The tapeats sandstone and bright angel shale contain fossils of trilobites. trilobites were marine organisms similar to modern-day horseshoe crabs. no other layers contain trilobite fossils. which conclusion best explains these observations? a. the area became dry land after the bright angel shale period. b. an asteroid hit the earth after the bright angel shale period. c. the trilobites became extinct after the bright angel shale period. d. the trilobites migrated away from the area after the bright angel shale period.