I have learned that two important lineages of fish are the bony fishes (Osteichthyes) and the cartilaginous fishes (Chondrichthyes). Many websites mention an advantage of the cartilaginous skeletons of sharks and other chondrichthyans: they're lighter than bony skeletons.

But it has been harder for me to find out if we know of any things bony skeletons do for fish that cartilage skeletons can't do, or don't do as well.

My research and hypotheses

I found an old Manual of Geology by Samuel Haughton (1865) that says

[a bony] skeleton only affords an advantage over a cartilaginous skeleton by allowing a greater variety of points of attachment for the muscles of the Fish, and so admits of more powerful motions.

But maybe other people have discovered more things since this was written. Also, I'm curious if this is just a theoretical argument, or if we have actual evidence that bony fish are capable of using their muscles more effectively than cartilaginous fish.

I have three main hypotheses at present:

  1. Haughton was right, and a bony skeleton is just more effective at serving as an attachment point for muscles.

  2. Bony skeletons are more effective at protecting the fish.

  3. There are no advantages to bony skeletons; bony fish just happened to evolve this way. (As far as I can tell, the early evolutionary history of fish has been a bit unclear and I haven't been able to determine if there is current consensus about whether bone or cartilage skeletons came first, and whether chondrichthyan cartilage skeletons are an inherited primitive feature or an innovation.)


As a fish morphologist, here's my best attempt. I really like the first reference I listed at the bottom, too.

1) Ancestral Condition:: The cartilaginous skeleton of chondrichthyes (sharks, skates, rays, chimaeras) is almost certainly a derived feature--a synapomorphy-- defining the clade. There are a few lines of evidence for this. First, the cartilage in a shark skeleton is a very specialized form of calcified cartilage, unique to the group. Second, in other gnathostome and pre-gnathostome ancestors (Osteostracans, other armored jawless fishes) had big bony head shields. The groups most closely associated with sharks (Placoderms, paraphyletic acanthodians) has both bone and cartilage in their skeletons. Basically I guess my point is that, while cartilaginous skeletons were ancestral for gnathostomes, that cartilage was very different from chondrichthyan cartilage, and bone turned up in many stem fishes before chondrichthyan cartilage. So the ancestor for sharks and bony fishes probably had both cartilage and bone but not the specialized calcified cartilage of sharks

2) Bone as a Plesiomorphic (ancestral feature), but not the entire skeleton: If we take what I've said above, it certainly suggests that your point 3 is in the right direction w.r.t. split between chondrichthyes and osteichthyes. The chondrichthyan skeleton is light...But the osteichthyes definitely took endochondral bone and ran with it--which brings us back to your original question: why did they replace their entire skeleton with bone? Why is always a tricky question in evolution, but we can speculate:

3) Potential adaptive value of a bony skeleton:

  • bone is better at transmitting muscle force than cartilage, so in that sense, you are correct
  • when fish were coming into prominence and diversifying, there were a lot of big, terrifying, predatory arthropods. Many early fishes have big bony armor plates-- which may have served to project them from their chitonous predators
  • More evidence for that -- when big scary predators went extinct, fishes began to pare down their bony armor (most teleosts have highly reduced skeletons).

Without a time machine we can't be certain of these things, but I'm pretty sure what I've laid out is fairly close to the common consensus.

Relevant lit:

  • $\begingroup$ Thanks for the edits, @kmm . I am not good at markdown. Or spelling, apparently! $\endgroup$
    – Kara
    Apr 7 '17 at 2:35

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