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Hypothetically, if an animal such as a cephalopod were to become terrestrial, and walking gradually evolved from crawling, would hardened muscle be likely to evolve as support, with articulation to allow for bending. If so, how would these bones differ from vertebrate bones.

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Use a hydrostatic skeleton

New bones are really hard to evolve. I'll wager that all vertebrates share the same basic skeletal structure (head, spine, 4 limbs, tail) regardless of environment, and I'll wager that true "bone" endoskeletons haven't evolved independently since the first time. Therefore, turning muscle into real bones probably isn't gonna happen. But we don't need real bones anyway.

Cephalopods manage to move without bones to push and pull on. They use muscular hydrostats, the same structures that power elephant trunks and human tongues. In essence, their muscles push and pull against each other using hydraulic movement. Since water can't be compressed, and it fills each muscle cell, the system has something to push on, and their limbs can move.

Many organisms, such as earthworms and sea anemones, have whole hydrostatic skeletons. Instead of using hydraulics to power individual organs, they base their whole structure around fluid-filled cavities that act like bones. If such a cavity were structurally sound and filled with a high-pressure fluid, it could act rigid enough to support a larger creature.

I propose that instead of evolving bone skeletons, cephalopods on land will develop their hydrostatic muscles into fully-fledged hydrostatic skeletons, providing the rigidity required for locomotion on hard surfaces. Such a structure might not support something big or strong, but it's a start.

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If you want a horrifying, semi-nightmare-inducing way for your creatures to develop "bones," you could try calcification. basically something about the atmosphere reacts with their bodies and causes accelerated calcification of certain tissues. Maybe not muscle tissue, as then they just can't move, but it results in a slowly hardening skeletal structure that could potentially be walked around on. With extreme pain as limbs begin to lock up and stop moving as well.

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