# Evidence of aquatic adaptation in the early Carboniferous: study of aerenchyma in Periastron reticulatum and a new cladoxylopsid from the Montagne Noire (France)

Selhma Froul, Anne-Laure Decombeix, Thibault Durieux

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Selhma Froul, Anne-Laure Decombeix, Thibault Durieux. Evidence of aquatic adaptation in the early Carboniferous: study of aerenchyma in Periastron reticulatum and a new cladoxylopsid from the Montagne Noire (France). 9. Agora Paleobotanica meeting, Agora Paleobotanica, Jul 2025, Paris, France. hal-05184201

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https://hal.inrae.fr/hal-05184201v1

Submitted on 24 Jul 2025

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# Evidence of aquatic adaptation in the early Carboniferous: study of aerenchyma in *Periastron reticulatum* and a new cladoxylopsid from the Montagne Noire (France)

Selhma FROUL<fnref n="1" />*, Anne-Laure DECOMBEIX<fnref n="1" />, Thibault DURIEUX<fnref n="2" />

Aerenchyma is a plant tissue that enables internal gas transport. It forms either as part of the normal development or in response to environmental stimuli, and is considered an adaptation to hypoxic conditions (submersion of all or part of the plant). It can develop either schizogenously, through cell separation and expansion of intercellular spaces, or lysogenously, via apoptosis, leading to the formation of air-filled lacunae within the cortex of stems, roots, and/or leaves in extant plants <fnref n="1" />. Fossil evidence for the emergence of aerenchyma remains scarce. The earliest known occurrences date from the Early Devonian, in the cortex of the lycopsid *Asteroxylon* from the Rhynie Cherts <fnref n="2" />, and from the Late Devonian in the stem of the iridopterid *Keratophyton* from Australia ³, in the stem of the cladoxylopsid *Pietzschia levis* from Morocco ⁴, as well as in the petiole of *Periastron reticulatum* and *Aerocortex kentuckiensis*, which are known from the Late Devonian to the early Carboniferous in Kentucky, Germany, and France ⁵. In this context, the present study focuses on the presence of aerenchyma in several early Carboniferous specimens from the Lydiennes Formation (Montagne Noire, France). This particularly well developed aerenchyma has been observed in the cortex of a stem or branch attributed to a new cladoxylopsid related to *Pietzschia levis*, and in the petiole of *Periastron reticulatum*. The presence of aerenchyma in a foliar organ suggests an adaptation to an aquatic environment ⁶, implying that the plant was at least partially submerged. These findings provide new insights into the evolution of aerenchyma and of the aquatic habit in vascular plants, and enhance our understanding of early Carboniferous floras in France.

1. Jung, J., Lee, S. C., & Choi, H.-K. (2008). Anatomical patterns of aerenchyma in aquatic and wetland plants. Journal of Plant Biology, 51(6), 428-439. https://doi.org/10.1007/BF03036065

2. Kidston, R., & Lang, W. H. (1920). XXIV.On Old Red Sandstone Plants showing Structure, from the Rhynie Chert Bed, Aberdeenshire Part III. Asteroxylon Mackiei, Kidston and Lang. Earth and Environmental Science Transactions of The Royal Society of Edinburgh, 52(3), 643-680. https://doi.org/10.1017/S0080456800004506

3. Champreux, A., Meyer-Berthaud, B., & Decombeix, A.-L. (2020). Keraphyton gen. Nov., a new Late Devonian fern-like plant from Australia. PeerJ, 8, e9321. https://doi.org/10.7717/peerj.9321

4. Soria, A., Meyer-Berthaud, B., & Scheckler, S. E. (2001). Reconstructing the Architecture and Growth Habit of Pietzschia levis sp. Nov. (Cladoxylopsida) from the Late Devonian of Southeastern Morocco. International Journal of Plant Sciences, 162(4), 911-926. https://doi.org/10.1086/320773

5. Beck, C. B. (1978). Periastron Reticulatum Unger and Aerocortex Kentuckiensis, n. Gen. et sp., from the New Albany Shale of Kentucky.

6. Decombeix, A.-L. (2024). Down by the water-Aerenchyma in Devonian and early Carboniferous plants. Xth International Organization of Palaeobotany Congress, Prague, Czech Republic.

<footnote n="1">¹ UMR AMAP, Univ. Montpellier, CNRS, CIRAD, INRA, IRD, Montpellier, France</footnote>

<footnote n="2">² Trinity College Dublin, Dublin, Ireland</footnote>

<footnote>* selhafroul@outlook.fr</footnote>