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This is part of the same world I am making that this question I once asked is set in: How to evolve a reptile into a wyvern, 100 million years in the future?

I might call these questions related to this world the "100 million years into the future" questions.

In that same futuristic Earth I imagine sharks that walk (or at least crawl) on land with some fully disconnected from sea life, that can breathe air directly and thus either dig underground or climb up in trees. Most of their species have evolved from today's Epaulette shark which is known to walk on the sea floor in reefs and occasionally comes out of water to crawl back into water.

These "land sharks" however are not your good old Shark Week Sharknado style nightmare that terrorizes everything, instead I imagine these sharks as rather small and "peaceful" ones. These sharks would mostly be herbivore and insectivore and they are preyed upon by terrestrial predators of that time like the wyverns from my other question.

What would drive the Epaulette shark to evolve into descendants that can breathe in the air and evolve into creatures that would never see the ocean again and would these sharks be able to consume vegetation or insects as their main source of food on land?

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    $\begingroup$ An obvious answer would be for prey to become scarce or move to the land. $\endgroup$ – Starpilot Aug 13 '18 at 9:16
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Epaulette sharks are known for their terrestrial habits, which can "crawl" across the land like this:

enter image description here

Much like an ancient Tetrapodomorph, they crawl with their pectoral and pelvic fins. They can survive for impressive periods in environments with little to no dissolved oxygen, such as the land.

Now we know it's possible, we should address how fully terrestrial sharks would evolve.

Why they would do it:

Our friend the epaulette lives in very shallow reefs, which are exposed to the air when the tide goes out. Where most other competitors would retreat to deeper waters, it stays, crawling across the land to feed where other predators can't go, and its prey stands little chance of escape in such enclosed tide pools.

If, in the future, a shark like this would live in an environment of many pools with areas of land connecting them, it might crawl between pools in search of prey.

Another idea is that it might become a water-to-ground predator, like a crocodile. If a potential, unexploited prey source lived on beaches, it could start out by beaching itself like a killer whale, but then evolve to be able to stay on the land a bit longer. After millions of years, the terrestrial habit becomes more and more refined, until they are fully able to go on land.

How they would breathe:

Bony fish have an organ called a swim bladder, which is an offshoot of the digestive system. It's believed that when fish colonized the land and became tetrapods, their swim bladders evolved into lungs.

Sharks lack a swim bladder to rig up, so we're gonna need some other method of air-breathing. Some arthropods have something called a book lung.

The way a book lung works is that oxygen passes through a slit, and then proceeds through a multitude of folds. Blood circulates through the lung, exchanging carbon dioxide for fresh oxygen.

Now, this is where it gets really speculative. Sharks have an organ in their gut called a spiral valve, which looks like this:

enter image description here

If they were somehow to bifurcate this spiral valve, keeping one as part of the digestive system, while another would form some sort of book lung equivalent, then they might be able to breathe air, through holes that are actually modified gills. Again, remember that this is the kind of wacky speculation that I enjoy, but others may frown upon.

What they would look like:

When we look at the epaulette shark and how it moves, it's clear that they're using their pectoral and pelvic fins to walk. So did the first tetrapodomorphs, so it's probable that a terrestrial shark would have four legs.

If an epaulette-like shark was to colonize the land, I predict that:

  • It would have four limbs with digits
  • Its dorsal fins would be lost
  • Its tail would atrophy

I think it would have digits because, looking at a shark skeleton, they have rays of cartilage in their fins which would become useful for walking. They wouldn't need their dorsal fins (Unless they were display organs) because they wouldn't swim much, and they don't need such a long, powerful tail on land.

One thing that would have to happen is ossification. Obviously, shark skeletons are made of cartilage, and cartilage might not be suitable for supporting a land animal of the shark's size. So, I'd be surprised if terrestrial sharks didn't ossify their cartilaginous supports and gain a full-fledged calcium phosphate skeleton.

How they would match your description:

In this scenario, I spoke of the sharks coming on to the land as predators, but a descent from carnivory to omnivory to herbivory is certainly possible. After all, plants carpet the land far more than the water, so it's an abundant food source for the shark to exploit.

As for insectivory, that's also possible. Depending on how they catch the insects, they might develop a good sense of smell and sharp claws. As for size, epaulettes are pretty small, about the size of a dog.


Anyway, those are my thoughts on how terrestrial sharks would evolve. I hope I answered to your satisfaction.

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  • $\begingroup$ Just a minor remark: land has a lot of oxygen (we use it), it's just not usable by the shark breathing system $\endgroup$ – L.Dutch Aug 13 '18 at 12:02
  • $\begingroup$ @L.Dutch True, but there is no dissolved oxygen, as that only occurs in water. $\endgroup$ – SealBoi Aug 13 '18 at 12:41
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With evolution you can nearly always get from A to B given enough time.

100 million years is enough time. The main problem is that the land is already covered in tetrapods that are very well adapted to terrestrial life. There aren't many niches for sharks to fill that can't be more quickly taken by tetrapods.

So the first thing to do is to wipe out land life. Perhaps something involving a gamma-ray burst would do it.

With us gone you've got some space for a shark to come out of the sea and live on land. In this scenario with large terrestrial life gone there an explosion of arthropod life, this provides a food source. Where there is food, life will follow.

Initially this might take the form of sharks beaching themselves to grab something to eat then getting back to the water. Sharks that can maneuver on land will be at an advantage, as can any ability to remain in air for longer.

Pretty soon, the sharks will stop looking much like sharks, and evolve into a range of animals as diverse as those of the Permian. That can include herbivores, carnivores. Small and peaceful as well as large and aggressive. Why not also some flying animals? All that is certain is that they won't look much like sharks anymore, but may retain things like a cartilaginous skeleton.

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  • $\begingroup$ I can see the descendants of sharks and cephalopods battling to control the surface! $\endgroup$ – GrandmasterB Aug 14 '18 at 4:43

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