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Living FossilLiving fossil is a term for any living species (or clade) which closely resembles a species otherwise only known from fossils. These species all survived major extinction events, and generally retain low taxonomic diversities. There is a subtle difference from a lazarus taxon. A lazarus taxon is a taxon (either one species or a group of species) that suddenly reappears either in the fossil record or in nature (i.e., as if the fossil had "come to life again"), while a living fossil is an species that (seemingly) hasn't changed during its very long lifetime (i.e., as if the fossil has always lived). The mean species turnover time (the time a species lives) is about 2-3 million years. So, a living species that was thought to be extinct (the coelacanth fish for instance) is not a living fossil by strict definition, it is a lazarus species. Coelacanths disappeared from the fossil record some 80 million years ago (upper Cretaceous). If, however, Cenozoic Latimeria (thus belonging to the Latimeria genus) fossils were to be found, Latimeria chalumnae would be considered a true living fossil. Of course, species do not just appear out of thin air, so all living lazarus species are nonetheless considered living fossils (excluding disappearing and reappearing red list species). Some living fossils are species that were known from fossils before living representatives were discovered (the most famous example of this are the coelacanth fishes, Latimeria chalumnae and Latimeria menadoensis). Others are a single living species with no close living relatives, but which is the survivor of a large and widespread group in the fossil record (a well-known example of this is the ginkgo tree, Ginkgo biloba). 260px
Note the similarity between the 170 million year old fossil Ginkgo sp. leaves on the left, and the living plant on the right. Examples - Plants
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- Animals
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History The term was first coined by Charles Darwin in the Origin of Species, when discussing Ornithorhynchus (the platypus) and Lepidosiren (the South American lungfish). On page 49, he wrote: "...All fresh-water basins, taken together, make a small area compared with that of the sea or of the land; and, consequently, the competition between fresh-water productions will have been less severe than elsewhere; new forms will have been more slowly formed, and old forms more slowly exterminated. And it is in fresh water that we find seven genera of Ganoid fishes, remnants of a once preponderant order: and in fresh water we find some of the most anomalous forms now known in the world, as the Ornithorhynchus and Lepidosiren, which, like fossils, connect to a certain extent orders now widely separated in the natural scale. These anomalous forms may almost be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition." Other definitions There are quite a lot of (ambiguous) definitions about living fossils: - A living species that lived trough a large portion of geologic time
- To prove this, all living specimens must belong to the same fossil species. This rules out Limulus, Peripatus, Latimeria, Sphenodon, Didelphis, the platypus, and many others. To allow some flexibility, the genus could be used. It should be noted that paleontological taxonomy relies on hard part morphology (the paleontological species concept), so there is a bias towards longer species turnover times, and relationships can only be inferred partially.
- The living specimens must not belong to the same fossil species (or even genus).
- There must at least be some physiological resemblance. The coelacanth for example, is a marine fish. The Mesozoic coelacanth species lived in salt and fresh water. Osmoregulation in Latimeria is handled by ureum retention. Ureum retention is considered as an indication for fresh water ancestry. This means the coelacanth lineage evolved from freshwater to saltwater. The resemblance between Peripatus and Aysheaia (an early Cambrian animal from the Burgess shales) is striking (as of now, both are classified in the Tardipolypoda (Tardigrada and Onychophora)), were it not that Aysheaia was a marine animal, while Peripatus lives in tropical leaf mould.
- A living species/clade with many primitive characteristics
- This is a more neutral definition. However, it makes not clear whether the the taxon is truly old, or it simply has many ancestral characteristics (plesiomorphies).
- Some paleontologists consider 'living fossils' with large distributions (such as Triops cancriformis) not to be real living fossils. In the case of Triops cancriformis (living from the Triassic until now), the Triassic specimens have lost most of their appendages, and they haven't been examined since 1938.
- An organism's living fossil status can be rejected if the (smallest) clade the species belongs to is species rich, as this would imply (recent) speciation.
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