A shoot at the root? Unique development and evolution of the stigmarian apical meristem

Stigmaria rooted late Palaeozoic lycopsid trees, and its development and homologies have long been debated. Micro-computed tomography of stigmarian apex and distal axis cast fossils reveals exceptional and surprising three-dimensional preservation of the internal system of rootlet anatomy, including immature traces that articulate to the apical plug prior to integrating proximally with the stele, reorientation of traces from longitudinal to radial behind the apex, and trace anastomoses. This record of development indicates that the stigmarian apical meristem has characteristic developmental features typical of vascular plant shoots, consistent with apical production of the plant morphogenic hormone auxin and its transport away from the apex. Importantly, the stigmarian meristem structure differs fundamentally from that of the small rooting stocks of related plants, inviting reconsideration of homology between Stigmaria and other lycopsid rooting structures. The shoot-like developmental pattern of Stigmaria challenges the foundational assumption that roots develop by auxin transport towards the apex, offering a rare example in the fossil record of a lineage experimenting with developmental architecture in ways unseen in living plants. The employed methods give potential new life to thousands of cast fossils that may, too, preserve internal anatomy.