To determine which organisms likely existed earliest based on the fossil record, we typically refer to the geological timescale and the known appearance of these groups in the fossil record:
- Ammonites (A) first appeared in the late Paleozoic era and became prominent in the Mesozoic era.
- Ferns (B) began to appear during the late Devonian period, around 360 million years ago.
- Gastropods (C), which include snails and slugs, are believed to have first appeared in the Cambrian period, roughly 500 million years ago, but they became more diverse later.
- Trilobites (D) are among the earliest-known groups of arthropods, appearing in the Cambrian period and diversifying throughout the Paleozoic era.
Given this context, the earliest organisms among the options provided are D. trilobites.