A new wood-boring ichnospecies is described from transgressive (lagoonal) deposits of the Lower Cretaceous Sparky Formation (Mannville Group) in west-central Saskatchewan, Canada. Apectoichnus lignummasticans isp. nov. is a trace fossil that occurs in a thin coal bed and which was emplaced in an in situ xylic substrate (woodground). The ichnofossil is thin, elongate, unbranched, and straight to gently curved with a circular cross section and uniform diameter. Apectoichnus lignummasticans is similar in many respects to modern borings in wood that are produced by marine isopods, such as Limnoria lignorum, for feeding and refugium. The recognition of A. lignummasticans in the rock record aligns with the modern observation that fossilized wood-boring assemblages should display higher ichnofossil diversities than commonly reported. Additionally, the stratigraphic occurrence of A. lignummasticans in association with other evidence of marine deposition reaffirms that certain wood boring morphologies (i.e. ichnotaxa) are useful as indicators of marine transgressions.