So, with that large, looming shadow swimming towards you, what do you look out for? If you can see its tail, simply look at the orientation of the tail fluke, is it horizontal or vertical? Failing that, look at how it moves, does it undulate its body up and down, or does it move its body from side to side? Dolphins and whales have a horizontal tail fluke, which means they have to undulate their body up and down in order to propel themselves through water, so if you see either the horizontal tail fluke or the undulating movement, you have a dolphin or whale coming towards you.
Sharks, on the other hand, have a vertical tail fluke and so must flex their body from side to side for propulsion. If you see the vertical tail fluke or side to side motion, then what is coming towards you is a fish and so might be a shark.
|Posture changes in dinosaurs.|
As our fishy ancestors used a side by side motion to propel themselves through water, so did our earliest terrestrial ancestors and so do reptiles today. Snakes are an extreme example of this sort of movement, but the side to side motion is still there. During the Mesozoic era things began to change, as our ancestors (and convergently in dinosaurs too) developed a more upright posture, instead of the sprawling gait of reptiles. With an erect posture the more effective way to rapidly move is to flex the spine up and down whilst running, rather than side by side (see image below).
Many vertebrates have some of their vertebrae fused to facilitate particular movements, so future evolution can often be restricted to working within the confines of that movement. As dolphins evolved from terrestrial mammals, their semi-aquatic ancestors also used this up and down movement and so adapted this to movement in the water. Side to side motion, like that of a shark, would require a larger number of changes when there was the simpler solution of up and down movement (though note that evolution does not have the foresight, it simply uses what is available - quick fix solutions often work in evolution). The motion of whales and dolphins is testament to their ancestry, having descended from active land mammals.
During the Mesozoic another group secondarily took to the waters and adopted the torpedo shape of dolphins and sharks. These were the ichthyosaurs, descending directly from terrestrial reptiles. As their ancestors used the side to side motion, so did the ichthyosaurs when they swam, also possessing a vertical tail fluke. They had some unusual traits for reptiles, giving live birth and being warm blooded, but their swimming motion gives away their reptilian status. This also quite ably demonstrates that they are not dinosaurs as many laymen mistakenly think, for dinosaurs did not have a sprawling posture which uses the side to side motion. So, if you are somehow in Mesozoic waters with a shape swimming towards you, it may be too late before you can discern whether or not it is an ichthyosaur or a shark.