Rhynie chert fossils demonstrate the independent origin and gradual evolution of lycophyte roots.

ABSTRACT

Mapping fossil traits onto the land plant phylogenetic framework indicates that there were at least two independent origins of roots among extant vascular plants - once in lycophytes and independently in euphyllophytes. At least two rooting structural types are found among extinct species preserved in the Rhynie chert. First, species that lacked roots and developed horizontal axes that developed rhizoids. Second, the rooting axes of Asteroxylon mackiei resembled the roots of extant lycopsids but lacked root hairs and root caps. These two rooting structures preceded the evolution of the roots of extant lycophytes comprising axes on which root hairs and root caps developed. These data demonstrate the defining root characters evolved gradually in the lycophyte lineage.