Why did sea mammals evolve horizontal tails while most fish evolved vertical tails? They both have the same function. Is it related to swimming mechanics as a consequence of the rest of the anatomy?
My understanding is that it is related entirely to sea mammals having evolved from terrestrial mammals. The backbone flexion from terrestrial limbs was orientated towards up and down movement.
Fish evolved swimming with a side-to-side motion so their tails are flattened vertically. The earliest tetrapods continued to “swim” side-to-side on land, as salamanders and many lizards still do. Some groups, like dinosaurs and mammals, evolved more upright stances, with the legs kept underneath the body and their spines evolved to be more flexible up-and-down than side-by-side. The fact that ichthyosaurs have vertical tails like fish probably means that their terrestrial ancestors never evolved the upright stances; they were more lizard-like than dinosaur-like.
Largely the same reason that we use swimming flippers. or more pertinently, a monofin, that is horizontal. If, as Kurt Vonnegut suggests, we evolve into a form of seal, our descendants will also have horizontal flippers.
One of the characteristics of mammals vs. reptiles is that mammal spines have processes on the sides which limit sideways movement. The side-to-side flexing seen as reptiles walk compresses the lung on the inside of each step. Paleontologists believe that up-and-down spinal flexion provided improved lung function needed for higher metabolic levels, as well as being better suited to a more upright movement, with legs under the body rather than to the sides. Whale and other marine mammal tails developed from this inherent movement restriction.
Very interesting aspect of the whale tail I don’t recall hearing previously. Not just the flexibility of the spine but the effect on lung function as well. This reminds me that birds appear to benefit from improved lung and heart function and a higher metabolic rate as a result of flapping their wings in flight. I don’t recall all details but it seemed possible that birds didn’t have to expend energy breathing and beating their hearts in flight. The wing motion was compressing and expanding their chests in sync with the wings.
Birds (and their dinosaur ancestors) have a different breathing mechanism, which uses multiple air sacs to enable oxygen extraction both when inhaling and when exhaling.
An excellent source of information on the current science about mammal development is The Rise and Reign of the Mammals by Stephen Brusatte.
Yeah, mammals are the last surviving sub-sub-sub-(many)-group of a formerly much broader clade called the Synapsids. And it’s as if the synapsids were determined to do everything differently from the sauropsids (reptiles, dinosaurs, birds). Not only do mammals usually keep their testicles outside of their body (a really bizarre adaptation when you think about it), but for some reason mammals reject their nitrogenous waste as urea when all sauropsids vent uric acid instead.