Osteichthyes in the context of "Dermal denticle"

Vertebrates (/ˈvɜːrtəbrɪt, -ˌbreɪt/), also called craniates, are animals with a vertebral column and a cranium. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.

The vertebrates make up the subphylum Vertebrata (/ˌvɜːrtəˈbreɪtə/ VUR-tə-BRAY-tə) with some 65,000 species, by far the largest ranked grouping in the phylum Chordata. The vertebrates include mammals, birds, amphibians, and various classes of fish and reptiles. The fish include the jawless Agnatha, and the jawed Gnathostomata. The jawed fish include both the cartilaginous fish and the bony fish. Bony fish include the lobe-finned fish, which gave rise to the tetrapods, the animals with four limbs. Despite their success, vertebrates still only make up less than five percent of all described animal species.

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).

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.

Chondrichthyes (/kɒnˈdrɪkθiiːz/; from Ancient Greek χόνδρος (khóndros) 'cartilage' and ἰχθύς (ikhthús) 'fish') is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or bony fish, which have skeletons primarily composed of bone tissue. Chondrichthyes are aquatic vertebrates with paired fins, paired nares, placoid scales, conus arteriosus in the heart, and a lack of opercula and swim bladders. Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.

The class is divided into two subclasses: Elasmobranchii (sharks, rays, skates and sawfish) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class). Extant chondrichthyans range in size from the 10 cm (3.9 in) finless sleeper ray to the over 10 m (33 ft) whale shark.

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.

Ichthyology, from Ancient Greek ἰχθύς (ikhthús), meaning "fish", and λόγος (lógos), meaning "study", is the branch of zoology devoted to the study of fish, including bony fish (Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha). According to FishBase, 35,800 species of fish had been described as of March 2025.

A seahorse (also written sea-horse and sea horse) is any of 46 species of small marine bony fish in the genus Hippocampus. The genus name comes from the Ancient Greek hippókampos (ἱππόκαμπος), itself from híppos (ἵππος) meaning "horse" and kámpos (κάμπος) meaning "sea monster" or "sea animal". Having a head and neck suggestive of a horse, seahorses also feature segmented bony armour, an upright posture and a curled prehensile tail. Along with the pipefishes and seadragons (Phycodurus and Phyllopteryx) they form the family Syngnathidae.

Actinopterygii (/ˌæktɪnɒptəˈrɪdʒiaɪ/ ; from Ancient Greek ἀκτίς (aktís) 'ray, beam' and πτέρυξ (ptérux) 'wing, fins'), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that constitute nearly 99% of the over 30,000 living species of fish. The vast majority of extant actinopterygian species are teleosts, and by species count they dominate the subphylum Vertebrata, comprising over 50% of all living vertebrates. They are the most abundant nektonic aquatic animals and are ubiquitous throughout freshwater, brackish and marine environments from the deep sea to subterranean waters to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 mm (0.3 in), to the giant sunfish, at 2,700 kg (6,000 lb), and the giant oarfish, at 8 m (26 ft) (or possibly 11 m (36 ft)). The largest ever known ray-finned fish, the extinct Leedsichthys from the Jurassic, is estimated to have grown to 16.5 m (54 ft).

Ray-finned fish are so called because of their lightly built fins made of webbings of skin supported by radially extended thin bony spines called lepidotrichia, as opposed to the bulkier, fleshy fins of the sister clade Sarcopterygii (lobe-finned fish). Resembling folding fans, the actinopterygian fins can easily change shape, orientation and wetted area, providing superior thrust-to-weight ratios per movement compared to sarcopterygian and chondrichthyian fins. The fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the articulation between these fins and the internal skeleton (e.g., pelvic and pectoral girdles).