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