Explain how scientists can determine relationships

between ancient arthropods that lived 500 million years

ago and arthropods of today.

When animals first crawled onto land, one of the greatest obstacles they had to contend with was figuring out how to breathe. No longer bathed in oxygen-rich marine waters, their gills would surely have dried out. Scientists have analyzed fossils from 500-million-year-old rocks that show one way these early pioneers may have dealt with this problem--the first terrestrial animals carried a shell on their backs.