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The form of large swimming animals is fairly consistent across distantly related groups. As example, all aquatic mammals, save the quadrupedal semiaquatic forms, have adapted for a fish-like shape with a vertically-moving tail and two flippers on their chest

Mammals, clearly, have only 4 limbs, of which all four are on the torso. Hence, I must ask if this form would be the ideal for a similar fish-shaped animal which possesses a pair of cephalofoils somewhat similar to a hammerhead shark

enter image description here

Specifically, the animal has a fish-shaped body with a dorsal fin and a fluked tail like a dolphin. It ancestrally has 6 fully moveable limbs, one pair on the head, one pair at the front of the chest, and one pair between the torso and tail. Which of these limbs should be retained or lost to reach a hydrodynamic ideal for swimming within this ancestral limb configuration?

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    $\begingroup$ Note that aquatic mammals have all evolved horizontal flukes on vertically-moving tails because they started out with spines adapted for bending up and down to facilitate running on land by their terrestrial ancestors. Aquatic reptiles, which retain a spine specialized for sideways bending, tend to develop more fish-like vertical tails. $\endgroup$ Commented May 30, 2023 at 18:02
  • $\begingroup$ Are the title question and the question posed in the body text the same? They don't look the same. They don't read the same. I'm 99% sure they're not the same.... can't ask two questions. Could you edit the post to change either the title question or the body question to reflect the other? $\endgroup$
    – JBH
    Commented May 30, 2023 at 19:16
  • $\begingroup$ @JBH The question in the body-text is merely an expansion of that in the title. The only relevant change from reality here is the cephalofoils, hence any answer regarding the ideal must focus on the usefulness of the cephalofoils $\endgroup$ Commented May 30, 2023 at 19:30
  • $\begingroup$ I need you to change this one, Ichthys. "would they be useful?" (answer:yes/no) and "which should be retained or lost?" (answer:analysis) aren't related enough. Please click the edit link and ask just one question (not from your perspective, but from ours...). $\endgroup$
    – JBH
    Commented May 30, 2023 at 19:42
  • $\begingroup$ I still don't see the similarity between "moveable cephalofoils" and "retained or lost limbs." Maybe change your title to, "Configuring the cephalofoils on a delphine swimmer." $\endgroup$
    – JBH
    Commented May 30, 2023 at 20:32

2 Answers 2

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Convergent evolution

Take a look at this picture from https://cannonballread.com/2018/03/to-the-reference-shelf-with-you/ Convergent evolution

The common pattern is a smooth elongated body with a pointy tip. Three (except the penguin) fins for stabilization and steering. And a movable surface at the back for propulsion. Note that both the Cetaceans and the Ichthyosaur have evolved a third fin while their ancestors had no limb in that place. The Penguin failed to do so. (yet) One could also add the sword fish in this picture.

Sword fish (wikipedia)

Note that the rear fins of both the Shark and the sword fish are small. And the dolphins have lost them totally. The feet of Penguin operate more in conjunction with the tail when in water.

So unless the limb has a special purpose other than fast agile swimming it won't be there if it is not found on these animals. These traits seem to be the best for swimming.

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To bring the title back to the cephalofoils, the head limbs become the cephalofoils (or head thingies). But it's a stretch to put the eyeballs at the end of the head thingies. The middle limbs become front fins and the back legs are lost or become itty bitty back fins that help slightly in steering.

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