What Makes the Coelacanth’s Skeleton So Unique?

Long thought extinct, the coelacanth’s rediscovery in 1938 presented science with a living specimen from an ancient lineage. This “living fossil” offers a window into the deep past, and its skeleton holds clues about major vertebrate evolutionary events. The fish’s anatomy is so distinct from modern species that its skeletal system is a primary focus for researchers seeking to understand how vertebrates transitioned from water to land.

Coelacanth Skeletal Composition

The coelacanth’s skeleton is a mosaic of ancient traits, distinguishing it from modern bony fishes, or teleosts. Its endoskeleton is predominantly made of cartilage, as bone formation, known as ossification, is drastically reduced compared to other fish. This retention of a largely cartilaginous framework into adulthood is a primitive characteristic reflecting an earlier stage in vertebrate evolution.

This composition stands in contrast to teleost fishes, which undergo extensive endochondral ossification, where cartilage is replaced by bone. While some parts of the coelacanth, like the neural and haemal spines, are composed of a layer of bone surrounding a cartilage core, the overall reliance on cartilage is a defining feature. The main divisions of the skeleton—axial, appendicular, and cranial—all exhibit this unique blend of tissues.

The Unyielding Notochord

The central feature of the coelacanth’s axial skeleton is its massive, persistent notochord. In most vertebrates, the notochord is an embryonic structure that guides the formation of the vertebral column before being reduced to small remnants. In the coelacanth, this structure remains throughout the fish’s life as a large, unconstricted, hollow tube filled with an oil-like fluid, serving as the primary support for the body.

This tough and elastic tube extends from the skull to the tail. Along its length, it is equipped with neural arches on top to protect the spinal cord and haemal arches below to protect major blood vessels. The complete lack of vertebral centra, the solid bony elements that form the backbone in most vertebrates, means these arches do not fuse to form a segmented spine. This unyielding notochord is a functional snapshot of the skeletal state of ancient vertebrates.

Lobed Fin Skeletal Structure

The coelacanth’s fins contain a skeletal arrangement that provides a direct link to the limbs of land-dwelling vertebrates. Unlike the simple, ray-supported fins of most fish, the coelacanth’s paired pectoral and pelvic fins are fleshy, muscular lobes. These lobes are supported by an internal chain of bony elements that show a clear homology to the bones in tetrapod limbs. This structure includes a single, large proximal bone, akin to a humerus or femur, which articulates with two smaller bones.

This limb-like structure is also present in the second dorsal and anal fins, which are similarly lobed and mobile. The fins can be rotated and moved in an alternating pattern, similar to the way a four-legged animal walks. Another distinctive feature is the tail fin, which is a symmetrical three-lobed structure known as a diphycercal tail. A small, fleshy central lobe is supported by a direct extension of the notochord.

The Hinged Cranial Design

The skull of the coelacanth possesses a design not seen in any other living vertebrate: the intracranial joint. This hinge divides the cranium into an anterior portion (ethmosphenoid) and a posterior portion (otoccipital). This joint allows for significant mobility, enabling the front part of the skull to be elevated during feeding. This motion greatly increases the gape of the mouth, enhancing the fish’s ability to use suction to capture prey.

This hinged design is powered by a large pair of basicranial muscles that cross the joint. Studies suggest these muscles and the joint also contribute to a more powerful bite, allowing the coelacanth to grip its prey effectively. This cranial mobility is a feature found in many fossil lobe-finned fishes but has been lost in all other living vertebrates, including lungfishes and tetrapods, whose skulls are a single unit. The skull also houses a rostral organ used for electroreception.

Evolutionary Echoes in the Bones

The skeleton of the coelacanth is a living archive of vertebrate history, offering insights into the transition from water to land. Each unique feature reflects an ancestral condition, helping scientists piece together the evolutionary journey of tetrapods. The persistent notochord represents a state before the evolution of a segmented vertebral column, while the intracranial joint shows a cranial mobility lost as vertebrates adapted to land.

The skeletal structure of the lobed fins provides the clearest evidence of this connection. The arrangement of bones, homologous to those in tetrapod limbs, illustrates an intermediate step between a fish fin and a weight-bearing leg. Although genomic studies now suggest lungfish are the closest living relatives to tetrapods, the coelacanth’s anatomy remains an invaluable blueprint for understanding the adaptations necessary for the move to a terrestrial environment.