Updated: Apr 27
The "Chondrichthyans" differ from their "Osteichthyan" (aka bony fish) relatives because their skeletons are formed from partially calcified cartilage, rather than bone. These two groups diverged approximately 420 million years ago and within the cartilaginous fishes, subsequently two groups, called the "Elasmobranchii" (sharks, skates and rays) and "Holocephalii" (chimaeras), have arisen.
Chondrichthyan (Ancient Greek) χονδρ (khóndros) 'cartilage' & ἰχθύς (ikhthús) 'fish'
The cartilaginous skeleton of sharks, chimaeras and rays is composed of "chondrocytes" (cartilage-producing living cells) within a layer of fibrous material called the "perichondrium", that surrounds an extracellular matrix. The extracellular matrix is "calcified"; containing crystals of calcium phosphate (Dean & Summers, 2006).
Different types of cartilage are calcified to varying extents in separate areas of the body, because this gives the cartilage contrasting degrees of strength versus flexibility. This occurs by differing levels of mineralisation; more calcified cartilage contains higher quantities of crystals in the extracellular matrix. The areas supporting the extremities and appendages (known as the "appendicular skeleton"), have limited internal mineralisation. Whereas, the vertebrae have highly mineralised layers on the surface, covering an unmineralised core (Dean & Summers, 2006, Summers & Long, 2006).
In modern sharks heavily mineralised cartilage is found in the vertebral column and in the jaws. The calcified surface layer in these structures is made up of plate-like tiles called "tesserae". Mineralisation in plate-form means the structure is less likely to crack under strain, when compared to a continuous mineralised structure. "Tessellated cartilage" is found in the vertebra, which provides strength for muscle attachment and more powerful swimming. Tesslated cartilage is also found in the base of the tooth (known as the "peduncle"), where multiple layers of tesserae form powerful attachments for the teeth. Whatsmore, the outer layer of the teeth are sheathed in enamel (calcified cartilage with a high fluoride content and well defined structure of crystals in the extracellular matrix), which provides it great strength (Dean & Summers, 2006)
In several species of sharks (large species which have highly demanding feeding strategies), such as the great white (Carcharodon carcharias), highly mineralised tessellated cartilage, can also be found in layers throughout the core and at the corners of the jaw. This provides the jaw incredible strength for hunting large prey (Dean & Summers, 2006, Fratzl et al, 2016).
It is the calcified nature of the teeth and jaw which allow these structures to become fossilised. It is very common to find fossilised shark teeth, but rare to find entire skeletons because the cartilaginous structures with little mineralisation decompose before they can become fossilised. Fossilised shark teeth can even be found on the beach, if you know where to look!
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Dean MN & Summers AP (2006). Mineralized cartilage in the skeleton of chondrichthyan fishes. Zoology, 109, 164–168. Access online.
Summers AP & Long JH (2006). Skin and bone, sinew and gristle: the mechanical behavior of fish skeletal tissues. In: Lauder GV & Shadwick RE (Eds). Fish Biomechanics: Fish Phsyiology, 23, pp. 141–178. Academic Press, San Diego. Access online.