The largest known (365 mm-long) specimen of Odontopteris cantabrica Wagner from the Late Pennsylvanian Sydney Coalfield in Canada was re-examined as part of the on-going research project "Chemistry and Architecture of Carboniferous Seed Ferns" to refine the 3D plant-reconstruction concept further in the framework of natural classification. The micromorphology and functional-group content and distribution of O. cantabrica are investigated by methods of Fourier transform infrared spectroscopy. Results demonstrate that the specimen is naturally macerated, showing little or no cellular features. Two types of stomatal structures occurred, however: anomocytic on abaxial pinnule surfaces, and cyclocytic amphistomatically on the rachises. Hence, the frond was entirely photosynthetic. Chemometric analysis of the infrared data resulted not only in a 3D chemical model, but also confirmed the hypothesis of the chemical-architectural relationship in extinct medullosalean fronds. Further demonstrated is the predictive power of chemometrics for reconstruction, hence frond interpretation, in the absence of certain frond parts. Inferred from the collective data is a basally bifurcate and larger frond, of larger size than previously believed, which could indicate a self-supporting (arborescent) or semi self-supporting habit for the O. cantabrica plant. The concept of natural classification for plant fossils, underpinned by a holistic data approach, is proposed based on data from Neuropteris ovata var. simonii, Alethopteris ambigua, and O. cantabrica.