5. SWIMMING EVOLUTION IN ICHTHYOSAURS
As I pointed out in Introduction,
early ichthyosaurs were lizard-shaped, and fish-shaped ichthyosaurs then
evolved from among them. Unsurprisingly, the way ichthyosaurs swam also
changed with the body shape evolution.
Many living fish undulate or oscillate their body axes for swimming. They are usually categorized into four swimming types, depending on how much of the body stays more or less stable (figure below). Anguilliform, or eel-like swimmers undulate their entire body, whereas thunniform, or tuna-like swimmers move only the posterior part of their body, keeping the anterior part steady.
Lizard-shaped ichthyosaurs shared some characteristics of living catsharks (see the section on Vertebrae): they have slender trunk, slender backbone, and high numbers of vertebrae. Catsharks are eel-like swimmers, so it is likely that lizard-shaped ichthyosaurs were eel-like swimmers too. This is not surprising since many lizards today swim in eel-like manners when in water.
Fish-shaped ichthyosaurs, on the other hand, resembled mackerel sharks, such as the Great White, with thick body trunks and thick backbone, as well as crescent-shaped tails. These characteristics suggest that many fish-shaped ichthyosaurs were tuna-like swimmers. I can tell you more about this in the future.
Eel-like swimming provides good combination of maneuverability and acceleration, and is sufficient for those vertebrates living in the area of continental shelf. Tuna-like swimming is known in cruising vertebrates that invade open oceans, and provides energy-efficient swimming at a steady speed. So, it looks like life styles changed drastically between lizard-shaped and fish-shaped ichthyosaurs, from dwellers of continental shelf to cruisers of the ocean.