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So, in my last question, I asked "How could an animal run fast using blade-like feet that sink into spongy ground?"

The premise was that on my planet, forests grow not from soil but from spongy, living mats. The creatures here have feet like blades, that sink into the squishy ground. However, while I did eventually find a solution that would make a blade-footed animal superior to a normal-footed one in terms of speed, a lot of people were saying that traditional, spread-out feet that rest on top the ground would be superior to bladed feet in such an environment. What environment would suit blade feet instead?

enter image description here

Preferrably, the terrain must be:

  • Biological
  • In a state that the blades sink into it
  • At least semi-regenerative (So the ground isn't permanently chopped up by footprints)

It could be:

  1. Moss
  2. Fungus
  3. Soft sponge (As in Porifera)

Or something else like that. One idea I had was that the organism that constitutes the ground has a soft, spongy exterior about 2-3 feet deep, and below it, there's a hard core. The blades of the animals would cut through the sponge and touch the hard bit, preventing them from slicing through the ground like butter and having trouble pulling their legs back out again, as this horrendously rough drawing shows:

enter image description here

(Don't worry, I'm better at drawing in real life.)

The evolution of blade-like feet would also bring a host of other advantages and uses to the creatures. They could be used for slashing prey, spearing fish, sensing vibrations, climbing and marking territory on tree (Alien tree analogue, that is.) trunks.

So, could any of you explain why bladed feet would evolve?

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    $\begingroup$ Perhaps you could consider something shaped like like prosthetic blades ? These don't necessarily require special ground to use on and might be better suited to a biological origin than pointy sink in the ground blades. $\endgroup$ – StephenG May 12 '18 at 11:42
  • $\begingroup$ I'm sure there are examples in nature, I can't google anything right now. Go for an additional and more practical mode of transportation, e.g. wings. Have your animals land on other animals (or whatever organism) with a bit of momentum so blade-style feet make sense. Or just give them claws. Cats can retract their sharp claws if needed, for example a velociraptor has a toe designated to basically a blade. All those things are designed to "walk" (more general be used) on animals, so perhaps consider that route (even though I'm sure e.g. koalas use them against trees) $\endgroup$ – Raditz_35 May 12 '18 at 12:57
  • $\begingroup$ 3-D attack? Basically your creature runs on walls and even on the ceiling as it was on a flat floor by sinking its feet. Like a spider through its web but it doesn't need to create the web. Jungles are very irregular: animals there are small, they crawl or they run through the trees. Being big and on the floor is tiring if you are encountering obstacle after obstacle. A jumping creature that doesn't care about running on the floor would have advantage in pursuits. $\endgroup$ – Alberto Yagos May 12 '18 at 13:12
  • $\begingroup$ I seriously doubt that there is any environment that would favor blade-like feet that are used for locomotion. When we see blade-like appendages - claws - on feet, they are generally used as weapons, and can be retractile for travel, as with cats. Perhaps the closest the current world comes to a blade-like foot is the cassowary: blogs.scientificamerican.com/tetrapod-zoology/… $\endgroup$ – jamesqf May 12 '18 at 18:57
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Not All 4 Need to be Blades

A body morphology like the one used for one of the invasive alien species in the half life spin-off Opposing Force would work.

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Having all 4 feet be blades is really really bad. It limits the creatures to a single fragile biome and is very very calorie wasteful as a form of locomotion. Ever notice how a cat will walk around something stupid instead of stepping over it (like a pencil, or towel,etc etc). What it is doing in conserving calories a fraction at a time. Because that's how important calories are in nature!!! The reason why animals with horror movie style adaptations don't exist is because the calories to make a gaping maw full of razor barbed tentacles or a natural nerve-gas producing organ are so ludicrously calorie expensive to maintain that the animal could never catch enough prey or eat enough plants to sustain something like that. Having to wrench all 4 blades out of soppy spongy material would be wasting hundreds of calories an hour, which is like lighting $100 bills on fire hourly as far as natural selection is concerned, its just unsustainable.

Its called evolutionary over-specialization. Animals with ridiculous traits like the saber toothed tiger, gargantuan sized cave bear, or reliance on a single biome or food type like the panda go extinct very easily. Creatures that are versatile and adaptable with a wide array of options for prey, environment, and climate will survive whilst ones that specify too distinctly will go extinct or become a evolutionary dead end.

This creature can dig the front two in for a sudden burst of speed kind of like a catapult, but it can switch over to caloric economy mode and just run on two feet when it doesn't need to be attacking or chasing something. It has a very powerful adaptation for its environment but it isn't ludicrous and impossible to sustain.

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  • $\begingroup$ The sponge/fungal mats cover virtually all vegetated land on the planet. $\endgroup$ – SealBoi May 12 '18 at 13:49
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    $\begingroup$ If you really want to go the star-wars single biome planet route I'm not gonna stop you, but truly good world-builders are a bit more creative and thoughtful than that. I'l give you this though, at least you're a step above "generic lava planet, generic desert planet, generic ice planet." I really like your idea though, it kind of reminds me of the alien "world" xen from half life. $\endgroup$ – TCAT117 May 12 '18 at 13:53
  • $\begingroup$ @SealBoi while I respect what TCAT117 is saying, it doesn't make a bad worldbuilder to have a single-biome planet. It's an often necessary detail due to the Law of Conservation of Detail. In stories where the story takes place on multiple worlds, single biome planets allows each planet to have it's own individual feel. The only real reason for having a realistically biomed planet is either if you are writing a story that takes place only on a single planet, or if you are writing Hard Sci Fi $\endgroup$ – Sydney Sleeper May 12 '18 at 13:58
  • $\begingroup$ Are you calling Earth a single-biome planet for having all of its plants growing out of soil? $\endgroup$ – SealBoi May 12 '18 at 14:00
  • $\begingroup$ If I'm coming off as the reality checker too hard I must preface that I have spent the last 5 days on an atomic rocket's website binge. I think youre Idea is pretty neat SealBoi. $\endgroup$ – TCAT117 May 12 '18 at 14:01
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After looking at the answers to your last question, I do agree that bladed feet would be less than optimal for a pursuit-type predator. Simply put, blades would require a lot of time and energy to extract after sinking into the spongy ground. However, who says this creature has to be a pursuit-type predator?

What if mole-like creatures travel underneath the biological floor, either eating the sponge or using it to endure sub-optimal conditions (heat-wave, for example). These blade-foots are designed to seek out the moles, spear them with their limbs, and then extract them and eat them for nourishment.

Now you have a good reason for the blades and you could use their lack of speed as justification for some interesting behavior. Without the ability to escape their own predators, blade-foots instead travel in packs for protection and lash out at any creature that gets close, like a nightmarish combination of preying mantis and cassowary. From a human standpoint, they would be an incredibly dangerous and aggressive species.

If this seems interesting to you, I would consider marking the front blades substantially longer that the blades on the back legs, since they would be the ones best at striking targets to eat. Barbs or some kind of serration would help at pulling the impaled prey to the surface as well.

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  • $\begingroup$ Yes, this would work for an individual species or a few, but the problem is that I was planning for the vast majority of the planet's megafauna to have blade feet. Perhaps they evolved from creatures like the one you spoke of and retained the blades? Also, you might want to check out the 'inverted opposable dew claw' answer. Allegedly it would be superior , speed wise, to duck feet. $\endgroup$ – SealBoi May 12 '18 at 13:14
  • $\begingroup$ Ah, I see. Well, a majority of Earth creatures have five digits cause of evolution, so there is some precedent for that solution. The dew claw idea is a great solution and could be used for an actual pursuit predator. I have no clue why herbivores would have these blades, however, or creatures that specialize in climbing. Still, if you can get this motif to work, it would be interesting. $\endgroup$ – Pinion Minion May 12 '18 at 13:39
  • $\begingroup$ In herbivores, blades can be used for slashing through dense vegetation, sensing vibrations in the ground, uprooting trees or tapping into their sap, marking territory, fighting with eachother, or climbing. $\endgroup$ – SealBoi May 12 '18 at 13:47
  • $\begingroup$ That all seems plausible enough. Hey, another way you could justify the blades is making the organic floor mildly caustic. Standing on it for too long might peel away at skin as the organic carpet eats away at the animal's feel. So enamel-coated spikes became a valid alternative to simple feet. $\endgroup$ – Pinion Minion May 12 '18 at 14:03

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