunnels, which can continue 600 feet into the side of the mountains, are only sufficient for crawling. During the rainy season, tunnels partially filled with standing water are frequently encountered, and miners must crawl half-submerged through the dark confining space to reach the treasure-laden veins. During wet weather, when the soil is saturated, cave-ins and landslides sometimes bury and take the lives of unfortunate miners. Workers are equipped with only the most basic toolsa hammer, chisel, candle, and sack. Flickering candles serve to illuminate the dark confines of the tunnel, producing barely enough glow to follow the gray layer of sedimentary rock encasing the amber. Days are spent lying supine, crouching or kneeling in that small space, chiseling away at the matrix rock and picking out the nuggets of fossilized resin that are exposed. Many small fragments fall out as the rock is dislodged, and valuable specimens are often lost or damaged.
At the end of an exhausting day, the miners return home and, filled with anticipation, spread out their finds in the last rays of the afternoon sunlight. Morsels with rare or spectacular fossils are infrequent and these are placed aside. Eventually all of the marketable items end up at a workshop where young boys will polish them. Standing in front of noisy machines that run from dawn to dusk, the polisher holds the amber against sandpaper, removing dirt and sharp irregular edges and smoothing the surface. The hot, humid air seethes with powder, thus many of the workers wear masks. If a fossil is present, an attempt will be made to get as close as possible without wasting too much of the sample, since this gem is often sold by the gram.
Specimens are then turned over to a middleman who shows them to a variety of buyers searching for the best price. The scientific value of amber is, of course, due to the types of life forms it contains. If a fossil is large or rare, it can be a significant source of income to the owner. A vertebrate such as a frog or lizard can bring 30-40 thousand dollars, and large, well-preserved scorpions can be sold for as much as 15 thousand. The demand for choice fossils is so intense that many are now bought directly from the miner's sorting table.
It is difficult to say how many hands various fossils pass through before they eventually reach paleontologists. Many valuable and scientifically important items end up in the hands of private collectors. Some people will allow researchers to examine their personal collections to document new finds and describe new species. However, in many cases, individuals are unaware of significant fossils in their possession or are reluctant to share them, with the result that many remain undiscovered. There is no doubt that the great majority of rare and scientifically important specimens are in private collections, however, since most museums can little afford the exorbitant costs of unique amber fossils.
Why Select Amber for Study?
What is so singular about amber that merits using it to reconstruct an ancient forest? If you examine other types of preservation such as compression fossils, you will find that most insects are two dimensional, uniformly colored, and lack fine morphological details. In contrast, amber fossils provide edifying clues to the past for the following reasons: first, they are three dimensional; second, their original color patterns are often preserved; and third, the matrix has protected them from much of the abuse that other fossils receivethus the preservation is outstanding. In addition, the organisms usually expired so swiftly that many are revealed in almost lifelike circumstances. Their accompanying organisms also had little chance of escaping. Thus there is evidence of past symbiotic associations that are rarely preserved under other circumstances. Finally, these fossils encompass plant, invertebrate, and vertebrate remains, all of which are important in a study like this one.
In each photograph presented here, you will see a clear picture of a frozen moment in the amber forest when it existed on a land now known as the Dominican Republic. Earlier we described what our bee may have seen in the final few hours of its life. By fitting these candid snapshots together like pieces of a jigsaw puzzle, we can make a kaleidoscope of life in its environs. The elegance of these fossils adds to their general enjoyment and fascination. Portrayed will be ancient life forms from a realm that no longer exists in the Greater Antilles, nor anywhere else in the world today! Such extinct forms can answer questions about paleobiodiversity, evolution, and biogeography. They can also enlighten us about past climates, insect-plant interactions, and parasitic relationships.
Amber fossils will be used to describe the primeval forest in relation to tropical forests as they exist today. There is no precedent for employing this technique for reconstructing a past tropical ecosystem, since aside from fossilized resin, the remains of organisms in tropical forests are few and scattered. High decomposition rates and low fossilization potential account for the meager number of actual fossils. In addition, insufficient rock exposure would render finding them almost impossible. The little that has been written to date about extinct tropical forests is based mainly on pollen records.
How Biased Are Amber Samples?
Can a selection of amber fossils portray a true sampling of past life in an extinct domain? One cannot hope to obtain evidence of all life forms that existed in that ancient forest. The majority of creatures preserved are those that lived or foraged on the bark of trees, such as worker ants, bark lice, certain beetles, and small flies. Such forms became entangled in the sticky resin as they performed their daily activities. The next most common category of organisms found are winged insects that were flying through the forest and alighted or were blown by a gust of wind against some sap on an algarrobo tree. A few insects, like our stingless bee and predator bug, were attracted for other purposes. Such organisms give a fairly accurate view of life in and around the algarrobo tree. Other, larger fossils from this same habitat are uncommon due to the limiting size of the resin flow (a small flow might only partially entrap a large specimen, making its remains susceptible to decay) and because of the creature's strength and ability to extricate itself from the sticky trap. Then there are the exceptional inclusions such as vertebrate remains, scorpions, orchid bees, and others. But rare does not just mean big, since fleas and ticks are as infrequent as frogs and lizards. These uncommon animals appear only occasionally and became entrapped by providence while searching for prey or by some unexplained misfortune. Based on experience, the likelihood of a rarity materializing is one in every five thousand pieces. Appendix A at the back of the book shows the frequency of types of organisms found in some three thousand pieces of amber. Obviously ants are the most abundant group represented, followed by gall midges, with bark lice close behind. Less frequent fossils include centipedes and grasshoppers. The more specimens examined, the more variety discovered. Each treasure is a scientific gem instructive in learning more about this former biome, yet unfortunately many of these are being sold as jewelry.
As we discuss the natural history of the extinct animals in amber, we will draw heavily on the principle of behavioral fixity in the fossil record. This principle is an important concept in the reconstruction of ancient worlds and states that the behavior, ecology, and climatic preferences of fossil organisms will be similar to that found in their present-day descendants at the generic and often family level. This well-documented principle is very helpful in deciphering the behavior of ancient organisms.
Thus it may be possible to indirectly infer the presence of a plant or animal that existed long ago by finding an organism that today is intimately associated with it. When we discover fig wasps, for example, we can be certain that fig trees existed even though we have not found their actual remains, since it is known that fig wasps can develop only inside the fruits of this tree.
Structure of Tropical Forests
Present-day tropical forests are immensely diverse and complex, teeming with an incredible array of plants and animals. Yet all tropical silva the world over share certain characteristics. Their lush vegetation consists mostly of evergreens, grouped according to size and growth habits. The vegetation is dispersed in more or less recognizable layers or strata. Beginning with the top, the canopy includes the bulk of the vegetation in tropical forests. Viewed from above, this layer forms a leafy roof appearing as an almost continuous undulating expanse of various patterns and colors. Flowers and fruits reaching up toward the sun provide splashes of vivid hues against a canvas with an amazing variety of textures. High above the floor, the branches abound with the many animals that seek food and shelter in this bountiful layer. The height of the trees in the canopy layer depends upon the type of forest, but in the amber forest it probably varied from 80 to 130 feet. Poking through this canopy layer to imposing heights of 160 feet are the giants of the forest, the emergent trees. Directly beneath the canopy is the subcanopy layer, which includes trees ranging from 40 to 80 feet. Underlying these layers, wherever patches of sunlight filter down into clearings or along the banks of wide river beds, is the understory layer, with residents growing from 20 to 40 feet. Among the understory vegetation, the inhabitants find refuge from the sun or rain. Still lower in the almost continuous shadows of the towering giants is the shrub layer, with plants spanning from 5 to 20 feet in height. In the sparse light of the forest floor are another set of plants adapted to the dim, still environment, seldom reaching over 5 feet. Linking these layers together and providing roadways for the wildlife are herbaceous vines and woody lianas that form a twisting, interconnecting web. Epiphytes, living on branches in all the strata, are salient components of the tropical forest. Gaps generated by storms and disease allow the genesis of secondary growth, which for a period contribute their own special habitat and life forms before eventually becoming part of the primary forest. It is assumed that the amber woods had these basic features as well.
Tropical forests today cover only 7 percent of the land surface, yet contain about 50 percent of all living species. How long these woodlands have prevailed is unknown, but it is clear that the flora and fauna that characterizes each one is indirectly dependent for its survival on soil and climatic conditions, especially moisture. An examination of the vegetation is the surest way of defining the forest type, and the plants determine what types of animal life can survive in that ecosystem. Beginning our journey into the past, we will now examine plant and animal clues in amber to elucidate the mysteries of the forest that was the home of our bee.
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