Final answer:
If marine algae and bony fish were found together in the same fossil layer, it would challenge our understanding of the fossil record.
Step-by-step explanation:
The pair of organisms that, if found together in the same well-defined fossil layer, would cause a major rethinking of our current understanding of the fossil record is marine algae and bony fish. This is because marine algae are photosynthetic protists, while bony fish are vertebrates.
The presence of both of these organisms in the same fossil layer would suggest a significant shift in our understanding of the timing and order of the evolution of photosynthetic organisms and vertebrates.