Shubin and his colleagues chose to focus on 375 million year old rocks because they were searching for the missing link between fish and early land animals ie the amphibians. It was known that 385 million year old rocks had fish without amphibian characteristics and 365 million year old rocks had amphibians not similar to fish so that is why 375 million year old rocks were looked for. In fact a fossil called Tiktaalik was found that provided the "missing link". These rocks were sedimentary rocks which is the rock type where fossils can be found. They found this fossil on Ellesmere Island in Canada's north.
