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The plants are organisms with soft cells that don't have walls. Their flesh is soft, with a gelatinous-seeming texture (rather than the fibre-based texture common to vegetables and herbs). Most terrestrial plants have a coat of hard chitinous scales that do not overlap, and an endoskeleton made up of a stiffened cartilaginous tissue, with the softer nutritious tissues in between. There are no soft-bodied plants on land, as the soft flesh can't stand on its own. The scales often have sharp hairs or other such defences between the scales when herbivores are common. Aquatic plants generally lack the endoskeleton, and have sturdier, less flexible flesh to make up for it. The plants' anatomy is composed of straight stems and flat leaves, both of which have all three of the tissues mentioned. They also vary in size from the scale of a small flower (around 10cm high) up to tree-like sizes (around 40m high), with the stiffness and thickness of the cartilage increasing with size. The scales similarly vary in strength from shell-like to almost not present, depending usually on the soil nutrients and amount of predators. The scales also come in a variety of sizes, grain-like to resembling an exoskeleton, based on similar criteria to the strength. The herbivores would vary in size roughly in the range of modern mammals, and have a set of limbs and mouthparts

What are some adaptations that would be common amongst the herbivores (specifically the foliovores) for gathering, consuming, and digesting these plants?

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    $\begingroup$ Plants born without cell walls simply die. It's like asking how would lions evolve if they ate zebras born without veins. $\endgroup$
    – user89947
    Sep 9, 2021 at 19:15
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    $\begingroup$ Have we decided on how soft cell land plants would be able to rise from the ground? $\endgroup$
    – Alexander
    Sep 9, 2021 at 22:07
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    $\begingroup$ Yeah? Probably missed that then. An improvement direction for the q then is to compare your nonexisting plant side by side so that difference would be more sticking out. Calfs drink milk, I guess your stuff can't be more mushy mushy or softer than that - rigth? $\endgroup$
    – MolbOrg
    Sep 10, 2021 at 20:09
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    $\begingroup$ How do the aquatic plants keep their leaves properly exposed (flat) for efficient photosynthesis without the endoskeleton? Are these "hard chitinous scales" providing structure in addition to predation resistance? $\endgroup$
    – legio1
    Sep 10, 2021 at 22:05
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    $\begingroup$ Plants do have soft cells! They just also have hard, dead structures of cellulose. And in some cases lignin, glueing the cellulose into ever harder stuff that we normally call "wood". But the actual plant cells, the living cells, are quite squishy! $\endgroup$
    – PcMan
    Sep 12, 2021 at 16:29

4 Answers 4

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A possible answer

NOTE: What follows is a possible solution, not the only solution. Evolution is frighteningly creative but is limited to what it has to work with.

Setting the stage

For simplicity, we’ll go medium sized. Our herbivore is a quadruped mammal about 80cm at the shoulder (if it helps, I’m picturing a large-ish goat). Its targeted food is one of these plants standing about 1.1 m tall (again, if helpful, I’m picturing something classically “tree shaped” just smaller).

On to eating

The first obstacle is the “sharp hairs”. It’s hard to know what these are exactly but I’m going to assume they function as thin thorns like those found on the prickly pear cactus. Many animals eat prickly pear, but most do so by carefully getting between the relatively widely spaced thorns. Based on the description, I get the impression that these hairs are much closer together so that strategy won’t work. Fortunately for us, there are also animals that just munch prickly pear without much care about the thorns. They do this with a few key evolutionary innovations. So, here’s our first set of changes:

  • Toughened lips and other soft mouth tissues to resist the hairs
  • Rotary mastication to push the hairs into falling over
  • Decent set of molars to grind up the hairs so they don’t pose a threat to the guts
  • Pain tolerance because the above will only get you so far
  • OPTIONAL - Prehensile lips and/or tongue to move things around

Next obstacle: hard chitinous scales. These range from very weak and small to “shell-like” and “resembling an exoskeleton”. On the small/weak end this is no problem, some good grinding molars and you’re set. But let’s go for the tougher end. Shell-like suggests a need to physically break or pry apart these scales. Here a comparison to clams and oysters is probably the best approach. In very broad strokes, eating clams and oysters requires tool use (e.g., bashing them on rocks), suction power (pulling them open) or brute jaw strength. Since herbivores aren’t usually bright enough for tool use and our “goat” doesn’t have anything to pull these scales off with, let’s go with jaws. Here I’m thinking we borrow from a decidedly not herbivore: bone-crushing dogs. Next change:

  • Serious bone-crushingly powerful jaws

Our herbivore now has a mouth full of juicy goodness. Now we have a choice: swallow this stuff in all its gooey, crunchy goodness, continue grinding, or find a way to spit out the scales. Given that our herbivore’s mouth may very well contain sizable pieces of broken “shell-like” scales, swallowing now has a lot of risk, so let’s not. (Note that if eating something with softer scales, swallowing is certainly an option.) Either grinding or spitting would work in our case and it’s likely that evolution will produce species that do both.

Option 1: grind it to dust: We already have the hardware for grinding (bone-crushing jaws and some good molars) so this seems the obvious solution. But grinding thorn-like hairs and grinding shell-like scales are very different things. I’m thinking shell grinding takes too much energy and causes too much wear and tear to be a good path.

Option 2: spit it out: This seems the safer option, but it requires some specialized hardware. Our herbivore needs to separate the problematic scales from the juicy goodness. If you’ve ever watched video of a hyena cracking open a bone to get at the marrow (or, if you’ve every tried it yourself), you’ve seen part of the solution. Our next set of adaptations: Dexterous tongue to maneuver the good stuff in and the bad stuff out Mostly closable mouth so suction can be used to gather up more goodness

On to the guts!

But first, a brief aside about the cartilaginous endoskeleton. The tools above will likely make quick work of this endoskeleton if its stiff enough to easily cut. If it’s not stiff enough to grind up, it can probably be swallowed and left to the guts.

Once swallowed, our meal will probably be a little easier to deal with than regular earth plants. The lack of cell walls means a lot less cellulose to deal which will make digestion a bit easier. Any herbivore lucky enough to be munching on one of the softer species of “plant” will also have some extra chiton to deal with and everyone will be dealing with the materials from the endoskeleton. Overall, these changes will be the least impressive. If these plants also include silica phytoliths (like real grasses) or lignin-heavy wood, the same processes used today will apply.

Ok, that was my stab at this. I would advise caution on one point: I’ve almost certainly forgotten or oversimplified something. It also goes without saying that I’m not an expert on herbivory.

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Either the Aggressive Anteater-like eating equipment...

The interior is gooey. There is hard and yucky stuff that you don't want to deal with both on the surface and on the inside of the plant.

So, just ignore all of that and go straight to the juicy bits.

The herbivores in question would probably have a needle-like beak that can pierce the outer shell, with a very long "mouth" that can shove a tentacle-like tongue inside the plant. The gooey bits would then be slurped out, leaving just the empty husk - which would probably be filled soon by the plant, if it manages to survive and heal it out.

Or the Vegan Vampire solution.

For teeth-enabled herbivores or omnivores, you would probably have a few vampire-like fangs that could pierce and suck out the juicy bits, without having to deal with the rest of the plant nor chewing the uncomfortable bits. For creatures of this type, they would probably use their front claws/paws to remove a bit of the bristly hair before going in for a bite.

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Completely different ones

The "hard walls" most plants have was an evolutionary pressure that herbivores had to adapt to (flat teeth for grinding, ruminant gut bacteria, etc.). What you are proposing is that we remove that evolutionary pressure. In that case, another evolutionary pressures would have dictated the ability to reproduce. There's no way for us to know which pressures would be most important in this case, because the lack of a problem isn't an evolutionary pressure.

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    $\begingroup$ The cell walls of living cells in plants are not all that hard... (The hard parts of plants are wood and other materials, such as the silica phytoliths of grasses. It is those "other materials" which were developed as a response to predation.) (What the cell walls do is that they don't allow the cell to expand. So that when the plants arranges for the cell to be full with water it becomes stiff like a plastic bottle when full. When empty, the plastic bottle is easily squished.) $\endgroup$
    – AlexP
    Sep 9, 2021 at 20:03
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    $\begingroup$ @AlexP learning more from your comments than a lot of answers and questions here $\endgroup$
    – user89947
    Sep 14, 2021 at 15:43
  • $\begingroup$ I've edited the question. Could you explain what specific adaptations would help eat these plants? $\endgroup$ Sep 14, 2021 at 22:30
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Terran herbivores' adaptations to terran cellulose and lignin which surrounds each cell in plants was to evolve a wide variety of mechanical and chemical means of breaking down that cellulose and lignin that makes them very different to carnivores, which must merely ingest the food they obtain, which may or may not involve chopping the food into bite-sized chunks.

In the OP's ecology, the line between sessile autotrophic 'plants' and motile heterotrophic 'animals' is far less clearly defined than is the case on Earth. This means that the line between 'herbivores' and 'carnivores' will be similarly blurred. The difference could well be that of speed, the carnivores more adapted to chasing down and catching motile prey while the herbivores will be more adapted to fleeing and/or self-defense.

However, when it comes to eating these soft, fleshy 'plants' with their armour, hard scales, cartilage and thorns, there are a number of obvious adaptations:

The 'herbivores' dentition will likely be very similar to that of a carnivore, in that they would be adapted to slicing up meaty flesh, cutting through tough skin, and scraping flesh from tough, less easily digestable surfaces. This would tend to make a herbivore much more likely to become an opportunistic carnivore. Where plants make themselves abrasive so as to wear down the teeth of the herbivores that eat them, the herbivores may counter with continually growing open-rooted teeth like a horse's molars, or infinitely replaceable teeth like a shark's.

Where the foliage that the herbivores may eat is covered in fine thorns or penetrating hairs, this may be dealt with in the same manner as giraffes which eat thorny acacia foliage deal with the same problem: an armoured mouth and gullet which can tolerate chewing and swallowing sharp, spiky food.

In order to get past thorns and penetrate armoured skins, there are a number of adaptations that could assist:

'Herbivores' heads and any other limbs which may be used to deal with thorny protective measures could develop tough - potentially armoured - skin of their own. If thorns are a problem, tough, horny, smooth surfaces will allow the herbivore to simply push past them. Eyes will be set well back from the mouth to keep them away from the thorns, or they may be armoured themselves.

Thorns may also be dealt with by breaking them off with a protected limb before using other, more sensitive organs and appendages to crack the 'plant' open.

Where a 'plant' has a particularly tough skin, herbivores may develop very powerful jaws in order to simply bite through into the soft interior, or they may develop hammer-like or military-pick-like limbs to respectively smash or puncture the plants' tough rigid or flexible carapaces prior to inserting their relatively slender feeding apparatus into the soft interior.

Limbs may be strong so that when a hole is punctured through the outer armour, the herbivore may use its strength to tear the plant's body open to provide access to the edible interior. An example of this may be the Palorchestids.

Not mentioned by the OP is the possibility that the plants may make their own flesh toxic. This would discourage opportunistic attacks by carnivores and unspecialised herbivores which may be poisoned, but some herbivores would evolve to be able to neutralize any toxins that the plants they feed upon may create. However, this may increase the metabolic cost of feeding upon these toxic plants, a problem that tends to lead to a large size and a leisurely lifestyle in order to reduce energy consumption per unit mass, large animals having a lower basal metabolic rate to smaller animals.

Many of these adaptations to herbivory in this environment would also lend themselves to carnivory, so true exclusive 'herbivores' and 'carnivores' may not actually exist, but instead each species may lie somewhere along the scale of omnivory between the two extremes.

Also not mentioned by the OP is the possibility of symbiotic relationships between 'plants' and certain 'animals' where the animals protect the plant in exchange for some incentive such as food and/or shelter. Eating such a plant would require dealing with both the plant's innate defences and its symbiotic defenders, though an animal evolved to do that may well simply eat both plant and its o(symbiotic defenders.

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