Trilobites are important to dating rocks between the Cambrian and the Permian because they define the lower boundary of which intervals?

• A. Lower boundary of the Permian Period
• B. Lower boundary of the Cambrian Period
• C. Lower boundary of the Cambrian Period and Paleozoic Era
• D. Lower boundary of the Triassic Period

Answer: (C)

Trilobites are classic time markers for the early Paleozoic because their presence and distinctive changes in the first part of the Cambrian are widespread and relatively rapid. Their fossil assemblages provide reliable signals that the rocks have entered the early Cambrian, and since the Cambrian marks the beginning of the Paleozoic Era, crossing into Cambrian rocks also means entering the Paleozoic. In other words, trilobites help identify the start of both the Cambrian Period and the Paleozoic Era at the same point in the rock record. They do not define the bases of the Permian or the Triassic, which lie later in the geological timeline, so the best interpretation is that trilobites help determine the lower boundaries of Cambrian rocks and of the Paleozoic Era.