The history of life on Earth is deciphered through the examination of fossils. Fossils are inorganic remains of plants and animals that reveal the structure of certain parts of the organism. By examining the way structures change in certain organisms over time, the natural history of a particular group can be reconstructed. Many organisms do not have structures suitable for fossilization, like the soft body of a snail. However, some portions of the snail's body (the shell) are easily turned into fossils over time. Snail shells, millions of years old, are discovered when they are exposed near the surface of Earth. By studying these fossils it is possible to partially deduce what snails, or any other fossilized lineage of organisms, looked like over millions of years of change. Living fossils are divided into two categories. The first includes organisms that are believed to have changed very little over time and that still retain a close resemblance to their older extinct relatives. Examples of these types of organisms are found all over the world. A familiar example of a living fossil of this type is the horseshoe crab . The structure and body of the horseshoe crab is very similar to ancient fossils of arthropods . Arthropods evolved to become several distinct and large groups of organisms. Insects, crustaceans (crabs, shrimp, and lobsters), and a wide variety of additional animals are all descendants of the early arthropods. Although the living arthropods share several features of the ancient arthropods, they are also quite different in major body structures and functions.
The horseshoe crab, however, is still very similar to the ancient forms of arthropods and helps us to understand how ancient arthropods looked and lived. This is why crabs are called living fossils. In the continental United States, the horseshoe crab is found only on the Atlantic seaboard.
Another example of this type of living fossil is the shark. Sharks do not appear to have changed very much over hundreds of millions of years. The rare fossils of sharks and their relatives show that they had bodies very similar to the species living today. Shark teeth are more commonly found as fossils since they are the only true bony part of the animal. By comparing the teeth of ancient and modern sharks it is possible to see that sharks also qualify as living fossils.
In the plant kingdom, the horsetail rush and the palmlike cycad are considered the last living relatives of their extremely ancient ancestors. The relatives of these plants are often found as fossils in regions where dinosaurs are discovered. They are believed to have been the food of some of these giant herbivores .
In the marginal areas of the sea that are adjacent to the land there is a single remnant of a once flourishing group of bivalves called the brachiopods . They once lived in the oceans by the millions. There were numerous species and they were distributed over the coastal regions of the entire planet. The brachiopods lived for many millions of years and were as common as clams are today. However, only one simple species has survived. It is a small brachiopod called Lingula.
The brachiopods appear initially to be similar to clams. In truth, however, they are quite different. Their soft tissues (internal organs) are nothing like those of a clam. In fact, brachiopods are so different from clams that they do not even belong in the same phylum, Molluska. If Lingula were not still living it is possible we would know very little about how the brachiopods lived, reproduced, or what their internal structure was like. This living fossil helps biologists to know a great deal about an ancient group that may have been a mystery.
The second category of living fossil is more exciting for researchers to discover. They are examples of species of plants or animals that were believed to have been extinct, but have been rediscovered in modern times. They are truly living fossils in that they are species of ancient organisms that have survived extinction by living in small pockets of land or ocean in which conditions may have been marginal, but livable. They have lived and reproduced continuously for hundreds of millions of years, but managed to escape notice until the last century. These organisms may once have been found throughout the world, but live today in very specific, geographically small areas.
One of the most famous examples of this type of living fossil is the coelacanth. The coelacanth is a member of a group of fish called the Sarcopterygians, which are believed to be the ancestors of the land vertebrates . They are commonly referred to as the lobe-finned fish because their pectoral and pelvic (front and back) fins are very thick and sturdy and look more like heavy lobes than fins. These fish are long and round and often use these fins to rest the weight of their heavy bodies on the seafloor. The structure of the fins of the Sacropterygians is very similar to that of the bones of all land vertebrates, including humans, leading researchers to believe that these ancient fish were the ancestors of the amphibians and all other tetrapods.
Until the early part of the 1900s, all that was known about the coelacanths was derived from the study of fossils. However, on a winter day in 1938, fishermen in a boat near the mouth of the Chalumna River, off the coast of South Africa in the Indian Ocean, found a strange fish tangled in their nets. As the story goes, a British woman living in the region recognized the fish as rare and unusual. The woman purchased the fish and sent it back to Britain where it was declared a marvel. A lineage of fish believed to have been extinct for over 65 million years was still alive! Examining this extraordinary fish was like looking into the past. It had all the characteristics of the fossil species, but was different enough to be given its own species name. The scientific name for this fish is now Latemeria chalumnae.
The excitement from the discovery of this fish has still not subsided. About 200 additional Latemeria have been discovered since the first one was caught. Scientists do not believe their populations are very large, and they apparently live in very deep water, which makes it difficult for researchers to study them. First, they are hard to find. Second, they are very difficult to keep in captivity for study because they require a high-pressure environment in order to remain alive. However, scientists are gradually learning more about this living fossil and are successfully keeping it alive in newly designed aquariums. In addition to this advance, another species, Latemeria menandosis, was recently discovered off North Sulawesi Island in Indonesia.
One of the important things scientists have learned by studying these living fossils is that they apparently hatch their young inside their bodies which is unusual for most fish. This piece of information has lead to speculation about how these early ancestors of amphibians may have reproduced. This may have led to the ability to possibly breed out of water without endangering or drying out their eggs. Unfortunately for the coelacanth today, this means that they cannot reproduce in large numbers and need all the help from conservationists they can get.
It is believed that there are only a few hundred of these fish alive at present. They are still in danger of extinction and in need of protection. However, if any animal qualifies for the title of "living fossil," the coelacanth is probably the most famous.
Another living fossil was discovered during the last century. This fossil was of a plant believed to have been extinct since the demise of the dinosaurs. The plant belongs to a group called the gymnosperms, or naked seed plants. They are part of a group that includes plants that make seeds without a seed coat. The living fossil we are talking about is called Gingko biloba.
For hundreds of years this small tree was known only from fossil impressions in clay. Its fossil record extends back in time almost 200 million years and the tree was once common all over North America and Europe. The Ice Age is believed to have made environmental conditions too harsh for the plant's survival. In more recent times, the gingko was considered a sacred tree in China, and a group of Buddhist monks cared for and nurtured the tree in a remote valley in China. This small group of trees was discovered by explorers and reintroduced to the world. Unlike the coelacanth, the gingko has flourished since being rediscovered and today gingkoes are found flourishing all over the world. They are a link to the days of the dinosaurs and can be found as ornamental trees in many communities. The gingko has become so popular it is even included in some botanical medicines.
Additional examples of living fossils exist in many groups of animals and plants. Some mollusks , such as the Nautilus, were another amazing discovery of a remnant species of an ancient group of cephalopods. Other modern relatives of the cephalopods include the octopus and squid. It too was discovered in deep ocean waters off the coast of Indonesia. One of the intriguing aspects of the nautilus is that it represents a very primitive form of the ancient cephalopods. Animals derived from this ancient group are called ammonites and have extremely complicated and beautiful shells. Some grow to extremely large size compared to the nautilus and measure several feet across. They are believed to be carnivores of the deep oceans and able to swim up and down columns of water in the ocean feeding on fish they encounter.
Until the nautilus was discovered, the Nautiloids were considered extinct and people knew very little about the soft tissue structure of these amazing animals. Biologists have created special environmental aquariums, and the nautilus has been successfully kept and bred. These specimens are providing scientists with increasing information about the life history of this group of mostly extinct animals.
No matter what the type, living fossils provide scientists with a window to the past. They help us understand how life on Earth evolved and what kinds of plants and animals inhabited the planet before humans. It is hoped that we will continue to discover more living fossils and learn more about the inhabitants of this planet.
see also Geological Time Scale; Morphological Evolution in Whales.
Brook Ellen Hall
Carroll, R. Vertebrate Paleontology and Evolution. New York: W. H. Freeman, 1982.