Why are sharks made of cartilage and not bone?

James,

Sharks and skates (the Elasmobranchii) and the chimaeras (the Holocephali) together form the class Chondrichthyes, the cartilaginous fish.

The earliest ‘true’ shark yet discovered was from early Silurian deposits (around 420 million years before present) in Siberia and is classed as a ‘shark’ due to the shape and form of the scales. Thus, ‘sharks’ have been around for a great deal longer than many other extant (living) creatures. It appears as though during the Jurassic period, the ‘Batoids’ evolved from a more typical sharky ancestor, to form the skates and rays.

The Chondrichthyes have a cartilaginous, rather than a bony skeleton because they are Chondrichthyes and not Teleosts (bony fish). This may not seem like much of an answer but it is important to make a distinction between ‘fish’ and ‘sharks’. Although Chondrichthyes and Teleosts share a common ancestor somewhere in the deep mists of time, they have evolved as groups along two separate lines. Therefore, having a cartilaginous skeleton is not a ‘primitive’ condition (although it has been around for a long time) compared to having a bony skeleton (being ‘advanced’), as is often assumed.

There are many advantages to having a cartilaginous skeleton, for a start it is much lighter than bone and more flexible whilst being strong and durable. These help the Chondrichthyes in reducing the weight of the animal, which aids buoyancy (as sharks do not have a swim bladder, as used by bony fish to create ‘neutral buoyancy’) and the flexibility of cartilage allows sharks to turn in a tighter radius than other fish. Where there are increased pressures placed on the basic cartilaginous skeleton, such as around the jaws, these areas are strengthened by calcium salts, creating ‘calcified cartilage’, similar to bone in terms of strength but without the added weight.

So, sharks are made of cartilage rather than bone because 1) there are many advantages to having cartilage over bone (although, for the Teleosts, there are advantages for them in having bone over cartilage) and 2) the evolutionary ‘pathway’ sharks have followed has not required them to evolve bone.

I hope this answers your question,

Dave.

Last edited by Dave Warburton (12th Oct 2007 08:35:17)

Although Chondrichthyes and Teleosts share a common ancestor somewhere in the deep mists of time, they have evolved as groups along two separate lines. Therefore, having a cartilaginous skeleton is not a ‘primitive’ condition (although it has been around for a long time) compared to having a bony skeleton (being ‘advanced’), as is often assumed.

Well.  It all depends on what kind of skeleton the most recent common ancestor of sharks and bony fishes had.  If it had a cartilaginous skeleton, then that is the primitive state, and the sharks and rays that retained that state do indeed have the primitive condition.  If the ancestor was bony, then the converse is true, and it is sharks that are advanced in this respect, not us.

Being primitive is nothing to be ashamed of!  As has recently been pointed out in response to a different questions, we are primitive as regards the number of our fingers and toes: in this way, horses, birds and snakes are all much more advanced than us.

Mike,

As to be expected due to your background, you've interpreted this as ancestral versus derived. The point I was making (or trying to make) to James is that although the common ancestor of Condrichthyes and Teleosts is likely to have had a cartilaginous skeleton, the point of putting apostrophes around the primitive and advanced is in regard to the common perception of primitive versus advanced i.e. "primitive is rubbish, advanced is great" blah blah blah (if you’ll excuse the gross generalisation).

Reading through what I wrote, it doesn't appear to me that there is any room for the interpretation that I implied that a less derived state is to be ashamed of; all I was saying is that although the presence of a cartilaginous skeleton may be ancient, then it is certainly not something that is barely clinging on to existence, having being superseded by bony fish.

Condrichthyes are (and have been) a very diverse and successful group and although their basic body plan may have existed for many millions of years, this in no way detracts from these creatures being skilled survivors.

Dave.