Which one of the following was not present during the Mesozoic Era of the geological time scale ?

The correct option is Bryophytes.

To understand why this is the correct answer, let's analyze the evolutionary timeline of plants in relation to the geological time scale:

1. Mesozoic Era: Often known as the "Age of Reptiles" or the "Age of Conifers," the Mesozoic Era spanned from about 252 to 66 million years ago. During this era, gymnosperms (like Ginkgos) flourished and dominated the land, while advanced seedless vascular plants like Ferns and Horsetails were also abundant and well-represented in the fossil record.

2. Origin of Bryophytes: Bryophytes (which include mosses, liverworts, and hornworts) are non-vascular land plants. Evolutionary and fossil evidence indicates that bryophytes evolved much earlier, during the early Paleozoic Era (specifically the Ordovician period, around 450–470 million years ago). While they survived through geological history into the Mesozoic and modern eras, they did not originate or first appear during the Mesozoic Era.

Therefore, among the options provided, Bryophytes represents the group of plants whose initial evolutionary emergence and primary establishment occurred long before the Mesozoic Era began.