Alright so, there's been a bit of questions towards aquatic mammals and why it keeps happening lately. I'm developing several essays around that at the time, and this blare is a small contribution not necessarily in proper order
Having recently acquired some insight into a non-cetacean aquatic lineage, Sirenia, I considered an adaptation I thought remarkable if it would ever occur; a cartilaginous skeleton. Not fully so, mind you, perhaps starting at low impact areas, and then exaggerating greatly. It would reduce the weight of the animal, for speed and flexibility, especially in tight spaces. Several amphibians have done this, but I'm not aware of it in mammals. It seems most likely to happen in the water, where less support from bony structures is needed
As for what could develop such a system, rodents, shrews (not great at my terminology), and bats seem likely. Bats are the weird one, though they already have lightweight skeletons, and keeping them would make sense
Clearly there exists in the universe a powerful force, akin to the phoenix force in the marvel universe, that powers baseball teams that embrace the will of the Cartilaginous Ones to crazy wins. Last year the Nationals worshiped at the altar of the almighty Baby Shark with the Sacred Dance do do do do do do. This year the devil rays (I will continue to refer to them as such as a flappy sea creature is a way better mascot than a burst of light) do the thing they did last night.
The Nats need to spend time this offseason studying this ancient force and learning out to channel it.
There are recent studies suggesting that contrary to popular belief, the cartilaginous skeletons of sharks and rays are not an ancestral trait, but a derived one. This is supported by the fact that sturgeons have a cartilaginous skeleton too. It's believed they developed it to control their buoyancy in the water, since they don't have a swim bladder like most bony fish.
I've recently been wondering if it would be possible for a tetrapod to develop such a skeleton, since all of them start out with cartilaginous skeletons in the womb which later ossifies as they develop, and indeed, tadpoles have cartilaginous skeletons as well.
One way I was thinking it could evolve was when I was reading Serina, and Sheather was exploring ways a bird could get around the limitation of calcified egg shells and develop live birth. If a bird lost its eggshell completely, there'd be no calcium for the chick to develop its bones, and thus it would be born with an underdeveloped cartilage skeleton. A lot of birds in Serina have this problem, but they circumvent it by feeding their chicks a high-calcium diet of insects allowing their bones to develop as they grow.
So let's say a type of bird develops soft eggs and later live birth. As I mentioned above, the chicks are born without developed skeletons, and rely on their parents to feed them a high-calcium diet to help ossify their skeletons. This bird later becomes fully aquatic, and they completely give up on ossifying their skeleton and later become what is essentially a shark-bird. Maybe they develop tough keratinous scales all over their body like on their feet to help compensate for their lack of bones for protection.
Wikipedia says cartilaginous fish are vertebrates and its got me f'd up.
