What showed their greatest diversity during the mesozoic era

Gymnosperms

Explanation:

The gymnosperms were a small, less diverse group during the Paleozoic era compared to the Mesozoic era.

Gymnosperms are vascular plants whose means of reproduction are by means of an exposed seed. Conifers are it dominant plant. They dominated in the Mesozoic era. Gymnosperms were gradually replaced by the more evolved angiosperms.

Gymnosperms are classified into 4 phyla.

Coniferophyta, Cycadophyta, Gingkophyta and Gnetophyta.