Living fossil

Overview

Living fossils have three main characteristics. They:

1. are members of taxa that exhibit notable longevity in the sense that they have remained recognisable in the fossil record over unusually long periods;
2. show little morphological divergence, whether from early members of the lineage, or among extant species, and
3. tend to have little taxonomic diversity.

Because such criteria are neither well-defined nor clearly quantifiable, the term "living fossil" necessarily is arbitrary in several respects and much misunderstood in popular media in particular, in which it often is used meaninglessly. In professional literature the expression seldom appears and must be used with far more caution. No definitive criteria for what qualifies as a living fossil are possible, because for one thing, the term is an analogy rather than an unambiguous or fundamental biological concept, and accordingly it is often used by independent parties in mutually exclusive senses.

One example of a concept often confused with "living fossil", is that of a "Lazarus taxon", but the two are not equivalent; a Lazarus taxon (whether a single species or a group of related species) is one that suddenly reappears, either in the fossil record or in nature, as if the fossil had "come to life again". In contrast to Lazarus taxa, a living fossil in most senses is a species or lineage that has undergone exceptionally little change throughout a long fossil record, giving the impression that the extant taxon had remained identical through the entire fossil and modern period.

The average species turnover time, meaning the time between when a species first is established and when it finally disappears, varies widely among phyla, but averages about 2–3 million years. So a living taxon that had long been thought to be extinct could be called a Lazarus taxon once it was discovered to be still extant. A dramatic example was the order Coelacanthiformes, of which the genus, Latimeria was found to be extant in 1938. About that there is little debate. However, whether Latimeria resembles early members of its lineage sufficiently closely to be considered a living fossil as well as a Lazarus taxon, has been denied by some authors in recent years.

Coelacanths disappeared from the fossil record some 80 million years ago (upper Cretaceous) and to the extent that they exhibit low rates of morphological evolution, extant species qualify as living fossils. It must be emphasised that this criterion reflects fossil evidence, and is totally independent of whether the taxons had been subject to selection at all, which all living populations continuously are, whether they remain genetically unchanged or not. This in turn gives rise to a great deal of confusion; for one thing, the fossil record seldom preserves much more than the general morphology of a specimen; to determine much about its physiology is seldom possible. To determine much about its noncoding DNA is hardly ever possible, but even if a species were hypothetically unchanged in its physiology, it is to be expected from the very nature of the reproductive processes, that its non-functional genomic changes would continue at more or less standard rates. It follows that a fossil lineage with apparently constant morphology need not imply equally constant physiology for example, and certainly neither implies any cessation of the basic evolutionary processes such as natural selection, nor reduction in the usual rate of change of the noncoding DNA. In short, not even the most dramatic examples of living fossils can be expected to be without changes, no matter how persistently constant their fossils and their extant specimens might seem.

Some living fossils are taxa that were known from palaeontological fossils before living representatives were discovered. The most famous examples of this are:

All of these were described from fossils before later found alive (2 species, 10 species, one species, and one species respectively).

Other examples of living fossils are single living species that have no close living relatives, but are survivors of large and widespread groups in the fossil record. Consider:

The fact that a living fossil is a surviving representative of an archaic lineage does not imply that it must retain all the "primitive" features (plesiomorphies) of its ancestral lineage. Although it is common to say that living fossils exhibit "morphological stasis", it is worth repeating that there is no point to assuming that any extant taxa might be strictly identical to their ancestors, much less their remote ancestors.

Some living fossils are relicts of formerly diverse and morphologically varied lineages, but the criteria for assigning living fossil status being so arbitrary, not all survivors of ancient lineages necessarily are regarded as living fossils. See for example the uniquely and highly autapomorphic oxpeckers, which appear to be the only survivors of an ancient lineage related to starlings and mockingbirds.

Evolution and living fossils

The term living fossil is usually reserved for species or larger clades that are exceptional for their lack of morphological diversity and their exceptional conservatism, and several hypotheses could explain morphological stasis on a geologically long time-scale. Early analyses of evolutionary rates emphasized the persistence of a taxon rather than rates of evolutionary change. Contemporary studies instead analyze rates and modes of phenotypic evolution, but most have focused on clades that are thought to be adaptive radiations rather than on those thought to be living fossils. Thus, very little is presently known about the evolutionary mechanisms that produce living fossils or how common they might be. Some recent studies have documented exceptionally low rates of ecological and phenotypic evolution despite rapid speciation. This has been termed a "non-adaptive radiation" referring to diversification not accompanied by adaptation into various significantly different niches. Such radiations are explanation for groups that are morphologically conservative. Persistent adaptation within an adaptive zone is a common explanation for morphological stasis. The subject of very low evolutionary rates, however, has received much less attention in the recent literature than that of high rates

Living fossils are not expected to exhibit exceptionally low rates of molecular evolution, and some studies have shown that they do not. For example, on tadpole shrimp (Triops), one article notes, "Our work shows that organisms with conservative body plans are constantly radiating, and presumably, adapting to novel conditions.... I would favor retiring the term ‘living fossil’ altogether, as it is generally misleading."

The question posed by several recent studies pointed out that the morphological conservatism of coelacanths is not supported by paleontological data. In addition, it was shown recently that studies concluding that a slow rate of molecular evolution is linked to morphological conservatism in coelacanths are biased by the a priori hypothesis that these species are ‘living fossils’. Accordingly, the genome stasis hypothesis is challenged by the recent finding that the genome of the two extant coelacanth species L. chalumnae and L. menadoensis contain multiple species-specific insertions, indicating transposable element recent activity and contribution to post-speciation genome divergence. Such studies, however, challenge only a genome stasis hypothesis, not the hypothesis of exceptionally low rates of phenotypic evolution.

History

The term was coined by Charles Darwin in his On the Origin of Species from 1859, 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.

Long-enduring

A living taxon that lived through a large portion of geologic time.

Queensland lungfish (Neoceratodus fosteri) is an example of an organism that meets this criterion, fossils identical to modern Queensland lungfish have been dated at over 100 million years making this species one of the oldest if not actually the oldest extant vertebrate species.

Resembles ancient species

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

This is a problematic criterion that has led to much controversy, because it often is implied to fail when there are indications that the living fossil actually or probably differs in some respects from extant species. For one thing, as a matter of elementary set theory the intersection of sets depends on shared attributes and only in special contexts is excluded by differences. When there are no differences, the relationshop is identity rather than resemblance. And as already stated, a living fossil need not be the same actual species as fossils, but only the same lineage or taxon, so there generally must be differences as well as resemblances. The argument also becomes tenuous when invoking physiological as well as morphological differences.

As an instance of such an argument, the coelacanth is a marine fish. The Mesozoic coelacanth species lived in salt and fresh water. Osmoregulation in Latimeria is handled by urea retention. Urea retention is considered to indicate fresh water ancestry. This would imply that the coelacanth lineage evolved from freshwater to saltwater. This has been argued to exclude extant coelacanth species from living-fossil status.

Again, the resemblance between Peripatus and Aysheaia (an early Cambrian animal from the Burgess Shale) is striking; both are classified in the Tardipolypoda, Tardigrada, and Onychophora. This is argued to be unpersuasive because Aysheaia was a marine animal, while Peripatus lives in tropical leaf mould. On the other hand whatever the facts of their respective evolutionary history or lineage might be, such a distinction is not cogent when one compares the Onychophora with say, the Isopoda, that include not only abyssal marine species and terrestrial species, but every gradation between.

As already remarked, no coherent definition of living fossils can demand that extant populations must belong to the same fossil species (or even genus). Certainly in principle there must at least be some morphological or physiological resemblance indicating a shared lineage, but whereas sufficiently persuasive morphological evidence sometimes is obvious, physiological changes are another matter; evidence for them need not be cogent even if the evidence is good, which in palaeontology it rarely is.

Nonetheless, physiological arguments have been widely mis-applied as evidence against living fossil status, given that many such adaptations are in evolutionary terms rapidly and repeatedly reversible; for instance, adaptation to fresh water followed by reverse adaptation to salt water is not unusual among related species. For example, some taxa, such as the monophyletic Salmonidae, apart from including both fresh-water and marine species, even include multiple species that annually migrate between fresh and salt water. In such cases to argue from fossils that suggest the presence of their ancestors in either or both conditions requires unusually definitive evidence to support any argument conclusively. And this is independent of considerations such as estuarine or littoral organisms. To exclude an extant species from living fossil status on the basis of comparison of its physiology with that of the fossils, one would need to demonstrate that the fossils were not ancestral, and that the actual ancestors did not resemble extant species.

Retains many ancient traits

A living taxon with many characteristics believed to be primitive.

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.

Relict population

Any one of the above three definitions, but also with a relict distribution in refuges.

Some paleontologists believe that living fossils with large distributions (such as Triops cancriformis) are not real living fossils. In the case of Triops cancriformis (living from the Triassic until now), the Triassic specimens lost most of their appendages (mostly only carapaces remain), and they have not been thoroughly examined since 1938.

Low diversity

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 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.

The two living species thus seem to represent 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 this theory to be tested.

Testable definition

- Proposed in 2017: lineage has a slow rate of evolution and occurs close to the centroid of the lineage's morphospace.

Examples

Some of these are informally known as "living fossils".

Bacteria

Protists

Plants

Fungi

Animals