Why is first seed plant and Gymnosperms kept in different periods if Gymnosperms were the first seed plants ?

Dear student,,Gymnosperms include the gingkoes and conifers that inhabit many ecosystems, such as the taiga and the alpine forests, as these are well adapted for cold weather. True seed plants became more numerous and diverse during the Carboniferous period around 319 million years ago; an explosion that appears to be due to a whole genome duplication event. The true seed plant refers to those plant seeds that contain a diploid embryo that germinates into a sporophyte. Storage tissue to sustain growth and a protective coat to give seeds their superior evolutionary.