Characterizing and distinguishing the earliest woody euphyllophytes based on secondary xylem anatomy: method...

The oldest vascular cambial growth (woody growth) has been recognized in several Early Devonian (ca. 410-395 Ma) euphyllophytes. Incomplete preservation of these fossils, in multiple cases, makes it difficult to evaluate their taxonomic diversity, in the absence of quantitative methods for distinguishing plants based on secondary xylem anatomy. In turn, this hinders understanding of their implications for the evolution of plant development. To develop and apply metrics that quantify secondary xylem anatomy and allow for conclusive comparisons, we investigated extant and Early Devonian fossil woody taxa.

We developed multiple metrics for quantifying tracheid size as a function of position in the secondary xylem and tested them against a dataset of extant gymnosperm wood. The four metrics that showed consistent trends within taxa and captured differences among taxa were then applied to previously recognized Early Devonian fossil woody taxa and to previously undescribed Early Devonian woody specimens to compare them and evaluate taxonomic diversity and placement of the new fossils.

The four new secondary xylem metrics, considered alongside other anatomical characters, differentiated the previously recognized Early Devonian taxa from each other, allowed assignment of two of the new fossils to the previously described species Franhueberia gerriennei and separated two new woody euphyllophyte taxa.

The metrics developed here for quantifying secondary xylem anatomy are effective in demonstrating conspecificity and in separating distinct woody taxa, especially in combination with data on the primary xylem and xylem rays. They provide a new method for assessing the taxonomic placement of fossils with incomplete preservation, opening up a new avenue for exploring fossil plant diversity and for characterizing anatomy, with implications for the evolution of plant development.