The fossil record of monocots is known primarily from fossil pollen; few large fossils are known from the Mesozoic and early Cenozoic, and so their evolutionary history has not been well understood. There are a number of well-preserved leaves from the Cretaceous which appear to be monocots, but these cannot be placed definitively with any one group. It is not until the end of the Cretaceous (Maastrichtian) that recognizeable fossils have been found; these include gingers, palms, and Dioscorea.
The sparse fossil record of most monocot groups may in part be due to the fact that monocots are primarily tropical, and most paleobotanists have collected in temperate and arctic regions. It is also true that leaves are found more frequently than other parts, and that many monocot groups are difficult to tell apart based solely on their leaves.
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Jan Muller. 1981. Fossil pollen records of extant angiosperms. Botanical Review 47(1):1-142.