Conversely, these earliest occurrences of secondary growth
in the euphyllophyte clade are ≥ 15 million years younger than
the major divergence of the Lycophytina and Euphyllophytina
(the oldest lycophytes, ≥ 426 million years old, are reported
from Ludlow rocks of Australia and Canada; Kotyk et al.,
2002 ; Gensel, 2008 ). Consequently, although lycopsids and
euphyllophytes share a mechanism for secondary vascular
tissue production ( Rothwell et al., 2008 ), the question still
remains whether that mechanism was inherited from a common
tracheophyte ancestor.