While working in the field, you find some sedimentary rocks that include hundreds of fossils of crinoids, numerous brachiopods, and a few trilobites. Realizing that these ancient organisms are some of the first with hard parts, and (when taken as a group) are not very similar to animal communities living today, in which unit of geologic time would you place these rocks?

Paleozoic era

Explanation:

The Paleozoic era refers to the time period ranging from 540 million years back to 250 million years back. It is comprised of six distinct periods, namely Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. During this era, a large number of species evolved after the huge Cambrian explosion. The environmental changes that occurred during that time favored the growth of numerous species, and species have undergone radiative adaption.

This era was characterized by the presence of Trilobites that were abundant in the Cambrian. The Crinoids were closely related to the sea urchins and starfishes and are important fossils. Brachiopods were also dominant during this time. Some of them are still found such as the Lingula, which is a living fossil.

These species covered a large geographical extent in the ocean water bodies and were extinct in the huge mass extinction event that took place on earth by the end of Permian.

iii. and iv. only is the right answer