Data from: An Early Devonian actinostelic euphyllophyte with secondary growth from the Emsian of Gaspé (Canada) and the importance of tracheid wall thickening patterns in early euphyllophyte systematics

<p>Secondary growth is a tracheophyte structural feature whose earliest known occurrence dates to the late Pragian-early Emsian.  <i>Armoricaphyton</i>, <i>Franhueberia</i> 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): <i>Gmujij</i> <i>tetraxylopteroides</i> gen. et sp. nov. is characterized by a mesarch actinostele with <i>Psilophyton</i>-type (P-type) tracheid wall thickenings and exhibits a new type of anatomical organization among Early Devonian wood-producing euphyllophytes.  <i>Gmujij</i> 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 <i>Gmujij</i> with younger euphyllophytes exhibiting secondary growth.  Comparisons show similarity between <i>Gmujij</i> and aneurophytalean progymnosperms, such as <i>Tetraxylopteris</i>.  However, its plesiomorphic P-type tracheids set apart<i> Gmujij</i> 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, <i>Gmujij</i> 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.</p>