Trilobites were abundant in Pennsylvania. Trilobites were among the first of the arthropods, a phylum of hard-shelled creatures with multiple body segments and jointed legs. Their fossils first appeared in early Cambrian period about 570 million years ago. The trilobite fossils show that during most of this time Pennsylvania was ___________ providing favorable conditions for the trilobites.
A) glacial terrain
B) a dry and sandy plain
C) mostly mountain ranges
D) covered by warm, shallow seas
