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Why is cartilage present in a vertebrate embryo replaced by bones in an vertebrate adult even though cartilage can also provide me the structural framework like as in Chondrichthyes and they seem to do well with it as Sharks are members of Chondrichthyes which are some of the most fearsome creatures of the world.

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Bones are complex organs, made of more than just a structural element. Cartilage is just a tissue, usually devoid of nerves and blood vessels.

Chondrichythes live in water (ἰχθύς ichthys is Greek for 'fish'); the difference in specific gravity and buoyancy is considerable, and cartilage is not robust enough to mechanically support your body out of the water.

Vertebrates have evolved to have a bone marrow, which is part of the bone. If the structural element of the bone (the dense mineral matrix) was replaced by cartilage only, then the hollow structure would make things even worse.

Additionally, cartilage has about the same density as water (Pan et al, 2016) whereas dense bone is 60-90% more dense, which means extra weight pulling aquatic animals down.

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  • $\begingroup$ Bones evolved in fish that lived in the same water as sharks, so the mechanical argument doesn't hold. Maybe land vertebrates would be hindered (but there are plenty of insects, etc. that do just fine). $\endgroup$ – kmm Feb 3 at 23:49
  • $\begingroup$ @kmm The mechanical argument only means that "if you live on land and have a significant mass, then you need bones", it does not mean that "if you have bones, you cannot live in the water". There is no contradiction here :) $\endgroup$ – Mowgli Feb 3 at 23:56
  • $\begingroup$ Then I don't understand the point you are making in the 2nd paragraph with respect to the poster's question. $\endgroup$ – kmm Feb 4 at 0:50
  • $\begingroup$ It's an example of why in certain contexts, vertebrates (like you and I) have a biological advantage because they have bones instead of cartilage. Which contributes to explaining why the presence of bones has been maintained in aerial vertebrate lineages. It does not explain everything but it explains why vertebrates are ecologically successful on land (which is where the majority of described vertebrates species live) so that helps answer the question $\endgroup$ – Mowgli Feb 4 at 0:58
  • $\begingroup$ bone marrow is unrelated to bone, marrow actually evolved before bone. ncbi.nlm.nih.gov/pubmed/3636583 $\endgroup$ – John Jun 21 at 13:57
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Short answer, bone is stronger and stiffer.

Sharks may be big but they have pathetically weak bites for their size and that even when they partially ossify their jaws. They just can't fully ossify the tissue. The main swimming action of "fish" works even with flexible connections because each muscle is linking to the next linearly, the cartilage and collagen is mostly subjected to tension. But things like jaws and limbs need to withstand shear forces and withstand flexion, which cartilage sucks at, but bone is quite good at. It is not coincidence that the first bones evolved in jaw.

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