petrified softwood
Petrified wood, also known as fossil wood, is wood that is preserved in the fossil record. Over time the wood will usually be the part of a plant that is best preserved (and most easily found). Fossil wood may or may not be petrified.
The study of fossil wood is sometimes called "wood", with a "palaeoxylologist" somebody who studies fossil wood.
The fossil wood may be the only part of the plant that has been preserved, with the rest of the plant completely unknown: therefore such wood may get a special kind of botanical name. This will usually include "xylon" and a term indicating its presumed affinity, such as Araucarioxylon (wood of Araucaria or some related plant) or Castanoxylon (wood of some indeterminate chinkapin) .