Living fossil is a term for any living species (or clade) of organism which closely resembles 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 reason for this is that 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.

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

• Ginkgo tree (Ginkgoaceae)
• Horsetails Equisetum (Equisetaceae)
• Metasequoia Dawn Redwood (Cupressaceae; a borderline example, related to Sequoia and Sequoiadendron)
• Sciadopitys tree (Sciadopityaceae)
• Whisk ferns Psilotum (Psilotaceae)
• Wollemia tree (Araucariaceae; a borderline example, related to Agathis and Araucaria)

Animals

• Coelacanth
• Coral (polyp)
• Crinoids
• Crocodylia (Crocodiles, Gavials and Alligators)
• Horseshoe crab (Limulus polyphemus)
• Laotian Rock Rat (Laonastes aenigmamus)
• Lingula anatina, an inarticulate brachiopod
• Monotremes (the Platypus and echidnas).
• Neopilina galateae, a monoplacophorid mollusc
• Nut clam (Ennucula superba)
• Onychophorans, for instance Peripatus
• Snout-nosed frog (Nasikabatrachus sahyadrensis)
• Triops cancriformis, a notostracid crustacean
• Tuatara (Sphenodon)

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:

• 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. With the proviso that we are using the special case of a paleontological species name, the definition holds together in context however.

• A living species morphologically and/or physiologically resembling a fossil species through a large portion of geologic time (morphological stasis)

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 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 taxon is truly old, or it simply has many ancestral characteristics (plesiomorphies).

• Either one of these 3, and also with a relict distribution in refuges.

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.

• Either one of these 3, and the clade has a low taxonomic diversity (low diversity lineages)

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.