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Recently I had a vision of a colossal underwater bird-like figure.

I would describe it as follows:

  • Roughly the wingspan of the length of a blue whale
  • Snow-white
  • Typical shape and feathering of a bird of prey (I may have mistaken something else for feathers)
  • Gliding into the depth, probably in the mesopelagic/twilight zone

Since such a creature couldn't possibly be an actual bird, unlike the aqua-bird, what is my giant mid-sea bird?

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    $\begingroup$ The wingspan of a blue whale... in the length or in the breadth of the whale? $\endgroup$ – L.Dutch Mar 28 at 12:50
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    $\begingroup$ @L.Dutch The wingspan of the creature is roughly the length of a blue whale. $\endgroup$ – A Lambent Eye Mar 28 at 13:06
  • $\begingroup$ @cobaltduck The light may have tricked my eye into thinking they were feathers... $\endgroup$ – A Lambent Eye Mar 28 at 13:07
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    $\begingroup$ "During the Late Eocene and the Early Oligocene (40–30 mya), some lineages of gigantic penguins existed. Nordenskjoeld's giant penguin (Anthropornis nordenskjoeldi) was the tallest, growing nearly 1.80 meters (5.9 feet) tall. The New Zealand giant penguin (Pachydyptes ponderosus) was probably the heaviest, weighing 80 kg or more. Both were found on New Zealand, the former also in the Antarctic farther eastwards." (Wikipedia, s.v. Pinguin) $\endgroup$ – AlexP Mar 28 at 20:02
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Let's start with something in the Real World (TM), then try to see whether we can manipulate its future evolution into your creature. The thing I have in mind is the humble gannet, birds of the Morus genus. As seen in this article at Media Drum World, when these birds feed, they spend quite a bit of time swimming to a bit of depth, and do so quite adeptly.

enter image description here

We begin with two-out-of-four of your features already in place: Snow-white, and typical shape and feathering of a bird of prey. We just need to figure out how to grow it much, much larger, and get it to abandon its life in the skies and greatly increase its diving depth. How? The usual evolutionary pressures: eat, don't get eaten, make more of your kind (i.e. sex).

As the fish dive deeper to escape, the mega-gannet must follow. But now the sharks, which already compete with the gannet at the bait ball, have a better chance to pick them off along side the fish. Time to get larger, too large for a shark to swallow. This will eventually make it too large to fly. Alongside this, the mega-gannet will probably become swifter at swimming. I'm uncertain whether the mega-gannet might develop a blubber layer, or if feathers can adapt to provide cold-water protection (penguins, for example, have both). Finally, those mega-gannet which are most successful at eating and not getting eaten will also be more successful at breeding, and these traits will augment in each generation.

Given the right circumstances and a few million years, anything can happen.

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    $\begingroup$ change in bone density and muscular structure. The change in medium the bird moves through would require different physiological changes. Penguins and chickens have a higher bone density, so its not unheard of for that adaptation to occur in birds. $\endgroup$ – Sonvar Mar 28 at 23:32
  • $\begingroup$ Interestingly, we might see these designs lay eggs on land akin to sea turtles $\endgroup$ – TrEs-2b Apr 5 at 21:33
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It's for sure the notorious Pinguinus Humongous.

It's a descendant from penguins, which, instead of feeding on small fishes, decided to go big and hunt for dolphins and other large sea mammals.

Its size is necessary for hunting those preys, and the feathers come from its ancestors being birds adapted to the sea environment.

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    $\begingroup$ Pinguinus, not Penguinus. And they are auks, not penguins. Otherwise it's fine. If you want penguins, that would likely be a descendant of Anthropornis, possibly Dinanthropornis colossicus. $\endgroup$ – AlexP Mar 28 at 20:06
  • $\begingroup$ Penguins don't tend to have a large wingspan, nor are the feathers particulary visible, or am I mistaken? $\endgroup$ – A Lambent Eye Mar 29 at 7:37
  • $\begingroup$ @ALambentEye, if you want to be streamlined underwater you cannot afford fluffy feathers and the resulting drag, especially if you rely on velocity to chase your meal $\endgroup$ – L.Dutch Mar 29 at 7:46
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Except for the feathers, your creature is rather like a manta ray: https://en.wikipedia.org/wiki/Manta_ray

So convergent evolution is your friend here. Just as the demands of hydrodynamics cause sharks, tuna, dolphins, and ichthyosaurs to all look much the same to a casual eye, your mesopelagic bird is descended from penguins that evolved into occupying the same environmental niche as manta.

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Giant nudibranch.

nudibranch source

These are ocean animals. They swim slowly along as I imagine your creature might. They can have a vaguely avian outline as seen here.

Known nudibranchs of course do not get to the size you want, but maybe they could. The molluscan body plan can scale up. Squids get big.

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  • $\begingroup$ What an intersting creature! Why might it change colour and become larger, or what would cause it to do so? $\endgroup$ – A Lambent Eye Mar 29 at 7:33
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    $\begingroup$ 1. Color - you could assert that default color for a mollusk is silvery white. Your creature is not trying to camouflage and it is not signaling to conspecifics with color so it is the default slug color 2. Something this big is probably a filter feeder like the whales and largest sharks. Size is an advantage for filter feeding and probably the bigger the better because you can filter more. An ancestor got into the filter feeding business and evolution scaled it up with time. $\endgroup$ – Willk Mar 29 at 11:21
  • $\begingroup$ As regard the scaling up - the typical Blue Sea Angel is as long as a human finger is wide. From less than an inch (~25mm) to nearly a thousand inches (~80 feet, ~25m) is a lot of scaling. Not saying it's impossible, just making it clear the magnitude in question. (+1 by the way) $\endgroup$ – cobaltduck Mar 29 at 20:11
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It's a giant ray.

Roughly the wingspan of the length of a blue whale

Rays can be big. The giant oceanic manta ray has a 7 metre wingspan, and with sufficient food availability they could evolve to be very big. Perhaps they could crystallize their cartilage skeletons with some harder mineral like calcium phosphate, calcium carbonate, or chitin, for better support.

Snow-white

You could have seen an albino/leucistic individual. Or maybe it's so big that it fears no predators, and thus needs no camouflaging pigmentation, remaining plain white. If it spends much of its time in the deep, it wouldn't have much use for coloration anyway.

Typical shape and feathering of a bird of prey

What with their huge wings, rays certainly can have a wholly avian profile from above. As for the feathers, perhaps take inspiration from the yeti crab?

enter image description here

The ray's dermal denticles are long and filamentous, and amongst these filaments it grows bacterial cultures which it then eats, perhaps by means of some long fin-sweeping proboscis.

Gliding into the depth, probably in the mesopelagic/twilight zone

The depth doesn't really matter, most large whales change zone every now and then, with some of them diving well below the twilight zone despite living in the sunny layers.

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