Portrait of a Temnodontosaurus (and the many wonders of Ichthyosaurs)
Ichthyosaurs are my most favourite prehistoric creatures, overtaking even the formidable Tyrannosaurus rex and the not–so–gentle giants of Triceratops horridus. Occupying a similar ecological niche to modern day whales, ichthyosaurs and their sister clades (mosasaurs, plesiosaurs and pliosaurs) were the reigning reptiles of the Mesozoic oceans, where they persisted for over 150 million years.
Ichthyosaurs are strange creatures: ancient reptiles which bear strong resemblances to today's mammalian dolphins. This similarity is no mere coincidence, though it does not imply relatedness—ichthyosaurs and dolphins look similar because both were shaped by the selection pressures of the aquatic environment, and both followed similar evolutionary trajectories to reach their ultimate body plans (convergent evolution). This trajectory is predictable and directional; we've seen it repeating in various aquatic lineages (from whales to penguins to even polar bears) and we can broadly tell that things in the water are trying to become more fish–like again. And as for the ichthyosaurs—they ultimately overtook dolphins on this trajectory by having the most fishlike form, probably, since fish themselves.
Diving In: cliffnotes of a third-year paleobiology report
Both ichthyosaurs and dolphins mark the remarkable transition from land to water, also called secondary aquatic adaptation (primary aquatic adaptation being the evolution of fish). Life, the fossil record tells us, has undergone several remarkable transitions: fish evolved into tetrapods through terrestrialisation, diversified, and then several lineages returned once more to the water. So in the story of ichthyosaurs and dolphins, both were once fish which colonised land, decided (after some million years) that it wasn't for them, and then returned again to the water, becoming more and more fishlike over time.
Water is very difficult to live in. It is more dense and viscous than air, and so animals must adapt for drag reduction, propulsion, locomotion, neutral buoyancy and stability. There are not a great number of ways in which to do this. Sharks (as fish, and top predators at that) exemplify all the necessary adaptations.
a streamlined, fusiform body shape (for drag reduction)
pectoral fins (for locomotion and stability)
a dorsal fin (for stability)
bony adaptations such as pachyostosis, the increase in bone sizes, or osteosclerosis, the increase in bone densities (to move up and down the water column, instead of floating or sinking)
thunniform locomotion (propulsion restricted to the tail, which allows rapid acceleration and sustained swimming, ideal for pursuit predators)
Ultimately, the most successful aquatic predators move towards this body plan, and ichthyosaurs were no exception.
The ichthyosaurs have an excellent fossil record. We have found enough species through time to track their evolution, and one of the most remarkable evolutionary changes in ichthyosaurs concerns point no. 2, the development of pectoral fins. Ichthyosaurs don't have fins; being formerly–terrestrialised creatures they lost their fins when they stopped being fish and instead developed digits: five digits as per the Hox five-finger standard. However, it is fins that are useful in the water; their flexibility is used for powerful locomotion and so ichthyosaurs began to re-evolve these amazing, fin–like appendages: flippers with first more phalanges or segments, and then more and more digits until these flippers approximated fins rather well indeed. And—the absolute coolest thing is—we can see and loss and re-evolution of fins very clearly when comparing transitional terrestrial and secondary aquatic species:
The left side shows the evolution of fish–like animals to terrestrial animals. The gap represents missing terrestrial lineages (we haven't found them... yet). The right side shows how ichthyosaurs evolved more and more fin–like flippers, with more digits and more phalanges (Utatsusauras is a primitive ichthyosaur; Icthyosaurus is a later and more adapted ichthyosaur).
You can see even more of these adaptations below, including the development of a dorsal fin, the development of a homocercal tail fin (like tuna or sharks, this is used for thunniform locomotion), hyperphalangy and hyperdactyly as above, and also more predatory adaptations: large eyes with sclerotic rings for deep water diving and groove–grown occluding teeth for snapping up prey (which for the Temnodontosaurus included other ichthyosaurs, as per the Attenborough and the Sea Dragon documentary). Ichthyosaur niches also expanded over time from shallow, lagoonal environments where they were ambush predators to transoceanic pursuit predators in the same capacity as whales and sharks today.
A thought question, to end things: who would come out on top, an ichthyosaur or a whale? I can tell you now that ichthyosaurs are more fishlike than whales, or indeed any other secondary aquatic lineages, but does that necessarily mean that they are the most superior predator in water? We know that the great white has been ousted from its throne by the killer whale; where would the ichthyosaur fall in the hierarchy outside of time?
I can't say that I would rightly know, but in the unlikely event that time collapses to a single moment and all the ocean's predators come together in one final battle before the end, I have one trump card yet that I'd pull out for just such an occasion.
