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I've been doing research for the worldbuilding of my planet and I'm going back to design the details of the basic body plan for all creatures. I was interested on using something different than Earth's bone to have fun utilizing different properties, and I was wondering if a use of chitin and silica instead of collagen and hydroxyapatite would be viable and what limits or characteristics it would have?

The planet where these creatures live has 80% of Earth's gravity and a denser atmosphere. I'd love to be able to make large flying creatures if possible. If chitin with obvious extra proteins and silica end up being too brittle please suggest other possible combinations that you think would be interesting and viable? I would love to keep chitin as the matrix for its light and flexible properties but I'm still debating on the biomineral (I do not wish to use steel).

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By a strange co-incidence, I was just writing an answer about silican biomineralization for a different question (about teeth).

There's obviously no reason why you can't use chitin as a structural compound, because it is done already. It is obviously also possible for silica-based biomineralization to occur, because that's also done already.

However.

Silicon has relatively poor bioavailability compared to calcium. Silicic acid and silicates can be found in water, but concentrations are relatively low on Earth... that's part of the reason why calcium compounds are so much more popular, because they're simply easier to source. This might be rather different on your world, because the evolution of silicon-bearing skeletons implies a much more common and well developed silicon biochemistry, and lots more organisms that need to consume it to build themselves.

Because silica is a pretty inert and stable chemical, turning biosilica back into digestible silicates is going to be an energy intensive activity to the point where it couldn't be used as food (without exotic stuff like fluorine biochemistry, which you probably don't want here) and so there might need to be some other process to encourage a silicon cycle in your biosphere... maybe the rain is more acidic in your world, perhaps. The best thing to do is perhaps to ignore this problem... you're presumably not writing a story about biochemistry, so I'd just gloss over the issue. I don't see any other major obstacles to having silica-based bones.

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  • $\begingroup$ Some sponges do have silica-based spicules. $\endgroup$
    – AlexP
    Jan 10, 2023 at 13:10
  • $\begingroup$ That's wonderful to hear, thank you! If you don't mind me asking a bit extra: this is probably dependant on the percentages between both but what color would an endoskeleton like this have? I'm guessing the "bones" would be brown-ish because of the presence of chitin but the silica may just make them completely white or pinkish? $\endgroup$
    – Umbrace
    Jan 10, 2023 at 13:42
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    $\begingroup$ @Umbrace "pure" chitin is a sort of translucent whitish color. Actual uses in exeskeletons and so on tend to have structures or other compounds that affect the colour, but for your beasties those structures and compounds are entirely up to you. Your bones can be more or less any color you liked, potentially even opalescent if you wanted such a thing. $\endgroup$ Jan 10, 2023 at 14:14
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    $\begingroup$ Silicon biochemistry will never be well developed, because it's just not very good at making the kinds of bonds which make carbon so great for life. en.wikipedia.org/wiki/… $\endgroup$
    – RonJohn
    Jan 10, 2023 at 22:10
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Found skeletons enter image description here

https://www.quantamagazine.org/the-curious-strength-of-a-sea-sponges-glass-skeleton-20210111/

Depicted: silica glass skeleton of deep sea glass sponge.

Some crabs find shells made by molluscs and then move in. A good shell might be used by successive occupants over generations. The occupants could never have made such a shell from scratch but they can maintain the one they have found to some degree.

So too your fliers. The durable glassy tubes that they use for skeletal elements cannot be produced by their own biology. They must be found, as a crab must find a shell Maybe they are from plants? Maybe they wash up on the beach? Maybe they are abiotic crystals that form deep in the earth? A reliable place to find them is in a mature adult of their own species.

In any case, to maximize their fitness your creatures must find these things and then incorporate them into their bodies which they do by integrating them and then augmenting them with chitin as you propose.

-- I could imagine that entities who were without their silica could be viable but not very good, and certainly not one that would be chosen as a mate. Perhaps males vying for a mate would make a nest bowerbird style, decorated with many pieces of collected silica skeleton for the young to use - good advertising!

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