Edit: Okay, truth be known that this question was part of a set of 5 related questions that I was recommended to break up into individual posts (in their original form, the comments had no trouble seeing where the whole thing was going). I think that's where the confusion here is coming from and why y'all are having trouble with my using the word "hydroxyapatite" to describe metal turned into a form that an endoskeleton can be built off of, which is why I've now removed it from the title.

Calcium in Earth-side bones also act as mineral reserves and our biochemistry relies on having serum calcium levels to, among other things: perform muscle contraction, oocyte activation, building strong bones and teeth, blood clotting, nerve impulse, transmission, regulating heart beat and fluid balance within cells.

So it's not a good option to go, 'Just say "A skeleton mineralized with titanium/iron"'. Because that's going to have ripple effects all throughout the organism. (This was touched in related question 3 in the original multi-question post, "What effects might X metal have for blood chemistry, cell depolarization, etc; Would K+, Na+ and so on channels still be effective or not? but I was hoping to have some options to stick in there to keep it specific as suggested.)

This is a weird question, I know. And if it can't be answered, then it's that kind of question that's too hypothetical to get a general answer on. A recommendation of some introductory biochemistry texts would be just as valuable to me, if not more so, if this is the case.

Part 1 of 4 (split up from a previous multi-part single-theme post)

I am attempting to make a "semi-hard" alien species for a sci fi story. While I have had a great time working out evolution, gross biology, culture and the like... I am weak in chemistry on anything other than identifying elements and their groupings and a drooling baby in biochemistry and would like aid in working out what's possible and the ramifications of those possibilities. This is pretty much not going to be in the story directly, but I want to have it so I can make realistic actions and decisions correctly.

Based on a 30% oxygen and other non-human toxic gasses (I'm not married to the 30% number, but I wanted more atmospheric oxygen available to support larger invertebrate and deeper sea life), somewhat higher gravity, large and varied animal sizes and the availability of metals from the crust. I need to know:

What metals would give me both strength and lightness in a hydroxyapatite-like form?

(I'm leaning towards titanium or iron foam or even some form of cobalt-bond? But I don't know how to make them 'living' while avoiding the iron 'oxidation in oxygen issue' or if titanium/cobalt is a feasible metal for this based on their earth-rarity, but rarity can be easily handwaived for an alien planet. Cobalt is being considered an option because it doesn't appear to react caustically with water and it's involved in earth-side cyanocobalamin production so maybe it can help with oxygenation in larger creatures on some level?)


The aliens in my story have sequential life stages, where they spend an "infancy" with particular bodily proportions, the "juvenile" stage happens when they reach particular growth and development in their limb/body proportions, and finally "adult" when they hit fertility. They can progress into an "old age" where they remain in the adult proportions, but overall increasing in size evenly until they can't maintain/sustain their growth and finally develop physical senescence and die (or something kills them as they begin to weaken). -It's sort of the life cycle of pterosaurs, where they fill different ecological niches as they grow-.

I need a skeletal system that can be functional at all of those developmental points, as though the animal is fully developed at that point, but be able to be adaptable and be regrown as needed (there are no cranium sutures that fuse and can't widen the skull later, for example).

This growth-type in mind, I wanted the aliens to have a very variable bauplan through their various genera. This includes giant semi-self-powered flyers (they can maintain their flight and land easily, but can't take off from the ground, they climb and drop into the first downstroke), indricothere-sized herbivores, whale and seal-like marine life...

The atmosphere is at 30% oxygen, circles a blueish star, has two moons (so the darkest at night it ever gets is like a full moon on Earth), gravity is stronger (at about 1.5-7 earth g), there is tectonic activity similar to Earth leaning more towards earthquakes, volcanic activity largely takes place at plates beneath the oceans and land areas tend to slowly rise from the sea and subduction pulls them under at the other end, also underwater usually [I'm hoping this creates enough metals to be surface-available without deep mining being required, please advise if this is incorrect and how to fix it].

Let me know if anything needs clarifying, I think I mentioned the important bits. My chemistry info came from here, but it's not for biology applications: https://www.chemicool.com I also found this png of a periodic table of elements for biology, cobalt, iron and titanium are also there so... that helps too? Periodic Table of Biological Elements

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    $\begingroup$ Brazilianite seems like a good starting point - it's similar in structure to hydroxyapatite, using sodium and aluminum instead. Only thing is, it's just slightly stronger and lighter - and their may be other problems with it as well. $\endgroup$
    – Halfthawed
    Sep 3, 2019 at 3:28
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    $\begingroup$ Are you certain that you understand how this new-fangled "chemistry" works? Why do you believe that there is any relationship between the properties of elemental iron or titanium and those of iron or titanium hydroxyphosphate? (Yes, both titanium hydroxyphosphate and iron hydroxyphosphate a.k.a. giniite exist. In particular, giniite is both soft and brittle.) $\endgroup$
    – AlexP
    Sep 3, 2019 at 12:27
  • $\begingroup$ @AlexP from the question, "titanium or iron foam" implies a metal foam, especially when followed by "iron 'oxidation in oxygen issue'" implying iron in the metal form. $\endgroup$ Sep 3, 2019 at 12:41
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    $\begingroup$ Why do you want something like hydroxyapatite? Is there something about hydroxyapatite that you need or do you just need a light, strong alien skeleton? $\endgroup$
    – Willk
    Sep 3, 2019 at 16:06
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    $\begingroup$ FWIW, consider reading up on some of the incredibly strong teeth that deep-ocean lifeforms have right here on Earth. Same fundamental chemistry as land-dwellers but strength-to-weight ratio is outstanding. $\endgroup$ Sep 3, 2019 at 17:17

4 Answers 4


Whilst I don't necessarily agree with all of Willk's conclusions in his answer, it does raise an important point: one does not need a whole load of bone to use as a calcium and phosphorous reservoir to still make hefty use of calcium and phosphorous in ones biochemistry. This may actually be an example of exaptation in non-cartilaginous vertebrates. You could also reasonably have stores of calcium and phosphorous in non-structural areas, etc etc, and whilst the energy cost may be higher (because you now need both bone minerals and calcium and phosphorous reserves) there may be other benefits, like bone density not being quite so adversely affected by illness or poor diet.

You also don't necessarily need mineralisation to make robust bodyparts either... beaks, claws and horns can all be made very tough, or indeed very hard, from protein alone. There's scope for stronger structural proteins than the ones found in terrestrial animals, certainly. Even on earth, there's a wide variety... beta keratins in birds and reptiles (and probably dinosaurs) are a somewhat harder than the alpha keratins you'll find on your body, for example.

Calcium is obviously conveniently bioavailable, and calcium phosphates and carbonates are also conveniently biosynthesisable, but there are other options, and they needn't involve any kind of metal at all, toxic or otherwise. Consider silicates, as found in phytoliths (synthesised by plants) or the silica frustules constructed by diatoms as cell walls. Although slightly harder to work with than calcium compounds, silicates are clearly both bioavailable and biosynthesisable, contain no toxic metals, are found in living organisms, and are particularly hard. Silica and hydrated silica can have Mohs hardnesses of 5-7, with plain old calcium hydroxyapatite being a mere 5. For an even harder option, consider silicon carbide (Mohs hardness 9-9.5) which has recently been biosynthesised in genetically engineered bacteria. Silicates in their bulk forms aren't particularly reactive or toxic, but in finely ground form (such as can be sometimes found in mineral deposits laid down by dead diatoms) they can cause pneumoconiosis. On earth, there is some bacterial breakdown of silicates via hydrolytic processes, and if silicate-skeletoned creatures (and organosilicon compounds in general) were more common on your world there may well be more things that can degrade it. The local biochemistry might not be toxic to humans, but you may find that it can rot glass!

Limpets make use of iron compounds (goethite, an iron hydroxide) with a Mohs hardness of 5.5 to make some of the hardest teeth in the world. Iron sulphides are biosynthesisable, suggesting the possibility of pyrite mineralisation (Mohs 6-6.5). Note that some chemical pathways involving iron sulphides can release hydrogen sulphide which is quite toxic to terrestial life. Such creatures might not be dangerous in life because of this, but they may smell bad in death and in a confined space (or in other circumstances, such as being corroded by acid) might produce hazardous levels of fumes.

So pick your protein for toughness, and mineral for hardness, and you'll be good to go. No need for exotic metal biochemistries.

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    $\begingroup$ I like the idea of bacteria that can rot glass (one of the other story points was that the planet is a bacterial synthesis goldmine because there's no fungi as decomposers, just very active bacteria)! I did consider silica originally, but my reading on that was largely about silica-based life and that's pretty non-human friendly. Ignoring that, I'm curious how silica holds up if it's scaled up into an endoskeletal bone? I wasn't sure if it wouldn't behave like glass and assumed it would shatter rather than break neatly like calcium bones. Please advise? $\endgroup$
    – Amut
    Sep 3, 2019 at 21:41
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    $\begingroup$ @Amut bones behave quite differently from pure hydroxyapatite, because they're a complex protein/mineral composite. Similarly, any silica/protein composite won't behave like pure silica, and there are many different forms of silicon dioxide which are all slightly different in their mechanical properties. Not even all kinds of glass shatter like glass! There's plenty of scope to handwave the properties you need without things becoming implausible. $\endgroup$ Sep 3, 2019 at 21:47
  • $\begingroup$ @Amut Silica has a huge variety of amorphous forms. Bones made out of stiffened amorphous silica (or silica gel, or silicates) make sense. $\endgroup$
    – user86462
    Nov 24, 2022 at 10:42

No mineralized skeleton. Use cartilage.

Cartilaginous skeletons work great for fish. The cartilaginous fishes including sharks and rays have been doing well for 500 million years. Arguably a bony skeleton is better for a land creature with legs, because the bone can support more weight. But for a flying creature cartilage is perfect - cartilage is much lighter and more flexible than ossified bone.

Why then, do flying vertebrates not have cartilaginous skeletons? Probably because there is no going back, and fliers were all land animals first. And all the land animals are descended from bony fishes, who were the ones to first colonize the land. Those fish had bony skeletons and so do all their descendants regardless of where they wound up.

  • $\begingroup$ Weeeeell... maybe not so much for the flying; you want the leading edge of your wings to be nice and stiff so it doesn't just fold up immediately and leave you plummeting to your doom. You might be able to get the same effect from a cartilage rod held in tension by muscles along its length, but it might not be any lighter than a dumb bone alternative. $\endgroup$ Sep 3, 2019 at 18:59
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    $\begingroup$ @StarfishPrime cartilage can be as stiff as bone, it's not all soft and bendable. I believe birds have bones rather than cartilage because bone is also a great storage for calcium, which is required for so many metabolic processes and also because their eggs have hard, calcium-based shells. $\endgroup$ Sep 3, 2019 at 19:08
  • $\begingroup$ @Renan the existence of cartilaginous animals, and ones with leathery eggs, suggests that calcium is useful but not essential for either purpose. Do you have a reference for it being as stiff as mineralised bone? $\endgroup$ Sep 3, 2019 at 19:09
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    $\begingroup$ Thinking about it, there might be some healing issues associated with cartilaginous skeletons. Hard to say without suitable terrestrial animals to compare with, but human bones can reknit quite quickly and reliably, but mangled cartilage (especially old mangled cartilage) seems to take forever to heal, if it does at all. That might not be an necessary issue of cartilage though, but one to consider. $\endgroup$ Sep 3, 2019 at 19:46
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    $\begingroup$ @Starfish Prime: Healing is an interesting question and I wonder if it is an idiosyncrasy with how our species (possibly our order?) handles cartilage - it is poorly vascularized in humans. Sharks and rays are famously able to recover from damage and so must be doing something different. I wonder if their cartilage looks different as regards vascularity? $\endgroup$
    – Willk
    Sep 4, 2019 at 13:25

Magnesium and aluminium

have insoluble, solid material forming phosphates and oxides and non variable oxidation states, which I would suggest be your criteria.

Iron and titanium are probably OK-ish despite their multivalence.


is certainly worth a look. It can act a bit like a cation (when not paired with oxygen), or as an oxide network forming species (borates are a little bit like silicates).

Weirder candidates include

zinc or scandium

Sc is too rare but otherwise probably OK. The rest of the light transition metals have too much redox chemistry to be a good idea.

Personally, my order would go silica/amorphous silicates, boron, aluminium, magnesium, titanium, iron, zinc, scandium, and I think that exhausts the options unless you consider heavier elements. I like Si/B/Al the best because they can all form extended oxide/hydroxide/phosphate materials with non stoichiometric chemistry, they're all relatively bioinactive, they can incorporate other ions (either anions or cations), their oxide surfaces can be functionalised, they don't have much redox chemistry. You can imagine their materials combining with organic scaffolds easily enough. Si leads on all these grounds.


No need for mineralization. Have the bones be a tough rigid biopolymer like wood. Nanocellulose is very tough and stiff. Perhaps have a sand aggregate for hardness or do a smidge of handwaving for carbon nanofibers in your wood bones. I dont mean carbon fiber, nanotubes, or graphene, but aggregates are biologically doable. This would be hard tough stiff flexible lightweight and have rapid healing since carbon nitrogen oxygen and hydrogen make up the majority of carbon based life. No need to deal with relatively tiny amounts of calcium phosphorous and iron in the diet.


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