Living organisms that are members of a taxon that has remained recognisable in the fossil record over an unusually long time span. We see evolution all around us, constantly, in every living thing. Yet in the deep oceans we find a number of “living fossils” reminiscent of creatures from prehistoric times. Throughout human history, fossils have been used, studied, and understood in different ways. Early civilizations used fossils for decorative or religious purposes, but did not always understand where they came from.

Martin, Hilary C et al. " Insights Into Platypus Population Structure And History From Whole-Genome Sequencing". Molecular Biology And Evolution, vol 35, no. 5, 2018, pp. 1238-1252. Oxford University Press (OUP), doi:10.1093/molbev/msy041 Any of the first three definitions, but the clade also has a low taxonomic diversity (low diversity lineages). [ citation needed] A living taxon morphologically and/or physiologically resembling a fossil taxon through a large portion of geologic time (morphological stasis). [29] Retains many ancient traits [ edit ] More primitive trapdoor spiders, such as this female Liphistius sp., have segmented plates on the dorsal surface of the abdomen and cephalothorax, a character shared with scorpions, making it probable that after the spiders diverged from the scorpions, the earliest unique ancestor of trapdoor species was the first to split off from the lineage that contains all other extant spiders.The Cretaceous Pierre Shale formation of the United States and Canada is well known for the abundant ammonite fauna it yields, including Baculites, Placenticeras, Scaphites, Hoploscaphites and Jeletzkytes, as well as many uncoiled forms. Many of these also have much or all of the original shell, as well as the complete body chamber, still intact. Many Pierre Shale ammonites, and indeed many ammonites throughout earth history, are found inside concretions. On the Origin of Species, 1859 [28] Other definitions [ edit ] Long-enduring [ edit ] Elephant shrews resemble the extinct Leptictidium of Eocene Europe.

The Cephalopoda". ucmp.berkeley.edu. Archived from the original on March 24, 2022 . Retrieved September 24, 2019. In his 1859 book On the Origin of Species, esteemed naturalist Charles Darwin coined the term “living fossil” to describe living organisms that appeared unchanged from their extinct fossil relatives. The term has since been used to describe long-enduring lineages, relict populations, groups with low diversity, and groups with DNA that has hardly changed in millions of years. Magnolia – a genus whose form is little changed since the earliest days of flowering plant evolution in the Cretaceous and possibly earlier [39] [1] Paleontologists are people who study fossils. Paleontologists find and study fossils all over the world, in almost every environment, from the hot desert to the humid jungle. Studying fossils helps them learn about when and how different species lived millions of years ago. Sometimes, fossils tell scientists how Earth has changed. But by applying a standard technique for dating commercial fish, French scientists calculated they actually live close to a century, according to a study in the journal Current Biology released on Thursday.

6. A toothless pipsqueak from Brazil

In contrast to index fossils, living fossils are organisms that have existed for a tremendously long period of time without changing very much at all. For example, the Lingulata brachiopods have existed from the Cambrian period to the present, a time span of over 500 million years! Modern specimens of Lingulata are almost indistinguishable from their fossil counterparts (Figure 11.8). Machalski, Marcin; Heinberg, Claus (2005-12-01). "Evidence for ammonite survival into the Danian (Paleogene) from the Cerithium Limestone at Stevns Klint, Denmark". Bulletin of the Geological Society of Denmark. 52: 2005–12. doi: 10.37570/bgsd-2005-52-08.

Ammonoid septa characteristically have bulges and indentations and are to varying degrees convex when seen from the front, distinguishing them from nautiloid septa, which are typically simple concave, dish-shaped structures. The topology of the septa, especially around the rim, results in the various suture patterns found. [3] Switek, Brian (21 March 2011). "The pelican's beak: Success and evolutionary stasis". Wired (repost). Wired Science. Vol.152. pp.15–20. doi: 10.1007/s10336-010-0537-5 . Retrieved 10 June 2013. By studying human fossils and ancient Australian cave paintings that were dated to the same time period, paleontologists hypothesized that human beings—the earliest people to inhabit Australia—may have contributed to the extinction of Genyornis. A fossil (from Classical Latin fossilis, lit. 'obtained by digging') [1] is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood and DNA remnants. The totality of fossils is known as the fossil record.Because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are very rarely preserved in any detail. Nonetheless, much has been worked out by examining ammonoid shells and by using models of these shells in water tanks. The ammonoids as a group continued through several major extinction events, although often only a few species survived. Each time, however, this handful of species diversified into a multitude of forms. Ammonite fossils became less abundant during the latter part of the Mesozoic, and although they seemingly survived the Cretaceous–Paleogene extinction event, all known Paleocene ammonite lineages are restricted to the Paleocene epoch (65–61 Ma). [24] [25] Evolutionary history [ edit ] Such criteria are neither well-defined nor clearly quantifiable, but modern methods for analyzing evolutionary dynamics can document the distinctive tempo of stasis. [6] [7] [8] Lineages that exhibit stasis over very short time scales are not considered living fossils; what is poorly-defined is the time scale over which the morphology must persist for that lineage to be recognized as a living fossil. Due to their free-swimming and/or free-floating habits, ammonites often happened to live directly above seafloor waters so poor in oxygen as to prevent the establishment of animal life on the seafloor. When upon death the ammonites fell to this seafloor and were gradually buried in accumulating sediment, bacterial decomposition of these corpses often tipped the delicate balance of local redox conditions sufficiently to lower the local solubility of minerals dissolved in the seawater, notably phosphates and carbonates. The resulting spontaneous concentric precipitation of minerals around a fossil, a concretion, is responsible for the outstanding preservation of many ammonite fossils. Nagalingum NS, Marshall CR, Quental TB, Rai HS, Little DP, Mathews S (11 November 2011). "Recent synchronous radiation of a living fossil". Science (published 20 October 2011). 334 (6057): 796–799. Bibcode: 2011Sci...334..796N. doi: 10.1126/science.1209926. PMID 22021670. S2CID 206535984.

Lehmann, Ulrich (1981). The Ammonites: Their life and their world. Cambridge University Press. ISBN 978-0-521-23627-0. A thin living tube called a siphuncle passed through the septa, extending from the ammonite's body into the empty shell chambers. Through a hyperosmotic active transport process, the ammonite emptied water out of these shell chambers. This enabled it to control the buoyancy of the shell and thereby rise or descend in the water column. Fossils form in five ways: preservation of original remains, permineralization, molds and casts, replacement, and compression. The great variety of body shapes and sizes indicate that trilobites occupied a variety ofecological niches. Blind trilobites may have burrowed and scavenged in mud on the sea floor, or lived at great depth in the sea where there was no light. Other trilobites are associated with trails on bedding planes (e.g. Cruziana), which show that they moved about on the sea-floor, perhaps scavenging or preying on other animals. These trilobites were part of thebenthos (sea floor dwelling lifeforms). Yet more trilobites, such as Cyclopyge, had large eyes that enabled them to see downwards as well as in other directions. This, together with their widespread distribution, has led geologists to think that they swam or floated above the sea floor.Other fossils, such as many found in Madagascar and Alberta, Canada display iridescence. These iridescent ammonites are often of gem quality ( ammolite) when polished. In no case would this iridescence have been visible during the animal's life; additional shell layers covered it. Of all living creatures, crocodiles perhaps bear the greatest resemblance to dinosaurs; in fact, they were contemporaries. The semiaquitic reptiles first appeared about 200 million years ago, or 30 million years after the dinos. Yet despite all the time that's passed since, there are only 24 species today, all of which look remarkably like their ancestors, and each other, except for some variation in skull shape. Compare that to the wild diversity of, say, birds, who in a much shorter interval produced around 10,000 species. Some living fossils are taxa that were known from palaeontological fossils before living representatives were discovered. The most famous examples of this are: