The genus Phaeomegaceros R.J.Duff, J.C.Villarreal, Cargill & Renzaglia (Dendrocerotaceae J.Haseg.), represented by the species P. fimbriatus (Gottsche) R.J.Duff, J.C.Villarreal, Cargill & Renzaglia is recorded for the first time in Mexico, North America, extending its distribution range considerably to the North. Recording distribution ranges could aid to better understand ecological and biogeographical patterns, and detailed population level descriptions of wide-range distributing hornworts could be helpful in determining intraspecific variation, which in turn would aid to define species limits, nowadays poorly understood in Anthocerotophyta Stotler & Crandall-Stotler. Anatomical and morphological characters of collected plants are described in details and illustrations in light microscopy and SEM are provided. P. fimbriatus can be distinguished from other Mexican hornwort species by spore ornamentation, antheridial number per chamber, number of layers of spore tissue in the sporophyte and presence of band-like thickenings in cells of gametophyte thallus. Mexican plants of P. fimbriatus presented smaller spores (28-36 µm in diameter, average 34 µm) than those previously reported (32-43 µm in diameter) for this species, which extends the species variation range of this character and might be pointing out geographical variation.