How can Metamorphosis evolve in alien vertebrates?

Question

For an alien planet I'm making I thought of some creatures which are descendants of non-metamorphosing creatures with solid bones. These creatures however have a very diffrent life cycle than their ancestors, they hatch from tiny eggs as larvae with no bones, only cartilage, then they eat for a while before finding a safe place to enter dormancy as their limbs and bones develop. My question is: How would such a creature evolve? Would it even be possible?

Answer 1

The answer is always parasites.

Your vertebrate species used to lay its eggs inside another species' larva, perhaps a large caterpillar, or something that looks like a slug. The egg would not hatch into a larva, but into an embryo of the creature's final form, hooking up to the host's circulatory system to draw nutrients from it, essentially turning it into an autonomous, self-feeding uterus.

Then, through the eons, parasite and host co-evolved, shaping each other, until they merged into some kind of symbiosis. This is not entirely likely, but with a bit of suspension of disbelief, why not. In the current iteration of this evolution, this new species lays eggs which hatch into the host, with the parasite embryo already implanted. This has many advantages: the parasite mother no longer needs to look for a host, so they are no longer dependent on the host population, only on a biome suitable for its development. In addition, the host will no longer try to evolve defenses against the parasite, since they have become one.

Bonus points if the host doesn't die and is kept as a pet afterwards.

Answer 2

Possibly long answer.

Well, if we gonna lose our bones, then we need to lose our bones first.

Evolution theory basically follows easiest solutions that require least amount of changes and just enough benefit to solve the problem.

Now, bones are good in land. It supports you, offers protection for your organs, allows stronger muscles to grow, provide safe places across the body for cell production. Like blood cells.

But, we need to lose the bones. Now, where would we need the least amount of bones? Bones are heavy. Hard to heal. If you truly don't need bones, then you will live in sea where you can ride currents and need faster reaction than land. Also, if you have ridicilously complex nerve systems, that would be really hard to heal. One more thing. Big changes requires big events. I am thinking of a apocalypse, or something similar that wiped out dinasours. Maybe the one wiped out trylobytes. Trilobytes? The greatest destruction Earth ever faced. More than 90% of life got destroyed. Now, in your setting, it is time for water!

Now your surviving group of aliens needs to live in water for millins of years. Or little lands and high levels of water. Hunting in water, travelling in water. Here comes the smoother skin, wider hands and feet, lighter bones. More agile bodies with lesser strength. Water is rising! We need to... Umm... hold our breath for longer durations. Better lungs, closer nose channels for keeping our lungs dry enough. Some better adaptibility for handling high levels of salt. You know, salty water. Now, metamorphosis, is tricky. You need some, major change for it to happen. You know what time it is? Another destruction!

Ok, maybe not. But you need the young ones to hide, shed some skin that later on turns into full metamorphosis. Basically, young ones and adults will change and differ more and more in each generation. Now, if they didn't raised the little ones, all cool. But if parents cared a lot about little ones, we need something that makes it possible for parents to recognize the little ones. Smell? Sound? Patterns on skin? Pick at least one.

Next, the species needs to adapt two, relatively distinct forms of environment. Why? Because... Water is dropping a bit. Water is all good, but kinda dangerous now. We lost most of our bones and now we are more like sharks or manta rays.

Shedding skin, using a breathing like dolphins, are all cool. But we need to go back to land. Why? Ehhh... A disease? Lets say the sun in these days was paritcularly lethal, and a common mutation was hardening skin, limiting the movement on water. But now what is cool? Moving on land!

So, go back to land again. With a few groups relatively similar mutation, this skin mutation is now permanent. But, sea one and land one are different. Now enough for metamorphosis maybe, so lets evolve more. Differ a lot more. Sea is more dangerous, and all the little ones growing exoskeleton,... (lets change the disease to accumulating calcium on certain muscles) When little ones regrow better bones, they rush to land. Safer, and they are beter adapted now.

Close enough? Well... still no cacoon... And no liquifying. But little ones now live in water, hiding and feeding, growing, and change juust enough to go on lands. With a bit, distorted or unique limbs depending on which muscles got hardened. Add a bit suspension of disbelief, and this may... work.

With more millions passed, now the "muscle hardening" became a relatively stable form. the little ones also start to heal a bit faster, due to the sea and way weirder, way dangerous ecosystem.

There should be an evolution on the breathing system going on its own as well. Millions, perhaps billions now, living in water, slowly turned the lungs to a more forgiving for water leakage. Granted higher capacity and if you shed skin enough, with thinner skin, you can exchange oxygen from water. You need to have somethings constant in metamorphosis. Caterpillars have their wings and most of the nerve system of a butterfly already inside them. But a metamorphosis, with cacoon, requires a dormant period. Maybe the life cycle of thes alienc creatures is long, and ocean now goes in cycles of fertile and famine. So creatures, especially the young ones, have metabolisms adapting to this. With cycles became fiercer, the metabolism adapts. And the skin shedding cycles start to overlap after the fertile parts of the cycles. With these, now little ones shed massive parts of skin while growing and changing. With juuust a bit luck, the muscle hardening part also starts in this time. And enough dormant periond, now stars to look like a cacoon. Well, still no liquify but, this is close enough I guess.

Here, an apocalypse and multiple massive chances on planet's ecosystems in span of perhaps billions of years. This should... work. At least can help you form a better progress.