Living fossil in the context of "Molecular evolution"

Extinction is the termination of an organism by the death of its last member. A taxon may become functionally extinct before the death of its last member if it loses the capacity to reproduce and recover. As a species' potential range may be very large, determining this moment is difficult, and is usually done retrospectively. This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly "reappears" (typically in the fossil record) after a period of apparent absence.

Over five billion species are estimated to have died out. It is estimated that there are currently around 8.7 million species of eukaryotes globally, possibly many times more if microorganisms are included. Notable extinct animal species include non-avian dinosaurs, saber-toothed cats, and mammoths. Through evolution, species arise through the process of speciation. Species become extinct when they are no longer able to survive in changing conditions or against superior competition. The relationship between animals and their ecological niches has been firmly established. A typical species becomes extinct within 10 million years of its first appearance, although some species, called living fossils, survive with little to no morphological change for hundreds of millions of years, though this claim has been disputed.

Metasequoia glyptostroboides, the dawn redwood, is a fast-growing, endangered deciduous conifer. It is the sole living species of the genus Metasequoia, one of three genera in the subfamily Sequoioideae of the family Cupressaceae. It now survives in the wild only in wet lower slopes and montane river and stream valleys in the border region of Hubei and Hunan provinces and Chongqing municipality in south-central China, notably in Lichuan county in Hubei. Although the shortest of the redwoods, it can grow to 167 ft (51 m) in height.

In 1941, the genus Metasequoia was reported by paleobotanist Shigeru Miki [jp] as a widely distributed extinct genus based on fossils, before attracting considerable attention a few years later when small populations were found alive in central China. It is a well-known example of a living fossil species. Modern dawn redwood appears identical to its late Cretaceous ancestors. The tree faces considerable risks of extinction in its wild range due to deforestation; however, it has been planted extensively in arboreta worldwide, where it has proved a popular and fast-growing ornamental plant. If the species had not been discovered when it was, it might have become extinct before being investigated.

Araucaria araucana, commonly called the pewen, monkey-puzzle, pehuen or piñonero, is an evergreen tree belonging to the family Araucariaceae and growing to a height of 30–40 m (98–131 ft) and a trunk diameter of 1–1.5 m (3.3–4.9 ft). Native to central and southern Chile and western Argentina, it is the hardiest species in the conifer genus Araucaria.

Because of the prevalence of similar species in ancient prehistory, it is sometimes called a 'living fossil'. It is also the official tree of Chile and of the neighboring Argentine province of Neuquén. The IUCN changed its conservation status to Endangered in 2013, because logging, forest fires, and grazing have caused its population to dwindle.

Equisetum (/ˌɛkwɪˈsiːtəm/; horsetail) is the only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds.

Equisetum is a "living fossil", the only living genus of the entire subclass Equisetidae, which for over 100 million years was much more diverse and dominated the understorey of late Paleozoic forests. Some equisetids were large trees reaching to 30 m (98 ft) tall. The genus Calamites of the family Calamitaceae, for example, is abundant in coal deposits from the Carboniferous period. The pattern of spacing of nodes in horsetails, wherein those toward the apex of the shoot are increasingly close together, is said to have inspired John Napier to invent logarithms. Modern horsetails first appeared during the Jurassic period.

The Heteroptera are a group of about 40,000 species of insects in the order Hemiptera. They are sometimes called "true bugs", though that name more commonly refers to the Hemiptera as a whole. "Typical bugs" might be used as a more unequivocal alternative, since the heteropterans are most consistently and universally termed "bugs" among the Hemiptera. "Heteroptera" is Greek for "different wings": most species have forewings with both membranous and hardened portions (called hemelytra); members of the primitive sub-group Enicocephalomorpha have completely membranous wings.

The name "Heteroptera" is used in two very different ways in modern classifications. In Linnean nomenclature, it commonly appears as a suborder within the order Hemiptera, where it can be paraphyletic or monophyletic depending on its delimitation. In phylogenetic nomenclature, it is used as an unranked clade within the Prosorrhyncha clade, which in turn is in the Hemiptera clade. This results from the realization that the Coleorrhyncha are just "living fossil" relatives of the traditional Heteroptera, close enough to them to be united with that group.

Sponge reefs are reefs produced by sea sponges. All modern sponge reefs are formed by hexactinellid sponges, which have an endoskeleton made of silica spicules and are often referred to as "glass sponges", while historically the non-spiculed, calcite-skeletoned archaeocyathid and stromatoporoid sponges were the primary reef-builders.

Sponge reefs were once a dominant landscape in the Paleozoic and Mesozoic sea, but are now very rare, and found only in waters off the coast of North America's Pacific Northwest region, more specifically southern Alaska, British Columbia and Washington. Sponge reefs were reported in 2018 within the strait of Georgia and Howe sound close to Vancouver. Although still common in the late Jurassic period, reef-building sponges were believed to have gone extinct during or shortly after the Cretaceous period, until the existing reefs were discovered in Queen Charlotte sound in 1987–1988 – hence these sometimes being dubbed living fossils.

Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans. Despite their name, they are not crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scorpions. The body of a horseshoe crab is divided into three main parts: the cephalothorax, abdomen, and telson. The largest of these, the cephalothorax, houses most of the animal's eyes, limbs, and internal organs. It is also where the animal gets its name, as its shape somewhat resembles that of a horseshoe. Horseshoe crabs have been described as "living fossils", having changed little since they first appeared in the Triassic around 250 million years ago, and similar-looking fossil xiphosurans extend back to the Ordovician around 445 million years ago.

Only four species of horseshoe crab are extant today, the Atlantic horseshoe crab (Limulus polyphemus), native to the eastern coast of North and Central America, as well as the mangrove horseshoe crab (Carcinoscorpius rotundicauda), tri-spine horseshoe crab (Tachypleus tridentatus) and Indo-Pacific horseshoe crab (Tachypleus gigas), which are native to South, South East, and East Asia.