Devonian in the context of Seed ferns

Marine life, sea life or ocean life is the collective ecological communities that encompass all aquatic animals, plants, algae, fungi, protists, single-celled microorganisms and associated viruses living in the saline water of marine habitats, either the sea water of marginal seas and oceans, or the brackish water of coastal wetlands, lagoons, estuaries and inland seas. As of 2023, more than 242,000 marine species have been documented, and perhaps two million marine species are yet to be documented. An average of 2,332 new species per year are being described. Marine life is studied scientifically in both marine biology and in biological oceanography.

By volume, oceans provide about 90% of the living space on Earth, and served as the cradle of life and vital biotic sanctuaries throughout Earth's geological history. The earliest known life forms evolved as anaerobic prokaryotes (archaea and bacteria) in the Archean oceans around the deep sea hydrothermal vents, before photoautotrophs appeared and allowed the microbial mats to expand into shallow water marine environments. The Great Oxygenation Event of the early Proterozoic significantly altered the marine chemistry, which likely caused a widespread anaerobe extinction event but also led to the evolution of eukaryotes through symbiogenesis between surviving anaerobes and aerobes. Complex life eventually arose out of marine eukaryotes during the Neoproterozoic, and which culminated in a large evolutionary radiation event of mostly sessile macrofaunae known as the Avalon Explosion. This was followed in the early Phanerozoic by a more prominent radiation event known as the Cambrian Explosion, where actively moving eumetazoan became prevalent. These marine life also expanded into fresh waters, where fungi and green algae that were washed ashore onto riparian areas started to take hold later during the Ordovician before rapidly expanding inland during the Silurian and Devonian, paving the way for terrestrial ecosystems to develop.

The Silurian-Devonian Terrestrial Revolution, also known as the Devonian Plant Explosion (DePE) and the Devonian explosion, was a period of rapid colonization, diversification and radiation of land plants (particularly vascular plants) and fungi (especially dikaryans) on dry lands that occurred 428 to 359 million years ago (Mya) during the Silurian and Devonian periods, with the most critical phase occurring during the Late Silurian and Early Devonian.

This diversification of terrestrial photosynthetic florae had vast impacts on the biotic composition of the Earth's surface, especially upon the Earth's atmosphere by oxygenation and carbon fixation. Their roots also eroded into the rocks, creating a layer of water-holding and mineral/organic matter-rich soil on top of Earth's crust known as the pedosphere, and significantly altering the chemistry of Earth's lithosphere and hydrosphere. The floral activities following the Silurian-Devonian plant revolution also exerted significant influences on changes in the water cycle and global climate, as well as driving the biosphere by creating diverse layers of vegetations that provide both sustenance and refuge for both upland and wetland habitats, paving the way for all terrestrial and aquatic biomes that would follow.

The Silurian (/sɪˈljʊəri.ən, saɪ-/ sih-LURE-ee-ən, sy-) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at 443.1 Ma (million years ago) to the beginning of the Devonian Period, 419.62 Ma. The Silurian is the third and shortest period of the Paleozoic Era, and the third of twelve periods of the Phanerozoic Eon. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by a few million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.

One important event in this period was the initial establishment of terrestrial life in what is known as the Silurian-Devonian Terrestrial Revolution: vascular plants emerged from more primitive land plants, dikaryan fungi started expanding and diversifying along with glomeromycotan fungi, and three groups of arthropods (myriapods, arachnids and hexapods) became fully terrestrialized.

The Carboniferous (/ˌkɑːrbəˈnɪfərəs/ KAR-bə-NIF-ər-əs) is a geologic period and system of the Paleozoic era that spans 60 million years, from the end of the Devonian Period 358.86 Ma (million years ago) to the beginning of the Permian Period, 298.9 Ma. It is the fifth period of the Phanerozoic eon. In North America, the Carboniferous is often treated as two separate geological periods, the earlier Mississippian and the later Pennsylvanian.

The name Carboniferous means "coal-bearing", from the Latin carbō ("coal") and ferō ("bear, carry"), and refers to the many coal beds formed globally during that time. The first of the modern "system" names, it was coined by geologists William Conybeare and William Phillips in 1822, based on a study of the British rock succession.

Sharks are a group of elasmobranch cartilaginous fishes characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the division Selachii and are the sister group to the Batomorphi (rays and skates). Some sources extend the term "shark" as an informal category including extinct members of Chondrichthyes (cartilaginous fish) with a shark-like morphology, such as hybodonts. Shark-like chondrichthyans such as Cladoselache and Doliodus first appeared in the Devonian Period (419–359 million years), though some fossilized chondrichthyan-like scales are as old as the Late Ordovician (458–444 million years ago). The earliest confirmed modern sharks (Selachii) are known from the Early Jurassic around 200 million years ago, with the oldest known member being Agaleus, though records of true sharks may extend back as far as the Permian.

Sharks range in size from the small dwarf lanternshark (Etmopterus perryi), a deep sea species that is only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (40 ft) in length. They are found in all seas and are common to depths up to 2,000 metres (6,600 ft). They generally do not live in freshwater, although there are a few known exceptions, such as the bull shark and the river sharks, which can be found in both seawater and freshwater, and the Ganges shark, which lives only in freshwater. Sharks have a covering of placoid scales (denticles) that protects the skin from damage and parasites in addition to improving their fluid dynamics. They have numerous sets of replaceable teeth.

Charophyta (UK: /kəˈrɒfɪtə, ˌkærəˈfaɪtə/) is a paraphyletic group of freshwater green algae, called charophytes (/ˈkærəˌfaɪts/), sometimes treated as a division, yet also as a superdivision. The terrestrial plants, the Embryophyta emerged deep within Charophyta, possibly from terrestrial unicellular charophytes, with the class Zygnematophyceae as a sister group.

With the Embryophyta now cladistically placed in the Charophyta, it is a synonym of Streptophyta. The sister group of the charophytes are the Chlorophyta. In some charophyte groups, such as the Zygnematophyceae or conjugating green algae, flagella are absent and sexual reproduction does not involve free-swimming flagellate sperm. Flagellate sperm, however, are found in stoneworts (Charales) and Coleochaetales, orders of parenchymatous charophytes that are the closest relatives of the land plants, where flagellate sperm are also present in all except the conifers and flowering plants. Fossil stoneworts of early Devonian age that are similar to those of the present day have been described from the Rhynie chert of Scotland. Somewhat different charophytes have also been collected from the Late Devonian (Famennian) Waterloo Farm lagerstätte of South Africa. These include two species each of Octochara and Hexachara, which are the oldest fossils of Charophyte axes bearing in situ oogonia.

The Paleozoic (/ˌpæli.əˈzoʊ.ɪk, -i.oʊ-, ˌpeɪ-/ PAL-ee-ə-ZOH-ik, -⁠ee-oh-, PAY-; or Palaeozoic) Era is the first of three geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma at the start of the Mesozoic Era. The Paleozoic is subdivided into six geologic periods, (from oldest to youngest) Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian. Some geological timescales divide the Paleozoic informally into early and late sub-eras: the Early Paleozoic consisting of the Cambrian, Ordovician and Silurian; the Late Paleozoic consisting of the Devonian, Carboniferous and Permian.

The name Paleozoic was first used by Adam Sedgwick (1785–1873) in 1838 to describe the Cambrian and Ordovician periods. It was redefined by John Phillips (1800–1874) in 1840 to cover the Cambrian to Permian periods. It is derived from the Greek palaiós (παλαιός, "old") and zōḗ (ζωή, "life") meaning "ancient life".

Fish began evolving about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.

The earliest jawed vertebrates probably developed during the late Ordovician period. They are first represented in the fossil record from the Silurian by two groups of fish: the armoured fish known as placoderms, which evolved from the ostracoderms; and the Acanthodii (or spiny sharks). The jawed fish that are still extant in modern days also appeared during the late Silurian: the Chondrichthyes (or cartilaginous fish) and the Osteichthyes (or bony fish). The bony fish evolved into two separate groups: the Actinopterygii (or ray-finned fish) and Sarcopterygii (which includes the lobe-finned fish).

The most recent understanding of the evolution of insects is based on studies of the following branches of science: molecular biology, insect morphology, paleontology, insect taxonomy, evolution, embryology, bioinformatics and scientific computing. The study of insect fossils is known as paleoentomology. It is estimated that the class of insects originated on Earth about 480 million years ago, in the Ordovician, at about the same time terrestrial plants appeared. Insects are thought to have evolved from a group of crustaceans. The first insects were landbound, but about 400 million years ago in the Devonian period one lineage of insects evolved flight, the first animals to do so. The oldest insect fossil has been proposed to be Rhyniognatha hirsti, estimated to be 400 million years old, but the insect identity of the fossil has been contested. Global climate conditions changed several times during the history of Earth, and along with it the diversity of insects. The Pterygotes (winged insects) underwent a major radiation in the Carboniferous (358 to 299 million years ago) while the Endopterygota (insects that go through different life stages with metamorphosis) underwent another major radiation in the Permian (299 to 252 million years ago).

Most extant orders of insects developed during the Permian period. Many of the early groups became extinct during the mass extinction at the Permo-Triassic boundary, the largest extinction event in the history of the Earth, around 252 million years ago. The survivors of this event evolved in the Triassic (252 to 201 million years ago) to what are essentially the modern insect orders that persist to this day. Most modern insect families appeared in the Jurassic (201 to 145 million years ago).

The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals. While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear. They are areas of active research and debate among palaeontologists at present.

Most amphibians today remain semiaquatic, living the first stage of their lives as fish-like tadpoles. Several groups of tetrapods, such as the snakes and cetaceans, have lost some or all of their limbs. In addition, many tetrapods have returned to partially aquatic or fully aquatic lives throughout the history of the group (modern examples of fully aquatic tetrapods include cetaceans and sirenians). The first returns to an aquatic lifestyle may have occurred as early as the Carboniferous Period whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds, and several modern amphibians.

Adam Sedgwick FRS (/ˈsɛdʒwɪk/; 22 March 1785 – 27 January 1873) was a British geologist and Anglican priest, one of the founders of modern geology. He proposed the Cambrian and Devonian period of the geological timescale. Based on work which he did on Welsh rock strata, he proposed the Cambrian period in 1835, in a joint publication in which Roderick Murchison also proposed the Silurian period. Later in 1840, to resolve what later became known as the Great Devonian Controversy about rocks near the boundary between the Silurian and Carboniferous periods, he and Murchison proposed the Devonian period.

Though he had guided the young Charles Darwin in his early study of geology and continued to be on friendly terms, Sedgwick was an opponent of Darwin's theory of evolution by means of natural selection.

Sir Roderick Impey Murchison, 1st Baronet (19 February 1792 – 22 October 1871) was a Scottish geologist who served as director-general of the British Geological Survey from 1855 until his death in 1871. He is noted for investigating and describing the Silurian, Devonian and Permian systems.

Pteridospermatophyta, also called pteridosperms or seed ferns, are a polyphyletic grouping of extinct seed-producing plants. The earliest fossil evidence for plants of this type are the lyginopterids of late Devonian age. They flourished particularly during the Carboniferous and Permian periods. Pteridosperms declined during the Mesozoic Era and had mostly disappeared by the end of the Cretaceous Period, though Komlopteris seem to have survived into Eocene times, based on fossil finds in Tasmania.

With regard to the enduring utility of this division, many palaeobotanists still use the pteridosperm grouping in an informal sense to refer to the seed plants that are not angiosperms, coniferoids (conifers or cordaites), ginkgophytes (ginkgos or czekanowskiales), cycadophytes (cycads or bennettites), or gnetophytes. This is particularly useful for extinct seed plant groups whose systematic relationships remain speculative, as they can be classified as pteridosperms with no invalid implications being made as to their systematic affinities. Also, from a purely curatorial perspective the term pteridosperms is a useful shorthand for describing the fern-like fronds that were probably produced by seed plants, which are commonly found in many Palaeozoic and Mesozoic fossil floras.

Tiktaalik (/tɪkˈtɑːlɪk/; Inuktitut: ᑎᒃᑖᓕᒃ [tiktaːlik]) is a monospecific genus of extinct sarcopterygian (lobe-finned fish) from the late Devonian Period, about 375 Mya (million years ago), having many features akin to those of tetrapods (four-legged animals). Tiktaalik is estimated to have had a total length of 1.25–2.75 metres (4.1–9.0 ft) on the basis of various specimens.

Unearthed in Arctic Canada, Tiktaalik is a non-tetrapod member of Osteichthyes (bony fish), complete with scales and gills—but it has a triangular, flattened head and unusual, cleaver-shaped fins. Its fins have thin ray bones for paddling like most fish, but they also have sturdy interior bones that would have allowed Tiktaalik to prop itself up in shallow water and use its limbs for support as most four-legged animals do. Those fins and other mixed characteristics mark Tiktaalik as a crucial transition fossil, a link in evolution from swimming fish to four-legged vertebrates. This and similar animals might be the common ancestors of all vertebrate terrestrial fauna: amphibians, reptiles, birds and mammals.

Placoderms (from Ancient Greek πλάξ [plax, plakos] 'plate' and δέρμα [derma] 'skin') are vertebrate animals of the class Placodermi, an extinct group of prehistoric fish known from Paleozoic fossils during the Silurian and the Devonian periods. While their endoskeletons are mainly cartilaginous, their head and thorax were covered by articulated armoured plates (hence the name), and the rest of the body was scaled or naked depending on the species.

Placoderms were among the first jawed fish (their jaws likely evolved from the first pair of gill arches), as well as the first vertebrates to have true teeth. They were also the first fish clade to develop pelvic fins, the second set of paired fins and the homologous precursor to hindlimbs in tetrapods. 380-million-year-old fossils of three other genera, Incisoscutum, Materpiscis and Austroptyctodus, represent the oldest known examples of live birth.

Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes). They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes living sharks, rays, and chimaeras. Acanthodians possess a mosaic of features shared with both osteichthyans (bony fish) and chondrichthyans (cartilaginous fish). In general body shape, they were similar to modern sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteians (gars, bowfins).

The popular name "spiny sharks" is because they were superficially shark-shaped, with a streamlined body, paired fins, a strongly upturned tail, and stout, largely immovable bony spines supporting all the fins except the tail—hence, "spiny sharks". However, acanthodians are not true sharks; their close relation to modern cartilaginous fish can lead them to be considered "stem-sharks". Acanthodians had a cartilaginous skeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with a dentine spine. As a result, fossilized spines and scales are often all that remains of these fishes in ancient sedimentary rocks. The earliest acanthodians were marine, but during the Devonian, freshwater species became predominant.

A tetrapod (/ˈtɛtrəˌpɒd/; from Ancient Greek τετρα- (tetra-) 'four' and πούς (poús) 'foot') is any vertebrate animal of the clade Tetrapoda (/tɛˈtræpədə/). Tetrapods include all extant and extinct amphibians and amniotes, with the latter in turn evolving into two major clades, the sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (extinct "pelycosaurs", therapsids and all extant mammals, including humans). Hox gene mutations have resulted in some tetrapods becoming limbless (snakes, legless lizards, and caecilians) or two-limbed (cetaceans, sirenians, some lizards, kiwis, and the extinct moa and elephant birds). Nevertheless, they still qualify as tetrapods through their ancestry, and some retain a pair of vestigial spurs that are remnants of the hindlimbs.

Tetrapods evolved from a group of semiaquatic animals within the tetrapodomorphs which, in turn, evolved from ancient lobe-finned fish (sarcopterygians) around 390 million years ago in the Middle Devonian period. Early tetrapodomorphs were transitional between lobe-finned fishes and true four-limbed tetrapods, though most still fit the body plan expected of other lobe-finned fishes. The oldest fossils of four-limbed vertebrates (tetrapods in the broad sense of the word) are trackways from the Middle Devonian, and body fossils became common near the end of the Late Devonian, around 370–360 million years ago. These Devonian species all belonged to the tetrapod stem group, meaning that they did not belong to any modern tetrapod group.