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In an alternate Earth, coral has been extinct for over 400 million years. In their place were the following:

  • Bivalvia (clams, oysters, mussels)
  • Cirripedia (barnacles)
  • Canalipalpata (bristle-foot or fan-head worms)

Together, they make up a new kind of reefbuilding habitat called a "worm forest". In the 400 million years of the first worm forests being established, the worms took on many forms as they colonized every aquatic habitat--fresh or salt, warm or cold, shallow or deep.

But in regards to one group of chordates, this poses a problem. In shallow seawater, the density of a worm forest averages around 53 worms per acre. To specify the perspective, certain species of worms grow to seven feet tall and measure four centimeters in tubular diameter, while others grow to 260 feet tall and four inches wide.

Most pinnipeds--seals, sea lions and walruses--have so much blubber in their bodies that they're basically rounded in shape and heavy in weight--in other words, fat. Which is no problem in a coral reef (corals don't grow that tall) or in a kelp forest (being made of softer green or brown algae), but what about in a forest of calcium-skinned worms? With 53 individual worms per acre of seawater, can any species of pinniped swim through this kind of habitat?

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  • $\begingroup$ I think I'll answer this in an hour or so, when I get my PC. +1 for pinnipeds. $\endgroup$ – SealBoi Jun 10 '18 at 8:40
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Well, as Andon said, you might want to decrease the spacing of those worms, because your pinnipeds will have almost 10 metres of space between each one. Unless your worms have branching structures.

Barnacles do not have branches, and they would have no reason to do so. Corals have branches because they are colonial organisms that multiply, so they need to expand the limestone core outwards to accommodate more polyps. Trees branch because they are photosynthesizing plants, and that's how plants grow.

The only thing reminiscent of branches that i could see barnacles evolving are external cirri, or feeding legs, but these are made of soft tissue so would not harm the pinnipeds.

The bivalve body plan is also an extremely successful structure in nature, so unless they went a radical change in ecology I doubt that it would change much. Normally, bivalves get nutrition by collecting detritus with their palp proboscidae. Even if they took on the niche of corals, which get energy from bacteria on the polyps that convert sugar into such, they would still not need any kind of structure to fill the 10 metres squared of space between it and the next worm.

Fan worms do have external space-filling structures, but again these are made of soft tissue. So, ultimately, it doesn't look like any of the calcareous animals in your reefs would be able to harm an animal unless you increase the density of them per acre.


Another thing - even if these worms were densely packed, never underestimate the agility of a pinniped.

Otariids are not rotund and have minimal blubber, because they mainly live in warm climates. They are shaped like torpedoes, with large flippers for maneuvres. They can turn on a dime and flip through the air, so they'd be absolutely fine in an underwater thicket.

The Phocids, blubbery though they may seem, are also shockingly agile. They have extremely flexible spines and can perform almost all the underwater maneuvres of an otariids.

The least agile of your pinnipeds are going to be Odobenids, which are much more cumbersome. However, walrus hide can withstand stabs and slashes from ivory tusks, and absorb the full force of a competing bull. In fact, the blubber of these animals is going to help them, not hinder them, in this environment.'


A final word: I think that your date is a bit too extreme. If calcareous reefs evolved 400 million years ago, that could change the course of evolutionary history. Paleozoic fish would evolve to cope with it, the same fish that are going to crawl out of water and become the first tetrapods. Therefore; nearly every chordate species on Earth will have evolved differently.

If early fish evolved to cope with this environment, they might have evolved hard chitinous plating, like some Devonian fish which did that for protection. But an entirely different ocean ecosystem could cause all fish to adapt, so nearly all fish may have hard coverings.

And one of the things that makes amphibians amphibians is their soft skin that requires to be wet frequently, which would not evolve because of the fish' hard skin, meaning that there would never be any amphibians.

And if there were no amphibians, there would be no reptiles. Without reptiles, there would be no mammals or birds. So, I suggest that you wipe out corals far further into the future, at least as recent as the K-Pg extinction. If you can find some natural cause of bleaching that would occur then, there would be a global coral extinction event.

65 million years - especially after a mass extinction - is plenty of time for your annelids, molluscs and arthropods to fill the niche and diversify. Typically, after extinction events, life recovers very quickly, and the survivors "explode" into all available niches.


Thanks for asking, hope the answer helps.

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  • $\begingroup$ "However, walrus hide can withstand stabs and slashes from ivory tusks, and absorb the full force of a competing bull. In fact, the blubber of these animals is going to help them, not hinder them, in this environment." Okay. So how will the worms make sure that they themselves don't get destroyed by a passing walrus? $\endgroup$ – JohnWDailey Jun 10 '18 at 13:01
  • $\begingroup$ @JohnWDailey I think that if they developed stronger calcium structures to protect them, but in a smooth shape, it would benefit both animals. Since there is no competition between walruses and sessile invertebrates, this could occur. $\endgroup$ – SealBoi Jun 10 '18 at 13:15
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According to wikipedia, "Pinnipeds split from other caniforms 50 million years ago (mya) during the Eocene." If "coral has been extinct for over 400 million years", then their replacement would have already been there when pinnipeds went back to the ocean. If their surroundings had a lot of these calcium obstacles then they would have evolved a type of skin that wouldn't be cut up by them, like the armoured skin of armadillos, for example.

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    $\begingroup$ Oh my god. Armoured seals! +1 $\endgroup$ – Joe Bloggs Jun 10 '18 at 11:06
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    $\begingroup$ To be honest, I don't think the seals would even need armour. They're maneuvrable and agile as they are, and the OP's specified density of worms per acre is very sparse. $\endgroup$ – SealBoi Jun 10 '18 at 12:32
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If we assume the OP slipped a digit when describing the worm forest (as noted, 53 worms/acre provides so much space between individuals that virtually anything could swim through it) and it is actually a densely packed conglomeration of individuals and multiple species filling all the niches, then pinnipeds will have evolved differently when they returned to the sea.

enter image description here

Visualizing the forest like this

Given the spacing is supposed to be very tight, and brushing against worms is discouraged (perhaps the worms secrete a poison to protect them from predation), then the hydrodynamic shape of pinnipeds will have to be substituted for something more flexible to move through the worm forest. Pinnipeds would become preferentially long and flexible to weave through the forest, and likely evolve a sinuous rippling motion to propel them through the water rathe than horizontal movement of the flukes. Indeed, they may actually never evolve flukes for that reason.

The new version of the pinnipeds will then resemble sea snakes or eels rather than what we see today, a long, thin body with a very flexible spine to support the sinuous rippling motion. Rather than flukes, they may evolve long dorsal and ventral fins to make their motion more efficient, and evolution may even redact their limbs, much like whales no longer have any external hind limbs and only a residual structure for the hips. The forelimbs will also become much more streamlined to lie flat against the body when swimming, to eliminate any projections or protrusions that might get snagged in the forest.

enter image description here

Modern Sea Snake

Interestingly, in an example of convergent evolution, the ancient Ichthyosaurs also underwent a similar process on their way to becoming highly specialized oceanic hunters. An early Ichthyosaur, Cartorhynchus, resembles the hypothetical Pinniped evolved for your conditions.

enter image description here

Cartorhynchus

A slightly different approach of convergent evolution was the ancient marine crocodiles. They too evolved to become more hydrodynamic (some species replaced limbs with flippers and became totally aquatic), and if worms are somehow poisonous, the armoured skin of a marine crocodile provides protection as well.

enter image description here

Prehistoric marine crocodile

Creatures have evolved to meet many strange conditions, so use evolution as your guide.

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  • $\begingroup$ Are you sure pinnipeds aren't flexible enough as they are? Having visited many aquariums in my pursuit of seals, I've been many times amazed by their acrobatic abilities. I think that the OP should be more worried about whales and dolphins in this environment rather than seals. $\endgroup$ – SealBoi Jun 10 '18 at 13:32
  • $\begingroup$ In his treatise [In the Wake of the Great Sea Serpent])amazon.com/Wake-Sea-Serpents-Bernard-Heuvelmans/dp/0809058146) Heuvelmans concluded that the most common type of sea serpent was in fact a pinniped. $\endgroup$ – Willk Jun 10 '18 at 13:47
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An acre is, with rounding, a square with a side length of just under 210 feet.

With 53 worms per acre, each worm has about 820 ft/sq - Or, a square with a side length of just under 29 feet.

Dispersed evenly, your worms will be ~30ft apart. Plenty of room for pretty much anything not whale-sized to fit through.

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Before addressing your question:

1: In a world without corals, the competitors of coral would take over the niche of corals. You propose just this but besides bivalved mollusks and worms there are others: sponges, tunicates, bryozoans etc. Before there were corals, stromatolites built reefs. The reef phenotype is an adaption to a certain type of habitat. With no corals, something else will assume that phenotype.

2: 53 worms per acre may be true for one kind of worm. There are many others. If corals disappeared this minute worms could do a lot of what corals do now. Here is a reef made by the sandcastle worm.

https://www.alamy.com/stock-photo-intertidal-sand-reef-made-by-the-sandcastle-worm-playa-guiones-beach-54746713.html enter image description here

In deep water, worms can grow large as you state. In rough, shallow water favored by corals and seals, that is trickier for them or anything else, and so these things converge on the coral body plan and lifestyle.

But this is a Dailey question: the mandate is towering worm forests and fat seals. Answer: seals would do fine.

1: Seals do fine in forests of kelp or gorgonians.

2: If the worms were so close that the seals did not fit, the seals would be smaller. Still fat, but smaller. Otters and porpoises are small marine mammals so a smaller body plan is fine for mammals and presumably pinnipeds.

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