Osteichthyes, or bony fish, are the most diverse group of vertebrates. This clade includes animals with skeletons primarily made of bone tissue, encompassing over 28,000 documented species and making them the largest class of vertebrates. Bony fish have conquered nearly every aquatic environment, from deep oceanic trenches to high-altitude freshwater streams. Their success is a testament to anatomical tools that evolved over 425 million years.
Defining Anatomical Features
The primary characteristic of Osteichthyes is their bony endoskeleton. This framework is composed of endochondral bone, which forms by replacing a cartilage model during development. This process creates a strong, lightweight skeleton that supports muscles and protects internal organs, contrasting with the cartilage skeletons of sharks. This bony structure extends to the head, where cranial bones cover the head and pectoral girdles.
A feature for respiration in bony fish is the operculum, a hard, bony flap that covers and protects the gills. Unlike sharks, which have multiple exposed gill slits, the operculum allows bony fish to pump water over their gills while stationary. By controlling the operculum and mouth, the fish creates a pressure gradient that draws a stream of water across the gill filaments for efficient gas exchange.
Another feature is the swim bladder, a gas-filled sac in the body cavity. This organ functions as a hydrostatic device, allowing the fish to control its buoyancy and maintain a desired depth without expending significant energy. The fish can adjust the amount of gas in the bladder to become more or less buoyant. In some lineages, this structure was modified to become a lung for breathing air, a feature seen in lungfishes.
The Two Major Lineages
Osteichthyes is divided into two lineages, distinguished by the structure of their fins. The first and largest group is Actinopterygii, the ray-finned fishes. This lineage includes over 99% of all living fish species, with examples ranging from tuna and eels to guppies. Their fins are supported by slender, bony spines known as rays, which spread out from the base in a fan-like structure.
The second lineage is Sarcopterygii, the lobe-finned fishes. These fish possess fleshy, limb-like fins supported by a central axis of bone. This internal bone structure is homologous to the limb bones of terrestrial vertebrates. Today, Sarcopterygii is a smaller group of fish, represented by the coelacanths and the lungfishes.
The differences in fin anatomy reflect the divergent evolutionary paths these two groups have taken. The fins of ray-finned fishes are maneuverable and adapted for many swimming styles, contributing to their aquatic success. The bone-supported fins of the lobe-finned fishes laid the groundwork for the transition to land.
The Evolutionary Leap to Land
The evolutionary journey from water to land was initiated by members of the Sarcopterygii. The fleshy, bone-supported fins of ancient lobe-finned fishes were a pre-adaptation for life on land. These appendages had the internal structure necessary to support an animal’s weight outside the buoyant environment of water. These fins were the evolutionary starting point for the development of legs.
Fossils from the Devonian period document the transition from these lobe-finned ancestors to the first tetrapods—four-limbed vertebrates. These early transitional animals possessed a mix of fish-like and amphibian-like features, including both gills and lungs. Their fins were also progressively more limb-like, and this lineage is the direct ancestor of all amphibians, reptiles, birds, and mammals.
This ancient connection means that every terrestrial vertebrate is a descendant of a branch of lobe-finned fish. Our arms and legs are modified versions of the fins seen in living coelacanths and lungfishes. We are, in a sense, a type of fish that evolved to walk on land, a result of the anatomy that first appeared in the Sarcopterygii.