Multiple choice What type of organism makes up the oldest known fossil? T-Rex from Mexico Ammonites from England Trilobites from Spain Blue-green algae from South Africa Reveal answer Fill a bubble to check yourself D Correct answer Explanation Stromatolites, which are formed by cyanobacteria (blue-green algae), represent some of the oldest known fossil evidence of life on Earth, dating back billions of years. The other options refer to much younger organisms from the Phanerozoic eon.