The fossilized roots of primeval trees turned out to be structured in a more peculiar manner than paleobotanists had anticipated. Three casts of Stigmaria subjected to micro-computed tomography revealed that their subterranean axes produced lateral rootlets following a pattern reminiscent of shoot genesis. This finding offers a novel explanation for how the colossal lycopsid trees anchored themselves within Carboniferous swamps.
Stigmaria are the fossilized remains of the underground structures belonging to ancient lycopsid trees. Such trees flourished in the Late Paleozoic and attained heights of scores of meters. Their root systems spread out over meters, and the surface of the casts displayed rows of circular and oval pits—evidence of fine lateral rootlets.
These casts, the Stigmaria, have been instrumental in deciphering the appearance of ancient coal swamps. As early as the nineteenth century, the discovery of Stigmaria adjacent to the stumps of Sigillaria demonstrated that these coal swamps constituted true forests in situ, rather than mere accumulations of water-carried trunks. The roots kept the trees grounded and maintained the plants’ positions even after death. However, the internal organization of the Stigmaria remained contentious. Their external morphology could not resolve the debate; ascertaining the internal anatomy was necessary to settle the argument.
In extant vascular flora, roots and shoots typically develop according to divergent principles. In a shoot, the hormone auxin is synthesized at the apex and travels downward, facilitating the arrangement of leaves and lateral organs. Conversely, in roots, lateral roots originate from internal tissues via a different mechanism. Therefore, the orientation of vascular traces within the fossilized organ could indicate which growth program governed Stigmaria.
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Utilizing micro-computed tomography, scientists examined three Stigmaria casts to determine whether their terminal tips exhibited characteristics similar to roots or shoots. The findings were published in the journal Proceedings of the Royal Society B: Biological Sciences.
Inside the casts, the researchers observed a network of vascular traces supplying the lateral rootlets. Near the very tip, these traces ran parallel to the main axis, connecting with the inner surface of the upper section. Some nascent traces approached the apex without yet reaching the central stele. Further down, they curved and proceeded radially toward the points where the rootlets attached on the surface.
This configuration strongly resembled shoot development. Lateral structures emerged adjacent to the growing tip, and their vascular pathways reorganized as the main axis extended. The study’s authors concluded that the Stigmaria apex functioned not as a typical root tip, but as a shoot-like meristem. According to their proposed model, this aligns with auxin movement originating from the apex, characteristic of shoots.
Another critical feature identified was the presence of anastomoses. This term describes the junctures where vascular traces from various lateral organs converge before connecting with the central stele. These were present in the three-dimensional models of the Stigmaria and bore resemblance to the vascular trace patterns observed in the shoots, leaves, and cones of arborescent lycophytes. This observation bolstered the conclusion regarding a shoot-like developmental program.
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Consequently, Stigmaria did not resemble “inverted shoots” but rather represented specialized subterranean appendages of the lycopsid trees. They fulfilled root functions but developed according to an unusual, shoot-like blueprint. This represented a unique organ of these ancient trees, originating from a modified shoot axis and lacking direct parallels among extant species.
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