What are some examples of living fossils
Living fossils, introductory article by V. Fahlbusch, part 2
Animals and plants - all in good time
It has been known for nearly two hundred years that organisms have changed constantly during the long periods of geological history. Reconstructing this from the fossil remains, which are often very sparse, is a difficult and far from finished task. But the rough features of the development are somewhat known to us from the fossil documents. One of the important findings of paleontology on the basis of the fossil remains is that every period of the earth's history has its own animal and plant society which is characteristic of it. Of course, there are considerable differences depending on the geographical areas, i.e. seas and continents with their many differences and differentiations. And there have always been very special societies of plants and animals, adapted to the respective habitat, which we call biocenoses. Above all, however, it is important to recognize that every time has its own animal and plant world, admittedly different according to habitat. If one examines the individual species or groups of species of organisms from different periods of the earth's history, it quickly becomes clear that they are ultimately the result of different speeds of development. This explains why, on the one hand, very original and, on the other hand, highly developed groups of organisms, often adapted to special environmental conditions, occur at the same time and side by side.
The specialty of geology / palaeontology that has made use of this peculiarity of the development of organisms as a function of time is biostratigraphy. She tries to classify the long periods of geological history on the basis of the development of animal and plant organisms or to determine the age of certain rock strata based on the fossils they contain.
Living fossils without fossils
After our detour to the speed of development of individual groups of organisms and their temporal distribution in the history of the earth, let us return to the paradoxical biological phenomenon of "living" fossils. Because it is no longer so paradoxical and contradictory. However, one must know that there is by no means a generally accepted or scientifically founded definition for the term "living fossil". Incidentally, that was already the case in Darwin’s time. The examples he deals with show very clearly what different "types" can be understood by them.
As already mentioned, today's lungfish belong to a time-honored tribe of fish. It is true that the few survivors show clear differences and different specializations among themselves: the South American Lepidosis more than the African Protopterus, and both together more than the Australian Neoceratodus. However, they all descend from common ancestors from the Devonian period (around 400 million years ago). At that time they were particularly varied and frequent. Measured against the long and widespread distribution in the history of the earth, the few present-day representatives are, so to speak, the last representatives of a fish group that was important in fossil history. For today's Australian lungfish Neoceratodus it should be added that it was not discovered until 1870, while its closely related and very similar forerunners from the Triassic (about 230 million years ago) through fossil remains of the genus Ceratodus had long been known to paleontologists. Thus, even if they are to be found "by chance" and "barely" alive in relic areas that are far apart from one another, they are actually more fossilized than today's, living world - truly leftover, still living fossils.
A similar example with a comparable fossil history, but thought to be extinct for around 100 million years and not rediscovered alive until 1939, is the coelacanth Latimeria. The exciting fossil history of this sister group of the lungfish and especially their living descendants will be discussed in more detail later because of their importance for the higher vertebrates.
It is completely different with Darwin’s second "living fossil", the platypus Ornithorhynchus. Together with the beaked hedgehog, it belongs to the small, very original, but zoologically extremely interesting group of egg-laying mammals, the monotremata. These are now only found in small populations in the Australian region and are critically endangered. They are characterized by numerous original features (egg-laying, shoulder girdle, no teats, shape of the sexual organs) and thus clearly differentiated from the mass of more highly developed mammals. In contrast to the lung fish discussed above, the fossil history of the monotremata is almost completely unknown. And yet they can be called "living" fossils with Darwin, because they show features that represent an original level of development that mammals must have gone through in the Mesozoic era. Despite all the special adaptations, today's monotremata can be viewed as a model of a former evolutionary stage of mammals. And in this respect, they may very well be called "living" fossils, even if fossil precursors themselves are not yet directly known.
|The platypus Ornithorhynchus, still occurring in Southeast Asia today. As an egg-laying mammal, it represents a very original stage of development within mammals. From Mohr: Collection of natural history panels.|
A similar, no less interesting example of a "living" fossil is the pointed squirrel, which is now found in the primeval forests of Southeast Asia Tupaia. Its body is reminiscent of squirrels, and on the one hand it has features that are reminiscent of more original mammals from the insectivore relatives (hedgehogs, shrews, moles). On the other hand, one can also observe characteristics that are otherwise known from semi-monkeys, i.e. primates. There are still doubts about the actual assignment of this animal, since it cannot be ruled out that the primate characteristics are so-called parallel developments that do not indicate a real relationship with the primates. Nevertheless, this animal also documents a stage of development that shows us, like a model, how the early development of primates from relatives of insectivores - i.e. in the fossilized state, as it were - could have taken place in the Upper Cretaceous around 80-100 million years ago . So you can too Tupaia regarded as a "living" fossil, although no direct fossil precursors are known (so far).
|Picky squirrel Tupaia, today's Southeast Asia. With features of native insectivores and primates, it shows a blueprint as one might expect in mammals 70 million years ago. From Mohr: Collection of natural history panels.|
Forgotten by evolution?
In spite of Ornithorhynchus and Tupaia as "living fossils without fossil ancestors" are the truly exciting representatives of those organisms that can also be described as "old timers of the animal and plant world", those that have remained unchanged for tens or even hundreds of millions of years without serious changes in their shape and way of life have survived and are documented by fossil remains. Is it they who have been "forgotten" by evolution in the shadow of the phylogenetic changes and apart from the "advantageous" adaptations to new areas of life, to new climatic or other ecological conditions?
|Examples of 'living fossils' and their temporal distribution in geological history, From Thenius 1965, modified. |
(click on picture for enlargement)
In fact, there is a not inconsiderable number of such "oldtimers" in both the flora and fauna. ginkgo with the plants, Lingula at the arm pods (brachiopods), the "pearl boat" nautilus in the molluscs, the horseshoe crab Limulus in the arthropods or the bridge lizard Sphenodon among the vertebrates are common examples of "living" fossils. Some will then be dealt with separately in their own chapters and shown with examples in the exhibition. They show that there are "living fossils" in all major animal and plant groups. Above all, however, they or very close relatives are documented by fossils over long geological periods. Over tens or even hundreds of millions of years they show little or no change in shape: Limulus for 150 million years and Lingula for almost 500 million years. These are truly gigantic periods of time in which these organisms have not undergone any significant development. They retained an original construction plan, without special adaptations to particular habitats or diets, "unspecialized": masters in enduring geological epochs.
What this means only becomes really clear when one looks at the great changes that can be observed in the relatives of these "living fossils" in often much shorter periods of time: fundamental changes in the habitat with great ecological differences, under changed climatic conditions clearly different diets, different modes of locomotion, and what other developments the individual groups of animals and plants may have experienced. These are the omnipresent processes of evolution, which in their complex mode of action have grasped and changed the organisms and ultimately led to the great diversity for which the catchphrase "biodiversity" has recently become established. Occasionally such highly and highly differentiated stages of development are referred to as "progressive" - a dangerous, easily misunderstood term for describing organisms.
Not so with the "living" fossils. Once acquired, they have retained organizational forms and ways of life (in the broadest sense of the word) and retained corresponding morphological features, often preserved almost unchanged over geological time on the fossil remains. This expresses extensive or even complete adaptations with which these organisms fit into the environment, regardless of short-term changes in the environment, which specially adapted forms are often unable to follow, so that all that remains for them is extinction.
"Living fossils" are characterized by permanent forms of organization. They belong to the diversity in the world of organisms with their constant changes, as it were as standards of development - anything but "forgotten by evolution".
The bridge lizard Sphenodon (Picture above) still lives on some islands near New Zealand. This primeval reptile has hardly changed its shape for 200 million years: this is how the closely related one differs Homoeosaurus (Right picture, Paläontologische Staatssammlung München) from the Upper Jurassic (150 million years old) only marginally from the bridge lizard
|last change by R. Leinfelder||copyright|
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