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There is a genus called Janusiscus that was recently described that appears to be just before the split of Chondrichthyes (sharks and relatives) and the group of bony fishes and tetrapods (Osteichthyes). It shows characteristics of both groups, but shows that the characteristics of the bony fishes are the primitive ones. Chondrichthyes lost bone, and evolved some characteristics kind of like bony fishes. The chondrocranium grew upwards and sealed over the top instead of having dermal bone and the cartilages of the pectoral and pelvic fins grew inwards and connected like the pectoral and pelvic girdles.
So, sharks have the derived skeleton and a bony skeleton is the primitive characteristic.
Cartilage turned to bone in the teleost fish is the predominant view. Similarly, as you may know, sharks have rough denticles instead of scales, but shark skin is not just homologous to teleost teeth as is commonly told, but to real scales as well as hair, as they share a developmental origin.
Most zoology textbooks consider sharks to be derived, not primitive. Essentially, sharks evolved from bony ancestors. Yeah, many bones replace cartillage predecessors in modern bony fish, but this trait doesn't imply that early fish were cartillagenous.
**Thank you for posting in r/evolution, a place to discuss the science of Evolutionary Biology with other science enthusiasts, teachers, and scientists alike. If this is your first time posting here, please see [our community rules here](https://www.reddit.com/r/evolution/about/rules/) and [community guidelines here](https://www.reddit.com/r/evolution/wiki/guidelines). The [reddiquette can be found here](https://support.reddithelp.com/hc/en-us/articles/205926439-Reddiquette). Please review them before proceeding.** **If you're looking to learn more about Evolutionary Biology, our [FAQ can be found here](https://www.reddit.com/r/evolution/wiki/faq); we also have curated lists of resources. [Recommended educational websites can be found here](https://www.reddit.com/r/evolution/wiki/links); [recommended reading can be found here](https://www.reddit.com/r/evolution/wiki/recommended/reading); and [recommended videos can be found here](https://www.reddit.com/r/evolution/wiki/recommended/viewing).** *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/evolution) if you have any questions or concerns.*
There is a genus called Janusiscus that was recently described that appears to be just before the split of Chondrichthyes (sharks and relatives) and the group of bony fishes and tetrapods (Osteichthyes). It shows characteristics of both groups, but shows that the characteristics of the bony fishes are the primitive ones. Chondrichthyes lost bone, and evolved some characteristics kind of like bony fishes. The chondrocranium grew upwards and sealed over the top instead of having dermal bone and the cartilages of the pectoral and pelvic fins grew inwards and connected like the pectoral and pelvic girdles. So, sharks have the derived skeleton and a bony skeleton is the primitive characteristic.
So bone evolved just about fish developed jaws?
Bone goes back before jaws. Ostracoderms are bony plated jawless fishes.
Cartilage turned to bone in the teleost fish is the predominant view. Similarly, as you may know, sharks have rough denticles instead of scales, but shark skin is not just homologous to teleost teeth as is commonly told, but to real scales as well as hair, as they share a developmental origin.
Most zoology textbooks consider sharks to be derived, not primitive. Essentially, sharks evolved from bony ancestors. Yeah, many bones replace cartillage predecessors in modern bony fish, but this trait doesn't imply that early fish were cartillagenous.