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Anytime one thinks "flightless bird" and "aquatic bird" put together, one would immediately think of Sphenisciformes, the penguins.

But during the Late Cretaceous, there swam a different kind of flightless aquatic bird, one with a greater distribution than the penguins' cold southern waters. They were the hesperorns, the "western birds".

enter image description here

As you can see, they were lankier and more streamlined than their penguin upstarts, which raises the question.

In an alternate Earth where penguins never existed or have yet to exist, colonies of hesperorns can be found in coastlines all around the world. (Wouldn't nip the existence of the pinnipeds in the bud, but they would be confined to the shadows.) There are hesperorns in Antarctica, a continent buried under a mile of steep, slippery ice. With that in mind, would the Antarctic hesperorns keep their ancestral sleekness, or would they have to shorten their body designs to make them more penguin-like?

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    $\begingroup$ A nice, simple and straightforward concept. An interesting twist on the possibilities of speculative evolution. Never heard of hesperorns before now, glad you helped me make their acquaintance. $\endgroup$ – a4android Jan 12 '17 at 3:32
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Heat loss is proportional to surface area, heat generation is proportional to volume

Penguins are stout because

  • Fat is good insulation

  • The closer you are to a sphere, the less heat you lose to the (cold) enviornment

The bird you picture there looks like a cormorant. Cormorants work just fine, and live in all sorts of primarily tropical and subtropical environments.

However, if these birds (or cormorants) were to adapt to an Antarctic lifestyle, they would need fat layers for insulation and lower their surface are to volume ratio to reduce heat loss.

In short, they would look like penguins.

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  • $\begingroup$ What you are saying is natural selection will favour the survival and adaptation of shorter, fatter hesperorns. The Antarctic hesperorn will develop into something closely resembling the penguins of Antarctica. $\endgroup$ – a4android Jan 12 '17 at 3:08
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    $\begingroup$ It is, but I was filling in the in- between the lines bit about natural selection. Too often folks, this isn't a reflection on yourself, talk about evolution as if it was a deterministic process. When it's more like nature rolling the dice a squillion times and sometimes coming up with a result. Ignoring that, you gave a good clear answer. $\endgroup$ – a4android Jan 12 '17 at 3:29
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    $\begingroup$ @a4android To anybody that is even slightly scientifically literate it is clear that when such expressions are used, they are just that: expressions. When everybody knows what a linguistic shortcut means it is perfectly correct to use it as a substitute for a more technically correct if longer alternative. Anyways... The sun will be rising soon. Oh sorry, I should say the rotation of the earth around its axis will soon progressively cause more of the sun to become visible above the horizon from my specific point of view. $\endgroup$ – AngelPray Jan 12 '17 at 3:58
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    $\begingroup$ @AngelPray In defense of a4android, and as a person who also posts on the more scientifically minded Stack Exchange sites, not everyone on the internet is even slightly scientifically literate. Your sunrise example is amusing, though. $\endgroup$ – kingledion Jan 12 '17 at 4:02
  • $\begingroup$ Given that there is Flat Earth Society and some of them are really serious, sun example is not as ridiculous as it sounds. $\endgroup$ – Mołot Jan 12 '17 at 6:39
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Really expanding on kingledion's answer, evolutionary pressures will lead to what is known as "convergent evolution".

The issue is the environment requires some very specific adaptations to fill the niche of aquatic predator. Since the prey is underwater, a streamlined hydrodynamic shape is required to be fast enough to successfully hunt. Since the waters and surrounding air are cold, the creature needs some means to retain body heat. Penguins who had these traits passed them on to their offspring, while those who did not starved or froze to death, and had fewer or no offspring.

We can see this in multitudes of natural examples. Sharks, Tuna, Ichthyosaurs and Dolphins come from four very different evolutionary paths and are entirely different species, yet have all evolved similar shapes in order to be top marine predators in their environments. Wolves have similar looks to extinct Thylacine, or Marsupial Wolf. A browse through the biology section of the library will turn up numerous other examples.

Going the other way, there have been species of penguins that lived in New Zealand some 25 million years ago that were large and skinny instead. Since they lived in warmer climates, their need for insulating fat or a low surface to volume ratio was limited, while obviously being large bestowed some sort of competitive advantage, perhaps in fending off other predatory species or being able to hunt larger prey.

So depending on how specialized the evolutionary niche is, expect your species to converge towards an "ideal" solution regardless of the starting point in evolution, given enough time and how rigorous the conditions actually are.

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Hesperorns are more seal-like than penguin-like, and so pinnipeds might never evolve. Also, penguins could still evolve in this timeline, but may be more terrestrial, as the sea would already be dominated by the hesperorns

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  • $\begingroup$ You might consider expanding upon your response. Why do you think pinipeds might never evolve? Why couldn't penguins evolve down south and these hesperorns elsewhere? $\endgroup$ – elemtilas May 15 at 8:24
  • $\begingroup$ Hesperons are effectively avian seals, so it is possible that their diversification would close off the niches that pinnipeds evolved to fill $\endgroup$ – Ichthys King May 15 at 9:30
  • $\begingroup$ Okay. Don't repeat to me what you wrote in your answer! My point is to encourage you to edit your answer to make it better! $\endgroup$ – elemtilas May 15 at 10:29
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1) Hesperornithes are arguably more adapted to marine life than penguins

They appear to be far less suited for terrestrial locomotion than penguins, at the very least. They don't need to become more penguin-like to become more adapted for aquatic life, they were doing fine with it on their own.

2) Hesperornithes don’t appear to be known from the Southern Hemisphere

All undoubted Hesperornithes come from Northern Hemisphere fossil sites. Some Hesperornithes have been reported from fragmentary remains from Southern Hemisphere sites but these identification are not very secure or widely accepted. Indeed, many of the purported Southern Hemisphere hesperornithes appear to pertain to Vegaviidae, a group of anseriform birds, which may have been the Southern Hemisphere's answer to Hesperornithes similar to how we see modern marine birds with similar features confined to one hemisphere (i.e., penguins versus auks). I can't think of many good examples of flightless marine birds present in both hemispheres.

In fact, penguins may have also been present alongside vegaviids in southern polar Cretaceous seas (at least early, unspecialized ones). Most studies of penguin phylogeny have suggested the group originated at or right before the K-T boundary, given the presence of several species of penguins in the earliest Paleocene of New Zealand. Though it's possible this is an explosive radiation due to the absence of other marine birds after the K-T like mammals, as no undoubted penguin fossils predate the boundary.

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