The only permanent feature of our world seems to be its variability. Riverbeds and ocean coastlines are changing, mountains are rising from the interior and fall down, new continents are born and die… And what can be said about the living flesh — so fragile and insignificant in the fourth dimension called time? Nevertheless, it is the living intricately organized matter, not the soulless inorganic world, that gives us examples of constancy and resistance...
The phenomenon of “living fossils” is amazing. It is known that the evolution of living creatures proceeds fast in terms of geological time. Indeed, formation rate of new taxa differs in different groups of organisms. If we do not take into consideration the world of microorganisms with its specific laws of speciation, at least among eukaryotes a certain species has existed, on average, several hundreds of thousands or millions of years, and then gives rise to a new one or goes extinct. In some groups of animals, especially protozoans or molluscs (for example, bivalves and gastropods, as well as ammonites), some mammals (for example, rodents), the rate of speciation is even higher. It is owing to the change of living forms that we notice how dynamically the image of the Earth’s organic world changed in the past geological epochs.
At the same time, there exist organisms which are an exception to the general rule. These are some types of animals and plants “frozen” in time, representatives of earlier widespread taxonomic groups. Such forms are called “living fossils” or, less frequently, “persistent types” (Schindewolf, 1993), or “relics” (Davitashvili, 1969; Yablokov, Yusufov, 1989).
“Living fossils” are quite rare. If we assign to them only those organisms whose structure is the same as that of their ancestors that lived in the early Mesozoic or even Paleozoic eras, then there would be no more than a few dozens. It is just a trifling quantity against those numerous hundreds of thousands of species living on the Earth, just a small fraction of a per cent of the general diversity. Nevertheless, their importance for science is hard to overestimate. The point is that a serious problem paleontologists are faced with concerns the reconstruction of different biologic features of fossils, for example, their physiology or reproduction processes. It is difficult to get an insight into the structure of ancient animals and plants, because only solid parts, that is, shells, testas, and bones, are kept intact in fossils… And what was the structure of soft tissues of the extinct species? That’s where “living fossils”, living museum specimens, carefully kept by nature, come to help paleontologists.
Eternity frozen in shells
The first representative of “living fossils” makes us recall antique mythology. A myth of ancient Greece tells us a story about the sculptor Pygmalion, who, disappointed in his contemporaries, carved his ideal of feminine beauty. A beautiful marble statue called Galatea was brought to life by the goddess of love and beauty Aphrodite, who took pity on the sculptor.
The name of Galatea was given to the research vessel that swept the Pacific Ocean near Costa-Rica in 1952. When sorting out a catch lifted from a depth of 3590 m, a strange mitriform shell with the top shifted to one of the edges was found. After a thorough examination of the specimen, a conclusion was made that it belonged to molluscs, not to modern groups widely represented in seas and oceans, but to monoplacophorans, believed to be extinct long ago.
The monoplacophorans differed from other molluscs by symmetrical muscles and some other “archaic” features. The monoplacophoran caught in the Pacific Ocean closely resembled its Cambrian (!) relative of the genus Pilina. Its modern great-great-great-“grandchild” was called, quite naturally, Neopilina, that is, new Pilina, the specific name N. galatheae being derived from Galatea, the vessel that witnessed the discovery. Eight modern species of monoplacophorans grouped in four genera have been described up to now. They live in the Pacific, Indian, and Atlantic Oceans, mainly at great depths (from 1000 to 6500 m), crawling the bottom with the help of a muscular foot.
Neopilina galatheae and its relatives serve as evidence that we know a lot about the organic world of the past. If, while studying modern fauna, we discover groups that are long known by their fossils, then our knowledge of “former biospheres” is commensurable to what we know about the surrounding world.
One of the most popular representatives of the “living fossils” is Nautilus, which lives in the seas washing the shores of Indochina, in the eastern part of the Indian Ocean and in the southwestern part of the Pacific Ocean, at depths of 100 to 700 m. Its beautiful mother-of-pearl shells attract jewelers and shell collectors.
The ancestors of the modern Nautilus, cephalopods from the Nautiloidea subclass felt at ease in Paleozoic seas. All the diversity reduced to one Nautilus, also called a “pearl boat”, represented in the modern fauna by several species.
In the Cambrian, there appears one more group of sea invertebrates whose modern representatives can be assigned to “living fossils” without reservation. These inarticulate brachiopods with their chitinous phosphate shells belong to the genus Lingula, meaning “little tongue”.
The simply structured bivalve shell of the lingula has an elongated oval shape and is small (about 5 mm long). From the side of the umbo, the foot with the help of which the animal anchors into the ground is protruded. From the other side of the shell, the lingula could protrude long “hands” with cilia making the water flow into the shell, which helped to filter and pass through small food particles.
Lingulas often live in the conditions not favored by other bottom dwellers, for example, in highly desalinated sea water or in near-bottom sites chemically contaminated, as a rule, by hydrogen sulfide. That is why it often happens that lingulas form homogeneous communities. Such lingulas’ “dormitories” can be found in modern seas as well as in fossil material. Paleozoic sedimentologists are well familiar with lingula clays in Upper Permian (Kazan’) deposits of European Russia. Literally stuffed with small lingula shells, they were formed, most likely, under the hydrosulfuric conditions. Thus, lingulas can be used as an important paleoecologic marker.
Sea scorpions’ brothers
While a non-specialist could experience some difficulties in determining the age of ordinary looking molluscs, the unusual appearance of the following representatives of “living fossils” eliminates any doubt.
First of all, we are speaking about the horseshoe crab, or limulus (Limulus polyphemus). In spite of its unattractive generic name (Polyphem was a cyclops who tried to kill Ulysses and his companions), the horse–shoe crab is a sweet and harmless creature. Its body consists of a cephalothorax in the form of a horse–shoe shaped half-spherical shell (hence the English name – a horse–shoe crab, even though it is not related to crabs), an abdomen, and a long tail shaped as a sharp sword (that is why in Russian it is called a sword-tail). The “sword”, however, is not a weapon; the crab uses it to bury itself in the ground.
Horse–shoe crabs live in seas of Indochina and in the Caribbean Sea, whose shores with sandbanks are perfect for spawning their young. Horse–shoe crabs can grow rather large, there are specimens no less than 1 m in length. The shells of dead individuals sometimes form such big accumulations on the shore that local people mill them for fertilizers. Limuli are also quite popular with the collectors of various sea wonders.
Surprisingly, many million years ago horse—shoe crabs lived on the territory of Russia. For example, the shell of a horse-shoe crab was found in the Lower Cretaceous sandstone near Moscow along with ammonite shells. Ancient remains of horse–shoe crabs were discovered in the Triassic sediments of the Volga region. The cephalothorax of a large horse–shoe crab was found in the Late Permian sandstones in the Ural region, and small ones are frequent in coal sediments of the Donets Basin along with the remains of land plants. There are examples of horse–shoe crabs from even more ancient sediments. Their closest relatives, “brothers”, were sea scorpions, who terrified the inhabitants of the Silurian and Devonian sea lagoons and wide river mouths.
The chronology of appearance of horse-shoe crab ancestors in the geologic record reveals a remarkable regularity. The most ancient horse-shoe crabs had a body with a great number of segments. For example, the Cambrian Aglaspis spinifera had eleven of them. In the forms that are young from the poin of viev of evolution, the number of segments steadily decreases due to the merging of some neighboring ones. As a result, the Permian horse-shoe crab Palaeolimulus avitus already bears resemblance to the modern one. Its abdomen is a single shield, although it still has visible cross furrows, marking the borders of the last six segments merged at the edges.
A similar evolutionary tendency is observed for many other fossils that first had a segmented body composed of identical metamers. In the process of evolution the number of segments is reduced, and their “specialization” takes place. The process of morphological transformations widespread among animals and plants is called oligomerization.
One more classical example of “living fossils” is tuatara (Sphenodon punctatus), a reptile from the group of Rhynchocephalia, whose closest relatives all disappeared in the Mesozoic era and whose immediate ancestors appeared on the Earth before dinosaurs. Now tuatara inhabits only small reserve-area islands near the New Zealand coast.
Under the canopy of the Mesozoic era
“Living fossils” are common in the realm of plants as well. Thus, in central Russia well-known mosses and horse-tails grow; they are spore plants preferring, as a rule, moist and shadowy places. Now both of them are not very tall herbaceous forms. However, their Paleozoic ancestors, which felt quite comfortable in forests, were real giants.
Calamites are often mentioned as almost immediate ancestors of modern horse-tails. These are huge articulate-stemmed plants of the Carboniferous period. This supposition is not quite true. The calamites were, undoubtedly, close to the ancestors of horse-tails in their taxonomic position, but the genealogical tree of horse-tails stems, judging by many characteristics, from another group of Paleozoic articulate-stemmed plants; namely, from the Tchernoviaceae family. Their reproduction zones consisted of numerous sporiferous organs located on several successive stem internodes. In the process of evolution, the number of generative zones of the Tchernoviaceae reduced (once again oligomerization!) to one zone which became the prototype of the strobile – a sporiferous spikelet of modern horse-tails.
The first place among the plant “living fossils” is, beyond any doubt, occupied by Ginkgo biloba. Generally speaking, it was this astounding tree that the term “living fossil” was first attributed to, and by the creator of the theory of evolution, Charles Darwin himself.
The modern ginkgo is quite a tall tree, reaching 30 m in height. It has a beautiful pyramidal crown and characteristic leaves with a rhombic or triangular blade on a long bipartite petiole (hence the generic name biloba, i.e., bi-bladed).
Today ginkgo can be encountered in the wild only in China, where there are several natural populations of the species, and in Korea and Japan, where it was cultivated as a holy plant and grown around Buddhist temples. Lately ginkgo has begun to enjoy worldwide popularity: It can be seen in Paris and Berlin, New York and Moscow, in botanical gardens and growing on streets.
The ancient ginkgo, quite similar to the modern one, was widespread in Mesozoic forests, especially in the Jurassic and Cretaceous periods. The seminiferous organs of the Mesozoic ginkgo were characterized by a host of seeds attached to the generative axis. The modern species has only two seeds, and only one of them usually ripens by the end of the generative season. In addition to the Mesozoic ginkgo, we know plants of the Paleozoic era which had leaves and seminiferous organs greatly resembling those of ginkgo. One of such plants, Kerpia macroloba, was found in the Permian sediments of the Ural region.
What is the secret of longevity of “living fossils”? Obviously, it is a very suitable combination of characteristics necessary for surviving in the constantly changing world. They include, first of all, an effective reproductive system which allows them either to produce a great number of offspring or to protect them from their enemies.
Secondly, of great importance is their relatively poor integration into communities of other organisms. Nearly all “living fossils” can be characterized as cenophobic; i. e., organisms that do not form stable relations with other components of the biocenosis. That is why when in the crisis periods of the ecosystem’s evolution many species go extinct, these “individualists” still have a chance to survive the unfortunate times and to settle in a new biotic environment. Thirdly, among “living fossils” there are many so-called eurybiontic organisms, which can live in a wide range of climatic and ecologic conditions.
“Living fossils”, organisms that have survived almost unchanged since the most ancient times, can tell a lot about the life of their remote ancestors known to paleontologists by their fossils. But the significance of these vestiges of the past far exceeds their pragmatic value. Is it not wonderful to walk along wet sidewalks on which, slowly turning in the air, the “hearts” of ginkgo leaves are falling? Their golden carpet rustles under our feet just as it did in a very distant past under the paws of giant dinosaurs and quick animal-like reptiles, first “drafts” of the future King of Nature…
Davitashvili, L. Sh. Causes of the Extinction of Organisms. Moscow, Nauka, 1969.
Yablokov, A. V., Yusufov, A. G. The Theory of Evolution (Darwinism). Moscow, Vysshaya Shkola, 1989.
Holder H. Naturgeschichte des Lebens. Berlin, Heidelberg, New York: Springer-Verlag. 1996.
Schindewolf O. H. Basic questions in Paleontology. Geologic time, organic evolution, and biological systematics. Chicago: University of Chicago Press, 1993.
The work has received support throngh grants of the President of Russian Federation (MD-1703.2005.5), the program of the Presidium of RAS “Origin and Evolution of Biosphere”, and from the Russian Foundation for Basic Research (NSh-1615.2003.5)
The author uses his own materials, photographs, and pictures. The author and editors would like to thank Kor Kwant (The Netherlands, http://www.xs4all.nl/~kwanten/) for kindly donated photographs of a modern ginkgo