Secondary growth is a tracheophyte structural feature whose earliest known occurrence dates to the late Pragian-early Emsian.  Armoricaphyton, Franhueberia and an unnamed plant from eastern Canada represent the only instances of secondary growth documented to date for the Early Devonian.  Here, we describe a new Early Devonian euphyllophyte exhibiting secondary growth, from the Emsian (c. 400-395 Ma) Battery Point Formation (Québec, Canada): Gmujij tetraxylopteroides gen. et sp. nov. is characterized by a mesarch actinostele with Psilophyton-type (P-type) tracheid wall thickenings and exhibits a new type of anatomical organization among Early Devonian wood-producing euphyllophytes.  Gmujij differs from coeval euphyllophytes, but lack of data on branching patterns, reproductive structures and other traditional diagnostic features preclude classification among previously recognized euphyllophyte groups.  We experiment with a numerical approach to tracheid size distributions, to compare Gmujij with younger euphyllophytes exhibiting secondary growth.  Comparisons show similarity between Gmujij and aneurophytalean progymnosperms, such as Tetraxylopteris.  However, its plesiomorphic P-type tracheids set apart Gmujij from younger actinostelic euphyllophytes, raising questions on the importance of tracheid thickening patterns as diagnostic characters separating major lineages.  Answers to this question hinge on whether transitions between wall thickening patterns require simple or complex changes in developmental regulation, a matter that requires improved understanding of regulatory programs that determine tracheid thickening patterns.  Irrespective of these, Gmujij adds a new component to the diversity of anatomically preserved plants known in the Early Devonian, an incompletely explored interval of tracheophyte anatomical diversification, contributing information that will help understanding of the evolutionary origins of secondary growth.

Direct measurements and summary statistics.