Why do many of the transitional fossils fish and tetrapods have flat heads.

In 2004, Shubin and colleagues found the fossil of a new species that shed light on a key transition in the evolution of tetrapods: the fin-to-limb transition. ... So finding a flat-headed fish was a good indicator that this fossil was part of the gradual evolutionary sequence from fish to tetrapods.

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.

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