Answer:
The animal with the most complete fossil record is likely the trilobite, an extinct marine arthropod that lived from the early Cambrian period (about 521 million years ago) until the end of the Permian period (about 252 million years ago). Trilobites were abundant and diverse, with over 20,000 species known from fossils, and their hard exoskeletons made them well-preserved in the fossil record.
Trilobites were important early animals in the evolution of complex life on Earth and played a key role in the diversification of marine ecosystems. They were also useful in dating rock formations and understanding the sequence of events in Earth's history. Because of their abundance, diversity, and good preservation, trilobites have provided scientists with a wealth of information about the evolution of life on Earth.