The onondaga contains an abundance of coral fossils, along with crinoids, brachiopods, and trilobites. Why are fossils of ferns, rushes, and primitive conifers unlikely to be found in these rocks

Due to presence of water in these rocks.

Explanation:

The fossils of ferns, rushes, and primitive conifers can not be found in these rocks because these rocks are present under the sea millions of year ago. In this onondaga contains an abundant amount of coral fossils, crinoids, brachiopods, and trilobites due to their aquatic habitat. These organisms are present millions of years ago when this area comes under the sea so that's why the fossils of these organisms are present in these rocks.