Dioon would therefore be the basal genus of the family. Species The genus is commonly divided into two groups by Norstog & Nichols of distinct morphology. The first includes D. mejiae, D. rzedowskii, and D. spinulosum, which are characterized by the greater size of their fronds, trunks, and cones. The second group contains D. califanoi, D. caputoi, D. edule, D. holmgrenii, D. merolae, D. purpusii, D. sonorense, and D. tomasellii, which are smaller overall, with shorter trunks, much shorter fronds, and smaller cones. A phylogenetic study from 1993 confirmed these clades with one difference - D. caputoi was assigned to the other clade Species accepted: Evolution The earliest fossil finds date back to the Eocene and were found on Kupreanof Island in Alaska. However, it is probable that Dioon dates back to the Jurassic (145.5 - 199.6 ka) or even further, since fossils of other, closely related but more developed cycads were already detected from this era. In the Pleistocene (0.011784-1.8 ka) and Holocene (today -0.011784 ka) eras species have probably carried out several north–south migrations and have followed the warm climatic conditions. One theory is that the recent dioon species have evolved from a number of vicarious events in the early Cenozoic, with different species representing different ecological conditions. These events must have followed each other for a very short time, which precluded an accumulation of synapomorphies, meaning that not many features could be re-acquired. The extant species of Dioon are attributed to three prototypes. D. mejiae is considered the most evolutionary advanced species of the genus. D. spinulosum and D. rzedowskii are attributed to a second prototype, which was probably better adapted to mesial conditions, such as areas of medium humidity, which enable them to establish on the Atlantic coast of Oaxaca. The remaining species presumably come from a third, to xerotic, dry areas, adapted to the original form. Uses In Honduras, the female cones of D. mejiae are picked to harvest the seeds. These are processed by about 33,000 indigenous people into tamales or tortillas and replace the cornmeal. The fronds are traditionally used on Palm Sunday. References External links Gymnosperm Database: Dioon Dioon on The Cycad Pages