A hypothesis is a tentative explanation based on some knowledge of the world (prior observations), but not confirmed by repeated observations or experiments. A theory is a body of knowledge based on multiple tests of a series of related hypotheses. Theories have explanatory power; in other words, they help us understand why things are the way they are. For example, the theory of evolution helps us make sense of the fact that all living organisms use DNA and all vertebrate forelimbs follow the "one, two, many" pattern; this is because all organism have a common ancestor that used that genetic material and all vertebrates have a common ancestor with that configuration of bones. Furthermore, theories should have predictive power such that we can use them to extrapolate the existence of unknowns from what we already understand. Examples include the prediction that whales and snakes descended from four-legged terrestrial ancestors because they have vestigial femurs, and the expectation that antibiotic and herbicide resistance will continue to increase as organisms evolve in response to the chemicals. For these reasons, theories should be used in formulating new hypotheses. Well-known theories include the theory of gravity, cell theory, and the theory of evolution. 1. Given what you know, hypothesize what kind of animal might have been the most recent ancestor of the Ichthyopterygia (the whole group of ichthyosaurs). In other words, what kind of animal that was not an ichthyosaur might have given rise to this group? Would it have been a terrestrial reptile? A fish? Or something else? It may help to think back to Tiktaalik again: it was one of the earliest animals with tetrapod characters, and could have been the most recent ancestor of all tetrapods. Explain your answer.
2. Assume for a moment that the ancestor of the ichthyosaurs was a terrestrial reptile. Give an example of a hypothesis you could make, based on the theory of evolution, about the animals that occurred on the phylogenetic tree between these terrestrial reptile ancestors and the ichthyosaurs (marine reptiles). Remember, we call these sorts of organisms transitional forms.
3. What would it mean to the theory of evolution if the types of transitional forms you hypothesized in your answer to question 2 above were never found? What would it mean if a very different type of animal, like a mammal or a fish, were found to have lived then?
4. Based on the origin of ichthyosaurs about 230 million years ago, predict when the transitional form you described in your answer to question 2 above would have lived. Since evolution is not a linear but a branching process, there could have been considerable overlap of earlier-evolved and later-evolved forms, so this question cannot have a clear-cut answer, but you can speculate.