[2] Rhizomnium dentatum is known from a small group of fossil gametophyte shoots which are inclusions in a transparent chunk of Baltic amber.

The holotype was first studied by a group of five researchers led by Jochen Heinrichs of the Ludwig Maximilian University of Munich.

The research group's 2014 type description for the species was published in the paleobotany journal Review of Palaeobotany and Palynology.

[2] The R. dentatum specimens the upper portions of a gametophyte shoots, with no lower structures preserved.

Overall the stems are up to 7 mm (0.28 in) long, with a noted zig-zag structure that separates it from the described extant species.