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loose confederation of seed plants from the Carboniferous and Permian periods (about 360 to 250 million years ago). Some, such as Medullosa, grew as upright, unbranched woody trunks topped with a crown of large fernlike fronds; others, such as Callistophyton, were woody vines. All had fernlike foliage; however, they reproduced by seeds, with ovules and pollen organs attached to the fronds. Gamete-producing structures in the seeds were surrounded by a hard inner integument and a fleshy outer layer. These features have led some authorities to speculate that these seeds may have been dispersed by animals. Some seeds were large. (Pachytesta gigantea, a seed of Medullosa, grew up to 7 cm [2.7 inches] long.) Pollen organs of seed ferns were also large and complex and were commonly made up of many pollen sacs fused into a large structure. Some authorities suggest that these large structures and the large pollen grains they contained were evidence of pollination by animals