A living fossil is a term used for a species that still exists today and looks similar to ancient species found only in fossils. However, scientists no longer use this term because it can be misleading. For a species to be called a living fossil, the ancient relatives it resembles must be much older than the time when the living group first appeared. Living fossils often belong to groups with few species, but this is not always the case. While the body shape of a living fossil may look similar to its ancient relatives, it is not the same species because genetic changes over time would alter their chromosomes.
Living fossils show little change in their appearance over very long periods, a phenomenon called "stasis" or "bradytely." Some books incorrectly say that living fossils have not changed much since they first appeared, with almost no changes in their molecules or physical traits. Scientists have shown that these claims are not accurate.
Small changes in living fossils are sometimes mistaken as no evolution at all, but these changes are actually the result of stabilizing selection, a type of natural selection that keeps traits stable over time. This process may be the most common way that physical traits remain unchanged.
Today, scientists and paleontologists no longer use the term "living fossil" because it is considered outdated.
Characteristics
Living fossils have three main characteristics, though some may only have two:
- They are living organisms that belong to a group that has remained clearly recognizable in the fossil record for an unusually long time.
- They show little change in body shape or structure, whether compared to older members of their group or to other living members of the same group.
- They usually have few different types of species within their group.
The first two characteristics are needed to call a species a living fossil. Some scientists also consider the third as an important feature, while others only note it as a common trait.
These rules are not clearly defined or easy to measure, but modern tools can study how evolution happens over time. Groups that stay the same for very short periods are not considered living fossils. However, the exact time needed for a group to stay unchanged to be called a living fossil is not clearly set.
The term "living fossil" is often misunderstood in the media, where it is sometimes used without meaning. In scientific writing, the term is rarely used and must be applied carefully because it has been used in different ways by different scientists.
A related idea is the "Lazarus taxon," but these are not the same. A Lazarus taxon is a species or group of species that appears suddenly in the fossil record or in nature, as if it had "come back to life." In contrast, a living fossil is a species or group that has changed very little over a long time, making it seem almost the same as its ancient ancestors. Even though their DNA is different from their ancient relatives, living fossils may look similar.
The average time a species exists before disappearing varies among different groups of organisms but is usually about 2 to 3 million years. A species once thought extinct can be called a Lazarus taxon if it is later found to still exist. For example, the Coelacanthiformes, a group of fish, was thought extinct but was rediscovered in 1938. While this discovery is not debated, some scientists argue whether the Coelacanthiformes should also be called a living fossil.
Coelacanths disappeared from the fossil record about 80 million years ago. If they show little change in body shape over time, their living relatives can be called living fossils. This definition is based only on fossil evidence and does not depend on whether the species has been affected by natural selection, which all living species experience.
The fossil record often only shows the general shape of ancient organisms, not much about their internal biology. Even species that look unchanged in the fossil record may have small changes in their biology or DNA. Noncoding DNA (parts of the genome that do not make proteins) changes over time even if the body shape stays the same. This means that a species with little change in body shape does not necessarily have the same biology or DNA as its ancient relatives.
Some living fossils were first known only from fossils before living examples were found. Examples include:
- Coelacanthiform fish (2 species)
- Metasequoia, a type of redwood tree (1 species)
- Glypheoid lobsters (2 species)
- Mymarommatid wasps (10 species)
- Eomeropid scorpionflies (1 species)
- Jurodid beetles (1 species)
- Soft sea urchins (59 species)
These groups were originally described only from fossils but are now known to include living species.
Other living fossils are single species with no close living relatives but are the last surviving members of large, ancient groups. Examples include:
- Wollemia nobilis, a type of tree
- Ginkgo biloba, a type of plant
- Syntexis libocedrii, a type of wasp
- Dinoflagellates, a group of microscopic organisms
All of these were first described from fossils before living examples were discovered.
Being a living fossil does not mean a species must keep all the ancient features of its ancestors. While it is common to say living fossils show little change in body shape, this does not mean they are exactly the same as their ancestors.
Some living fossils are the last surviving members of once-diverse and varied groups. However, not all ancient groups that have surviving members are called living fossils. For example, oxpeckers, a type of bird, are the only surviving members of an ancient group related to starlings and mockingbirds, but they are not usually called living fossils.
Evolution and living fossils
The term "living fossil" is used for species or groups of species that have changed very little in appearance over long periods of time. Scientists have proposed several ideas to explain why these species remain so similar for so long. Early research focused on how long a species could survive rather than how fast it changed. Today, scientists study how traits change over time, but most research has looked at groups that quickly diversified into many different forms, not those that stayed the same. Little is known about the processes that create living fossils or how often they occur. Some recent studies have found that certain groups evolve very slowly in terms of their traits and roles in nature, even though they produce many new species. This is called a "non-adaptive radiation," meaning species diversify without taking on very different roles in their environment. Staying well-adapted to a specific environment is a common reason for remaining unchanged in appearance. However, slow evolutionary rates have received less attention in recent research compared to fast rates.
Living fossils are not expected to change slowly at the molecular level, and some studies support this. For example, in the case of tadpole shrimp (Triops), one study states, "Our research shows that species with simple body plans can still diversify and adapt to new conditions. I think the term 'living fossil' should be avoided because it can be misleading." Some scientists suggest using the term "stabilomorph" to describe species that remain stable in appearance due to highly effective adaptation strategies. This stability reduces the need for many different physical forms and helps species survive over long periods.
Recent studies have questioned whether coelacanths, a group of fish, are truly morphologically conservative. Evidence from fossils does not support this idea. Additionally, research that linked slow molecular evolution to coelacanth conservatism was influenced by the assumption that these fish are "living fossils." New findings show that the genomes of two living coelacanth species contain genetic changes that occurred after they split into separate species, indicating recent activity of genetic elements. These findings challenge the idea that coelacanth genomes have remained unchanged, but they do not affect the idea that their physical traits have evolved very slowly.
History
The term "living fossil" was introduced by Charles Darwin in his book On the Origin of Species from 1859. He used it while discussing two animals: the platypus (Ornithorhynchus) and the South American lungfish (Lepidosiren).
A living fossil is a species that has existed for a long time in Earth's history. For example, the Australian lungfish (Neoceratodus fosteri), also called the Queensland lungfish, is a living fossil. Fossils of this species are over 100 million years old, and the modern version has existed for nearly 30 million years. The nurse shark is another example; it has lived for more than 112 million years, making it one of the oldest living vertebrates.
A living fossil may look similar to ancient fossils over a long period of time. This is called morphological stasis. Another definition is a species that has many traits considered to be older or simpler. However, this does not always mean the species is truly ancient, as it might have many older traits but not necessarily be very old. Some living fossils might have advanced features instead.
A living fossil may also have a limited range today, surviving only in certain areas. Some scientists argue that species with large ranges, like Triops cancriformis (a type of ancient water flea), are not true living fossils. This species has existed since the Triassic period, but its ancient forms had fewer body parts than modern ones. These ancient fossils have not been studied in detail since 1938.
Living fossils may also belong to groups with few species. For example, oxpeckers (birds that feed on parasites and blood from large mammals) look somewhat like starlings but are not closely related. They are part of a group that includes starlings and mockingbirds, but they are the oldest in this group. Studies suggest oxpeckers originally lived in Asia and later moved to Africa, where they now live in limited areas. Fossils of oxpeckers are rare because their small bones do not preserve well in some environments, but future discoveries might test this idea.
In 2017, scientists proposed a definition: a living fossil is a species that evolves slowly and stays near the center of physical differences among related species. However, some scientists have criticized the methods used to classify tuatara (a type of lizard) as a living fossil under this definition. The original researchers defended their work.
Examples
Some of these are informally called "living fossils."
- The dinoflagellate † Calciodinellum operosum
- The dinoflagellate † Dapsilidinium pastielsii
- The dinoflagellate † Posoniella tricarinelloides
- The coccolithophore Tergestiella adriatica
Plants
• Moss
• Pteridophytes: Horsetails – Equisetum; Lycopods; Tree ferns and ferns
• Gymnosperms: Conifers – Agathis (kauri in New Zealand, Australia, and the Pacific; almasiga in the Philippines); Araucaria araucana (monkey puzzle tree, along with other Araucaria species); Sequoioideae (redwoods, including Coast redwood, Sierra redwood, and Dawn redwood); Sciadopitys (a unique conifer found only in Japan, known in the fossil record for about 230 million years); Taiwania cryptomerioides (one of the largest tree species in Asia); Wollemia tree (Araucariaceae, related to Agathis and Araucaria, a borderline example); Cycads (evidence challenges this classification); Ginkgo tree (Ginkgoaceae); Welwitschia
• Angiosperms: Amborella (a plant from New Caledonia, possibly closest to the base of flowering plants); Magnolia (a genus that has changed little since the earliest days of flowering plant evolution in the Cretaceous and possibly earlier); Trapa (water caltrops, with many extinct species known from the Cretaceous); Nelumbo (lotus flowers, with species known only from Cretaceous fossils); Nymphaeales (water lilies, with fossils dating back to the Cretaceous); Sassafras (fossils from the late Cretaceous through the late Pleistocene); Platanus (sycamore, with abundant fossils in the northern hemisphere and many extinct species); Sycamore leaves and fruits are common in plant fossils, known from the Cretaceous onward; Platanus occidentalis (fossils found in North America during the Pliocene and Pleistocene); Nyssa (blackgum, with fossils dating back to the late Cretaceous); Liriodendron (fossils from the Cretaceous and Tertiary periods, with many extinct species; tulip trees were once in Europe during the Cretaceous and early Paleocene; Liriodendron tulipifera fossils were found in North Carolina); Liquidambar (sweetgums, appearing during the mid-late Cretaceous, with many extinct species in Asia, Europe, and North America; the genus was widespread in Europe and Asia during the Miocene; the American sweetgum is a living fossil, with fossils from the Miocene, Pliocene, and Pleistocene found in the eastern United States
Fungi
• Neolecta
Mammals
• Aardvark (Orycteropus afer)
• Amami rabbit (Pentalagus furnessi)
• Nesolagus (Asian striped rabbits)
• Chevrotain (Tragulidae)
• Chousingha (Tetracerus quadricornis)
• Elephant shrew (Macroscelidea)
• Giant panda (Ailuropoda melanoleuca)
• Baiji (Lipotes vexillifer) (one living species)
• Ganges river dolphin (Platanista gangetica)
• Indus river dolphin (Platanista minor)
• Hawaiian monk seal (Neomonachus schauinslandi)
• Koala (Phascolarctos cinereus)
• Laotian rock rat (Laonastes aenigmamus)
• Monito del monte (Dromiciops gliroides)
• Monotremes (platypus and echidna)
• Mountain beaver (Aplodontia rufa)
• Okapi (Okapia johnstoni)
• Opossums (Didelphidae)
• Clouded leopard (Neofelis nebulousa)
• Bush dog (Speothos venaticus)
• Maned wolf (Chrysocyon brachyurus)
• Red panda (Ailurus fulgens)
• Solenodon (Solenodon cubanus and Solenodon paradoxus)
• Shrew opossum (Caenolestidae)
• Spectacled bear (Tremarctos ornatus)
• False killer whale (Pseudorca crassidens)
• Pygmy right whale (Caperea marginata)
• Pacarana (Dinomys branickii)
• Rhinoceroses (Rhinocerotidae)
• Tapirs (Tapiridae)
Birds
• Pelicans (Pelecanus) – their form has changed very little since the Eocene and is even more stable than that of crocodiles
• Other birds (specific details not provided in the original text)
Insects
• Many insect genera and species, including:
– Rotoitid wasps (2 living species, 14 extinct)
– Sikhotealinia zhiltzovae (a jurodid beetle)
– Syntexis libocedrii (a cedar wood wasp)
– Tricholepidion gertschi (a silverfish with relatives from the Eocene)
– Other examples listed in the original text
Crustaceans
• Glypheidea (2 living species: Neoglyphea inopinata and Laurentaeglyphea neocaledonica)
• Stomatopods (mantis shrimp)
• Polychelida (deep sea blind lobster)
• Triopsidae (tadpole shrimp, also called notostracan crustaceans)
Molluscs
• Nautilina (e.g., Nautilus pompilius)
• Neopilina (monoplacophoran)