Convergent evolution of the developmental anatomy of leaf abscission: evidence from the arboreous lycopsid Sigillaria.

A striking feature of late Paleozoic arboreous lycopsids is the presence of leaf scars, set within leaf cushions, that blanket the trunk and branches. Whether these scars represent leaf loss through a passive process such as decay or tearing or leaf loss through an active process such as abscission has been a longstanding question in paleobotany-one marked by a notable paucity of direct anatomical evidence. Here we investigate using microscopy permineralized leaf cushions of the arboreous lycopsid Sigillaria approximata with and without articulated leaves in longitudinal section, as well as cushions showing different anatomical features associated with leaf loss. The developmental anatomy of Sigillaria leaf cushions indicates leaf loss through abscission. We describe an abscission zone present at the cushion-leaf interface that includes a protective layer and a separation layer, as well as a layer resembling abscission periderm immediately beneath the protective layer and cell files in the proximal leaf. These anatomical features correspond to those of leaf abscission in living euphyllophytes. Leaf abscission evolved independently in arboreous lycopsids and in euphyllophytes. The developmental anatomy of leaf abscission is highly congruent across clades, highlighting the strong shared constraint that active leaf shedding be coordinated with wound healing.