Actinopterygii

Lore & Background

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.

Reader's Guide

Actinopterygii are a class of bony fish that includes nearly all living fish species, with teleosts alone accounting for 96% of extant fish. Their evolutionary success is reflected in their dominance of vertebrate diversity—over 50% of all living vertebrates are ray-finned fish. They inhabit virtually every aquatic environment, from deep seas to mountain streams. The group's classification includes Cladistia, Chondrostei, and Neopterygii, with teleosts being the most diverse infraclass. Reproduction is mostly external fertilization, with sequential hermaphroditism and viviparity present in some lineages. Their legacy is central to understanding vertebrate evolution and aquatic ecosystems.

Did You Know?

• The largest ever known ray-finned fish is the extinct Leedsichthys from the Jurassic, estimated at 16.5 m (54 ft).

Deep Roots in the Silurian

The oldest known fossils of bony fish date back roughly 436 million years to the early Silurian period. These ancient specimens are particularly fascinating because they represent transitional forms—their tooth patterns fall somewhere between the rows seen in sharks and those of true bony fishes. Despite carrying the name 'bony fish,' these early basal representatives had not yet developed true ossification; their skeletons remained predominantly cartilaginous. What truly set them apart from other fish lineages was not bone but a pair of foregut pouches, outpouchings on either side of the esophagus. These structures served as supplementary respiratory organs in low-oxygen waters and would eventually diverge along two evolutionary paths: in ray-finned fish they became swim bladders for buoyancy control, while in lobe-finned ancestors they specialized into the lungs that characterize all land-dwelling vertebrates today. This single anatomical innovation thus underpins the respiratory systems of an astonishing breadth of life.

A Skeleton of Bone and Scale

Bony fish display a remarkably consistent cranial architecture: a stable arrangement of skull bones, a medial insertion point for the jaw-closing muscle, and broad dermal plates covering the head and pectoral girdles. The eyeball is cradled by a ring of four tiny scleral bones, though many modern species have lost or modified this feature. Inside the inner ear, large otoliths anchor the labyrinth, and the braincase is often split into anterior and posterior compartments by a distinct fissure. Equally distinctive is the scale system. Unlike the placoid scales of sharks, bony-fish scales sit beneath the epidermis rather than piercing through it. Three major types exist: cosmoid, ganoid, and teleost. All share a bony base, but they differ in layering. Teleost scales consist of a single bone layer and are further divided into smooth cycloid and spiny ctenoid varieties. Ganoid scales add lamellar bone topped by vascular bone and a cap of enamel. Cosmoid scales mirror the ganoid structure but insert a layer of dentin between the enamel and vascular bone.

The Tetrapod Connection and Taxonomic Revolution

For much of the twentieth century, ichthyologists treated Osteichthyes as a paraphyletic grouping that included only fishes, deliberately excluding the land vertebrates that evolved from lobe-finned ancestors. Under that older framework, the group was recognized by the presence of a swim bladder, three pairs of gill arches tucked behind a bony operculum, and a predominantly bony skeleton. The ray-finned fish formed a monophyletic subclass, but adding the smaller lobe-finned subclass made the whole assemblage paraphyletic because the common ancestor's descendants included tetrapods. A paradigm shift has since taken hold. Since 2013, widely cited ichthyology papers have published phylogenetic trees that treat Osteichthyes as a fully monophyletic clade encompassing tetrapods. In this cladistic view, the group is broadly equivalent to Euteleostomi. In paleontology the two terms are already synonymous; in ichthyology the distinction has narrowed to one of perspective. The lobe-finned fish, long seen as a small side branch, are now understood as the lineage from which every amphibian, reptile, bird, and mammal ultimately descended, making the 'bony fish' a group that includes us.

Diversity, Respiration, and the Whole-Genome Legacy

With 45 orders, more than 435 families, and roughly 28,000 described species, Osteichthyes represents one of the most species-rich vertebrate assemblages on Earth, and the vast majority of all living fish belong to this group. Respiration is universally gill-based, yet the group displays extraordinary flexibility: lungfish and certain other species breathe through lungs or vascularized swim bladders, while still others extract oxygen across their skin, intestines, or stomach lining. Most bony fish are ectothermic, their internal temperature tracking the surrounding water. However, some large marine species—opah, swordfish, and tuna—have evolved mechanisms that let them maintain body temperatures independently of the environment. A deeper genetic legacy also shapes the group: a whole-genome duplication event occurred in the ancestral Osteichthyes, providing the raw genetic material that likely fueled the extraordinary morphological diversification seen today. The lobe-finned fish, meanwhile, evolved articulated skeletal elements within their paired fins, the very structures that would become the limbs of tetrapods, and developed opercula that actively pump water over the gills, freeing them from the need to swim continuously just to breathe.

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