Sharks represent one of the oldest and most successful groups of vertebrates on Earth, with a lineage extending back over 400 million years. To understand their relationships, one must look beyond their superficial resemblance to other fish and examine their internal structure and evolutionary history. This reveals a distinct and ancient classification that separates them from the majority of other fish species.
The Cartilaginous Fish Group
Sharks belong to the biological Class Chondrichthyes, a group commonly referred to as the cartilaginous fish. This classification is defined by one of the most fundamental differences from other vertebrates: their skeleton is composed almost entirely of cartilage rather than true bone. Cartilage is a dense, yet flexible, connective tissue that is significantly lighter than bone. This lighter structure aids in the shark’s buoyancy and maneuverability in the water.
The skin of every member of this class is covered in tiny, tooth-like structures known as placoid scales, or dermal denticles. These denticles are structurally similar to teeth, featuring a pulp cavity, dentine, and enamel, and they give the shark’s skin a rough, sandpaper-like texture. Furthermore, cartilaginous fish lack a swim bladder, the gas-filled organ that most bony fish use for buoyancy control. Instead, sharks rely on a large, oil-filled liver, which is less dense than seawater, to help maintain their vertical position in the water column.
The absence of a swim bladder means that many sharks must swim constantly to prevent sinking and maintain a steady flow of water over their gills for respiration. This reliance on liver oil for lift, combined with their cartilaginous skeleton and dermal denticles, defines the Chondrichthyes class.
Closest Living Relatives
Within the Class Chondrichthyes, sharks share their closest living relatives in two major subclasses: Elasmobranchii and Holocephali. Sharks themselves are a part of the Elasmobranchii subclass, which also includes the rays and skates. This means that rays and skates are the most immediate and numerous relatives of sharks, sharing a recent common ancestor.
Members of the Elasmobranchii subclass are distinguished by having multiple external gill slits, typically five to seven pairs, that open directly to the water. Their upper jaw is generally not fused to the skull, allowing for a wide gape and flexible feeding mechanics. Rays and skates, which are essentially flattened sharks, display the same cartilaginous skeleton and dermal denticles, though they have adapted their pectoral fins into large, wing-like structures for propulsion.
The second subclass is Holocephali, which consists of the chimaeras, also known as ghost sharks or ratfish. Chimaeras diverged from the elasmobranch lineage over 400 million years ago, making them distinct cousins rather than immediate siblings. Unlike sharks, chimaeras have only four pairs of gills covered by a single, soft operculum, or gill cover, similar to bony fish. Their upper jaw is also fused immovably to the cranium, a feature called holostyly, which is a major anatomical difference from the flexible jaw of a shark.
Distinguishing Sharks from Bony Fish
The most common misconception is that sharks are simply a type of large fish, but their biology separates them into a class distinct from the vast majority of aquatic species. All other familiar fish, such as tuna, salmon, and goldfish, belong to the Class Osteichthyes, the bony fish.
The skeletal difference is the most obvious separation, with bony fish possessing a skeleton made of true bone tissue, while sharks have cartilage. This difference extends to their gill structure; bony fish have four pairs of gills protected by a hard, bony flap called an operculum. Cartilaginous fish, conversely, have their five to seven gill slits exposed to the environment, without a protective covering.
The covering of their skin also differs significantly, as bony fish possess scales like cycloid or ctenoid types, which are flat and overlap like shingles. Sharks, however, are covered entirely in placoid scales, which function more like a suit of tiny, hydrodynamic armor. Furthermore, their reproductive strategies show a deep divide; most bony fish engage in external fertilization, releasing eggs and sperm into the water. Cartilaginous fish, including all sharks, use internal fertilization, and their development can be oviparous, where they lay eggs, or ovoviviparous and viviparous, where the young develop internally and are born live. These biological distinctions in skeletal material, gill mechanics, skin covering, and reproductive method confirm that sharks occupy an evolutionary branch entirely separate from the bony fish.