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living fossil


n.

An organism, such as a coelacanth or the ginkgo, that is the sole surviving member of an otherwise extinct taxonomic group.


 
 
Animal Encyclopedia: Living fossil

Epiophlebia laidlawi

FAMILY

Epiophlebidae

TAXONOMY

Epiophlebia laidlawi Tillyard, 1921, Himalayas.

OTHER COMMON NAMES

None known.

PHYSICAL CHARACTERISTICS

Black with bright yellow stripes on the thorax and abdomen. One of the only two extant species of Anisozygoptera, it shares some characteristics with damselflies (forewings and hind wings similar in shape and venation and a well-developed ovipositor) and others with dragonflies (eyes separated by less distance than their width, a pair of superior caudal appendages and a single inferior one in the male, and broad-bodied larva with rectal breathing).

DISTRIBUTION

Confined to the eastern Himalayas in Nepal and India.

HABITAT

Breeds in streams between 6,000 and 11,500 ft (1,800–3,500m). Adults fly in clearings within dense bamboo forests.

BEHAVIOR

Larvae stridulate when disturbed. During the maturation period adults fly high above breeding areas. When mature, males fly slowly, low down and close to the stream; females skulk at the water's edge in the vegetation.

FEEDING ECOLOGY AND DIET

Nothing is known.

REPRODUCTIVE BIOLOGY

Males grasp females by the back of the head to form the tandem position. A solitary female inserts eggs into the stems of plants growing at stream margins. Larval period lasts from six to nine years, the longest known for any odonate.

CONSERVATION STATUS

Until 1980 considered Endangered but since then discovered at several new sites, appearing to be widespread and common. Given the necessary habitat protection, it can be considered safe.

SIGNIFICANCE TO HUMANS

None known.

 
Sci-Tech Encyclopedia: Living fossils

Living species very closely resembling fossil relatives in most anatomical details. The term is a relative one and, applied loosely, could embrace nearly all extant animals and plants. In its more restricted usage, the term applies to living species with four additional characteristics: (1) truly close anatomical similarity to (2) an ancient fossil species—generally at least 100,000,000 years old; (3) living members of the group are represented by only a single or at best a few species, which are (4) often found in a very limited geographic area. Examples are horseshoe crabs, ginkgo trees, and coelacanth fish.


 
Wikipedia: living fossil


Living fossil is an informal term for any living species (or clade) of organism which seems to be the same as a species otherwise only known from fossils and has no close living relatives. These species have all survived major extinction events, and generally retain low taxonomic diversities. A species which successfully radiates (forming many new species after a possible genetic bottleneck) has become too successful to be considered a "living fossil". The term is frequently misinterpreted, however.

Overview

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 averages about 2-3 million years. So, a living species that was thought to be extinct (e.g. the coelacanth, Latimeria chalumnae) is not a living fossil 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, other Cenozoic Latimeria fossil species were to be found, the coelacanth 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 coelacanthiform fishes Latimeria chalumnae and Latimeria menadoensis and the dawn redwood, Metasequoia, discovered in a remote Chinese valley. Others include glypheoid lobsters, mymarommatid wasps, and jurodid beetles, all of which were first described from fossils, but later found alive (2 species, 10 species, and one species respectively). 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, perhaps the best-known example of which is Ginkgo biloba (the ginkgo), though there are others, such as the Syntexis libocedrii (the cedar wood wasp).

Note that just because a living fossil is a surviving representative of an archaic lineage does not necessarily require that it retains all of the "primitive" features (plesiomorphies) of the lineage it is descended from; that is, they may possess one to many derived features (autapomorphies), that have evolved since the time of their lineage's divergence. All that is required is that they can be unambiguously assigned to an otherwise extinct lineage (rarely are they identical to the fossil forms). See for example the uniquely and highly autapomorphic oxpeckers, which are not "true" living fossils (as no fossils are known yet) but nonetheless appear to be the only survivors of an ancient lineage related to starlings and mockingbirds[1].

Fossil_Plant_Ginkgo.jpg Gingko-Blaetter.jpg
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

Fungi

Animals

History

The term was first coined by Charles Darwin in his The Origin of Species, when discussing Ornithorhynchus (the platypus) and Lepidosiren (the South American lungfish):

...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:

  • A living species that lived through 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. 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. However, with the proviso that we are using the special case of a paleontological species name, the definition does hold together in context.
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) 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 does not make it clear whether the taxon is truly old, or it simply has many plesiomorphies. Note that, as mentioned above, the converse may hold for true living fossil taxa; that is, they may possess a great many derived features (autapomorphies), and not be particularly "primitive" in appearance.
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.
  • Any of the first three definitions, but the clade also has a low taxonomic diversity (low diversity lineages)
Oxpeckers are morphologically somewhat similar to starlings due to shared plesiomorphies, but are uniquely adapted to feed on parasites and blood of large land mammals which has always obscured their relationships. This lineage forms part of a radiation that includes Sturnidae and Mimidae but appears to be the most ancient one of these groups. Biogeography strongly suggests that oxpeckers originated in eastern Asia and only later arrived in Africa, where they now have a relict distribution (Zuccon et al. 2007). The two living species thus seem to be representatives of an entirely extinct and (as Passerida go) rather ancient lineage, as certainly as this can be said in the absence of actual fossils. The latter is probably due to the fact that the oxpecker lineage never occurred in areas where conditions were good for fossilization of small bird bones, but of course, fossils of ancestral oxpeckers may one day turn up enabling to test this theory.

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.

References

  1. ^ Zuccon, Dario; Cibois, Anne; Pasquet, Eric & Ericson, Per G.P. (2006): Nuclear and mitochondrial sequence data reveal the major lineages of starlings, mynas and related taxa. Molecular Phylogenetics and Evolution 41(2): 333-344. doi:10.1016/j.ympev.2006.05.007 (HTML abstract)

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Copyrights:

Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
Animal Encyclopedia. Grzimek's Animal Life Encyclopedia. Copyright © 2005 by The Gale Group, Inc. All rights reserved.  Read more
Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Living fossil" Read more

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