I have wondered what a terrestrial ecosystem would look like dominated by cnidarians and ctenophores rather than insects and tetrapods. In order to fulfill the same niches I imagine they would need to be highly derived compared to their ancestral forms. Since their physiology is so different I am having difficulty figuring the path from, say, a jellyfish to a longlimbed Savannah grazer with a mass of tentacles for a face.


Well you need them to evolve some kind of hard tissue to support the body, which could give you animals superficially like echinoderms. specifically sea cucumbers or sand dollars and sea urchins depending on which way you want to point the mouth. But it really depends on how long they have had, the longer they have been evolving for land the less they will look like jellyfish.

  • $\begingroup$ Remember that cnidarians come in two forms: medusas and polyps. Medusas (jellyfish) probably wouldn't be able to support themselves like you said, but a land anenome or coral might be feasible. $\endgroup$ – FirstLastname Dec 2 '16 at 16:32
  • $\begingroup$ sure if you keep them really small but I assumed the OP wants megafauna. $\endgroup$ – John Dec 2 '16 at 22:25

If they can somehow separate the hydrogen or the helium from their food or enviroment to keep it in special bladders they could gain buoyancy and float out of the water (where they breed) into the air over dry land. If then they can detect a prey right under them and quickly release gas to drop straight down, they can substitute birds of prey in a ecosystem. Sea animals extract oxygen from the water with gills and such, maybe they can keep the H after taking the O2 (I don't know a lot about underwater respiration so ask someone who does before taking the idea into serious account)

  • 1
    $\begingroup$ Would it even be feasible for an animal to extract hydrogen/helium? We get most of our helium from natural gas deposits, while most of our hydrogen is paired up in water, which would require electrolysis or some other process to obtain. Might make for an interesting question on its own, assuming it hasn't been asked already. $\endgroup$ – FirstLastname Dec 2 '16 at 16:22
  • $\begingroup$ Electrolysis might work fine, it is possible for animals to produce significant electrical currents (eg electric eel/ray).\ $\endgroup$ – cometaryorbit Dec 4 '16 at 2:04
  • 1
    $\begingroup$ For the record: the oxygen gill-breathers collect is dissolved oxygen, it is "sifted" from the water, not chemically extracted. That said, earth plants do use hydrolysis - the free oxygen they produce is derived from water, not carbon dioxide; the "leftover" hydrogen is added to the carbon dioxide to produce sugar. It wouldn't be too much of a stretch for a jellyfish to develop a crude form of photosynthesis to fill their gas bladders. $\endgroup$ – No Name Sep 17 '17 at 16:49
  • $\begingroup$ Methane and ammonia are also lifting gasses, though not as powerful as hydrogen and helium, in Earth's atmosphere. And both of those are commonly produced by metabolic processes. $\endgroup$ – Logan R. Kearsley Jul 2 '18 at 18:08


Developing an extensive muscle system throughout their bodies, including their tentacles, could give them mobility--instead of supporting themselves with an endoskeleton or exoskeleton, they could go entirely without skeletons and simply use muscles to hold themselves up.


Perhaps instead of using skeletons or muscles, they could develop hydraulic systems to harden and soften key points in their bodies to allow for locomotion, holding, eating, etc. Perhaps some species could have very minimal, slow moving hydraulics, like plants use to turn towards sunlight, or perhaps some could be as fast as any land animal.

Thicker Outer Skin

To prevent drying out and keep them safe, they could develop a denser, dryer outer layer of tissue, like leather.

  • $\begingroup$ Have you read Life on Snaiad? The snaiadi, descended from sea cucumber/plant-like ancestors, have hydraulic muscles and fibrous bones. $\endgroup$ – Anonymous Dec 20 '16 at 14:20
  • $\begingroup$ No, I haven't. Sounds viable though. :) $\endgroup$ – Thom Blair III Dec 20 '16 at 14:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.