Fossil cutin of Karinopteris (Middle Pennsylvanian pteridosperm) from the “paper” coal of Indiana, U.S.A.

For the first time, a cutin-like, highly chemically resistant macropolymer has been isolated from rachises of Karinopteris sp. (lyginopteridalean pteridosperm, Middle Pennsylvanian). Samples are obtained from a cuticular or “paper” coal-shale, i.e., an organic-rich and highly clastic rock associated with the Upper Block Coal Member of the Brazil Formation, Parke County, west-central Indiana, U.S.A. Karinopteris specimens are preserved as naturally oxidized compressions, termed “fossilized cuticles”, and possibly represent vegetation of mineral substrate environments. Employing laboratory oxidation reactions, the fossilized cuticle of Karinopteris rachises is used to obtain the cuticle. After additional and long-term oxidation treatment, the cuticle yields the cutin-like macropolymer, here referred to as “cutin” for simplicity. The fossilized cuticle, cuticle, and cutin samples of Karinopteris sp. are chemically analyzed using semi-quantitative Fourier transform infrared (FTIR) spectroscopy. Cutin IR spectra of Karinopteris rachises are characterized by (a) a predominantly aliphatic composition as indicated by intense aliphatic (CH_al) CH stretching peaks at 3000–2700 cm⁻¹, which are assigned to methylene (CH₂) and methyl (CH₃) groups; (b) carbonyl (C=O) groups at 1730–1640 cm⁻¹, and aromatic carbon (C=C) absorption bands at 1600–1500 cm⁻¹. A comparison with the cuticle, the cutin stands out due to relatively higher values of CH₂/CH₃ and C=O/C=C, while displaying notably low values of CH_al/C=O and CC contribution. Specifically, the relatively low value of CH_al/C=O ratio obtained for the cutin of Karinopteris rachises is consistent with those found in the cutin of extant and fossil leaves. This lower CH_al/C=O ratio indicates the important role likely played by CO groups in creating a deformable and flexible structure in both the cutin and the cuticle. Such a reduced rigidity suggests a high level of rachis flexibility of the once-living Karinopteris plant, supporting the interpretation of a climbing or liana habit. Cutin isolation and its chemical characterization shed light on the probable biomechanical (flexibility) properties of Karinopteris rachises, thereby enhancing our understanding of the plant growth habit.