Roman use of the Contus

[Michael Kerr]

According to Philip Sidnell in his book Warhorse Cavalry in Ancient Warfare mentions that Roxolani used fir wood in making their shafts because of availability but fir could shatter into long splinters that would be a danger to rider & horse but Roman copies may have been made of ash or some other suitable wood. Are there any references of what type of wood the Romans may have used in making a contus?
Regards
Michael Kerr