Geochemical Support for a Climbing Habit within the Paleozoic Seed Fern Genus Medullosa

Abstract

A long-standing problem in paleobotany is the accurate identification of the growth habits and statures of fossil plants. Tissue-specific analysis of stable carbon isotope ratios in plant fossils can provide an independent perspective on this issue. Lignin, a fundamental biopolymer providing structural support in plant tissues and the second most abundant organic material in plants, is 13 C depleted by several parts per thousand, averaging 4.1&, relative to other plant constructional materials (e.g. cellulose). With this isotopic difference, the biochemical structural composition of ancient plants (and inferred stature) can be interrogated using microscale in situ isotope analysis between different tissues in fossils. We applied this technique to a well-preserved specimen of the Late Paleozoic seed plant Medullosa, an extinct genus with a variety of growth habits that includes several enigmatic yet abundant small-stemmed species widely found in calcium carbonate concretions (''coal balls'') in the Pennsylvanian coal beds of Iowa, USA. It remains unclear which of the medullosans were freestanding, and recent analysis of the medullosan vascular system has shown that this system provided little structural support to the whole plant. The leading hypothesis for small-stemmed medullosan specimens predicts that cortical tissues could have provided additional structural support, but only if they were lignified. The expected isotopic difference between lignified tissue and unlignified tissue is smaller than that expected from pure extracts, for the simple reason that even woody tissues maximally contain 40% lignin (by mass). This reduces the expected maximum difference between weakly and heavily lignified tissues by 60%, down to ;0.5&-2&. Analysis of the medullosan stem reveals a consistent difference in isotope ratios of 0.7&-1.0& between lignified xylem and cortical tissues. This implies low abundances of lignin (between 0% and 11%) within the cortex. This inferred structural biochemistry supports hypotheses that the peripheral portions of these medullosan stems were not biomechanically reinforced to permit the plants to grow as freestanding, arborescent trees. A number of climbing or scandent medullosans have been identified in the fossil record, and this mode of growth has been suggested to be common within the group on the basis of observations from comparative biomechanics, hydraulics, and development. Finally, this mode of growth is common in several clades of stem group seed plants, including Lyginopteris and Callistophyton, along with Medullosa. This study provides further support for ideas that place a great portion of early seed plant diversity under the canopy, rather than forming it.

The evolution of angiosperm lianescence: a perspective from xylem structure‐function

“…The tracheid structure of medullosan species, including large pit membrane areas and a high ratio of tracheid diameter to wall thickness, suggests that their vascular system provided little structural support to the whole plant, with support instead found in sheathing leaf bases. The recent finding that the cortex tissues of medullosan species exhibit very low lignin content, supports the idea that they were unable to grow free-standing (Wilson & Fisher 2011). The stem-leaf base structure of medullosan species somehow recall rattan palms, where leaf sheaths provide mechanical support and the senescence of sheathing leaf bases might release the flexible stem.…”

“…The only vessel-bearing lineage of extant gymnosperm, the Gnetales, comprise about 33 climbing species in the genus Gnetum (Won & Renner 2006). Some fern groups, such as Trichomanes (Dubuisson et al 2003) or the Lycopsids (Rowe & Speck 1997, 1998, or the extinct seeds ferns Medullosales (Dunn et al 2003;Krings & Kerp 2006;Wilson & Fisher 2011) provide examples of the climbing habit in tracheid-bearing species.…”

Wetland-Dryland Vegetational Dynamics in the Pennsylvanian Ice Age Tropics

“…A highly diverse and ecologically differentiated group of plants based on studies of compression foliage, they also comprise a variety of growth architectures, including lianas and small trees ( fig. 9D) of various configurations (Pfefferkorn et al 1984;Dunn et al 2003;DiMichele et al 2006a), a great many of which were likely ground cover or subcanopy plants (Wilson and Fischer 2011;Raymond et al 2013). Found in both clastic and peat substrates (Cleal and Shute 2012), much greater diversity has been described in the former, and no species have yet been described that are specific to peat substrates.…”

“…Thus, although tied to wetlands, possibly the higher nutrient parts thereof, the medullosans seemingly did not have the ability to colonize drier habitats or were limited in dispersal ability. The lyginopterids comprise several genera of small woody plants, including liana and thicket-forming and scrambling habits, typified by various morphological climbing aids (Krings et al 2003), and may have formed a significant component of the ground cover in many parts of the wetland habitat (Wilson and Fischer 2011;Tenchov 2012). They are common, diverse, and widespread, known from both mineral substrates and peat swamps, particularly in the Early and Middle Pennsylvanian, declining in the Late Pennsylvanian (Cleal 2008a).…”

Palaeoecology of Macroneuropteris scheuchzeri, and its implications for resolving the paradox of ‘xeromorphic’ plants in Pennsylvanian wetlands

“…Both of these traits are found in modern lianas (Burnham, 2009;Isnard and Silk, 2009). Though having limited wood to provide support, cortical sclerenchyma in stems and leaves appears to have offered structural support in many medullosan species, a conclusion also derived from biogeochemical studies of medullosan tissues (Wilson and Fischer, 2011). Theses morphological bases of inference are supplemented by historical-stratigraphic patterns; a vine-like habit has been inferred for the earliest medullosans (Dunn et al, 2003;Burnham, 2009) and these growth forms were carried forward in the clade as the medullosans diversified.…”