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There is a subtle difference between a "living fossil" and 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 a 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 lasts before it is replaced) varies widely among the Phyla , but 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 simply due to that definition (though it may still be one because it hasn't changed much), 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 Genus ''Latimeria'') fossils were to be found, ''Latimeria chalumnae'' would be considered a true living fossil, as that would fill in the gap where the species is "dead". Of course, species do not just appear out of thin air, so all living Lazarus species (excluding disappearing and reappearing Red List Species ) are nonetheless considered living fossils, if it can be shown they are not Elvis Taxa . Some living fossils are species that were known from fossils before living representatives were discovered. The most famous examples of this are the coelacanth fishes, ''Latimeria chalumnae'' and ''Latimeria menadoensis'' and the Dawn Redwood , ''Metasequoia'', discovered in a remote Chinese valley. 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''). Note the similarity between the 170 million year old fossil ''Ginkgo'' sp. leaves on the left, and the living plant on the right. Examples Some of these are ''informally'' known as "living fossils". ;Plants
;Animals
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 denoting living fossils:
: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. 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. Modern molecular biology has shown that genetic rates of change are relatively uniform and not well related to morphological change rates. So from a more molecular basis of interbreeding capabilities there are essentially no such thing as species that lived through a long geological time. With the proviso that we are using the special case of a paleontological species name, the definition holds together in context however.
:The living specimens need 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 to be an indication of fresh water ancestry. This means that the coelacanth lineage has evolved from freshwater to saltwater. : The resemblance between ''Peripatus'' and '' Aysheaia '' (an early Cambrian animal from the Burgess Shale s) 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.
:This is a more neutral definition. However, it makes not clear whether 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 (mostly only Carapaces remain), and they haven't been thoroughly 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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