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DiMichele, William [1], Bateman, Richard [2], Duijnstee, Ivo [3], Elrick, Scott [4], Looy, Cindy [5].

The stigmarian root system of the Carboniferous arboreous lycopsids.

The stigmarian root system, which anchored arboreous lycopsids and provided a water/nutrient conduit from substratum to their aerial shoot, has generated considerable recent interest. Organizationally it consisted of two anatomically and developmentally different organ types, the main rhizomorph and the rootlet appendages that it generated. The rhizomorph system split bilaterally but matured into multiple radially symmetrical primary axes that dichotomized infrequently and extended laterally meters outward from the base. The helically arranged rootlets of determinate growth dichotomized regularly. These were up to 1 m in length, hollow at maturity, and separated from the main axis by a pad of tissue. Originally homologized with leaves, recent studies of Isoetes and arborescent lycopod root hairs suggest stronger homology with roots of other vascular plants. Here, we argue that claimed rootlet abscission is a taphonomic artefact resulting from splitting of matrix along the surface of the rhizomorph axis. Rootlets are not positively geotropic. If unobstructed, rootlets develop around and perpendicularly to the main rhizomorph axis. Rootlets had weak penetrating power. Seen best in peat substrates, they preferentially penetrated and “congregated” in hollows or low-resistance areas, preferentially avoiding hard substrate elements. Rootlets may have penetrated the substrate initially when of small diameter, subsequently expanding to displace or compress the adjacent substrate. In addition to dissolved minerals, rootlets may have used hydrosoil CO2 (objections noted) as an alternative CO2 source. The abundance of, and hollow spaces within, stigmarian rootlets may have functioned in internal CO2 storage and enhanced the buoyancy of rhizomorphic systems in areas of open water. In soil the stigmarian system created a robust anchor to the substrate. Calculations indicate that massive force would have been needed to cause uprooting. We’ve documented hundreds of lycopsid tree stumps in both surface and underground exposures, but never one uprooted (despite the popularity of fallen trees in ecological reconstructions). Rarely, a fallen trunk is found partially attached to an in situ tree stump. The dead tree-stump/stigmarian system, hollowed out by decay, provided a conduit for air, water, and sediment to enter the substrate, introducing oxygenated surface water and chemicals into deeper, possibly dysoxic parts of the substrate. We conclude that these complex rooting systems played a role in late Paleozoic ecosystems unlike anything seen today. In order to understand those ecosystems, the stigmarian plants need to be understood as dynamic entities, both in life and after death.

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1 - National Museum Of Natural History, DEPT OF PALEOBIOLOGY, NHB MRC 121, Washington, DC, 20560, United States
2 - Royal Botanical Gardens Kew, Jodrell Laboratory, Richmond, Surrey, TW9 3DS, UK
3 - University of California, Berkeley, Integrative Biology, Berkeley, CA, 94720, USA
4 - Illinois State Geological Survey, 615 E. Peabody Drive, Champaign, IL, 61820, USA
5 - Integrative Biology, 3060 Valley Life Sciences Bldg #3140, Berkeley, CA, 94720, United States


Presentation Type: Oral Paper
Session: PL5, Paleobotany: Honoring Fran Hueber - Session III
Location: /
Date: Tuesday, July 20th, 2021
Time: 3:00 PM(EDT)
Number: PL5001
Abstract ID:306
Candidate for Awards:None

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