Paleontological evidence for membrane fusion between a unit membrane and a half-unit membrane.

Membrane fusion is of fundamental importance for many biological processes and has been a topic of intensive research in past decades with several models being proposed for it. Fossils had previously not been considered relevant to studies on membrane fusion. But here two different membrane fusion patterns are reported in the same well-preserved fossil plant from the Miocene (15-20 million years old) at Clarkia, Idaho, US. Scanning electron microscope, transmission electron microscope, and traditional studies reveal the vesicles in various states (even transient semi-fusion) of membrane fusion, and thus shed new light on their membrane structure and fusion during exocytoses. The new evidence suggests that vesicles in plant cells may have not only a unit membrane but also a half-unit membrane, and that a previously overlooked membrane fusion pattern exists in plant cells. This unexpected result from an unexpected material not only marks the first evidence of on-going physiological activities in fossil plants, but also raises questions on membrane fusion in recent plants.