Cretaceous gnetalean yields first preserved plant gum

Abstract

Some liquid plant exudates (e.g. resin) can be found preserved in the fossil record. However, due to their high solubility, gums have been assumed to dissolve before fossilisation. The visual appearance of gums (water-soluble polysaccharides) is so similar to other plant exudates, particularly resin, that chemical testing is essential to differentiate them. Remarkably, Welwitschiophyllum leaves from early Cretaceous, Brazil provide the first chemical confirmation of a preserved gum. This is despite the leaves being exposed to water twice during formation and subsequent weathering of the crato formation. the Welwitschiophyllum plant shares the presence of gum ducts inside leaves with its presumed extant relative the gnetalean Welwitschia. This fossil gum presents a chemical signature remarkably similar to the gum in extant Welwitschia and is distinct from those of fossil resins. We show for the first time that a water-soluble plant exudate has been preserved in the fossil record, potentially allowing us to recognise further biomolecules thought to be lost during the fossilisation process. A wide variety of vascular plants produce fluid exudates 1 e.g. resins and gums, with each group differing in chemical definitions (Table 1). Due to similarity in physical appearance distinguishing exudates based on chemistry is vital, for example gums and resins are visually similar resulting in these terms being used interchangeably 1. However, their chemical definitions are very different (Table 1); resins are composed of lipid-soluble terpenoids 1,2 , while gums are complex, highly branched (non-starch) water-soluble polysaccharides 3. A common example of this misunderstanding is the Eucalyptus tree, which is known as a gum tree, but nuclear magnetic resonance analysis of the Eucalyptus exudate shows its composition to be polyphenolic and is therefore actually a kino 4 (Table 1). Differences between gum and resin can also be seen in the functional roles within the plant. The main roles of resins are to respond to wounding, as a defence against pathogens and to dissuade herbivory by insects and other organisms 2,5,6. Gum is involved in food storage, structural support, and also for wound sealing, but there is no common role across taxa 3. Further confusion arises as some plants, e.g. Boswellia and Commiphora species, even produce exudates with a mixture of polysaccharide and resin components (the gum resins myrrh and frankincense respectively) 1. Until now only fossilised plant resin (ambers) 7 and latex filaments have been reported preserved in the fossil record 8,9. While the fossilisation of fluid exudates might seem unlikely, the fossilisation of resin is relatively common, and extends back some 320 million years to the Carboniferous 10 , but chemically confirmed gums have never been reported. The Early Cretaceous (~120 million year old) Crato Formation 11 of northeast Brazil (Supplementary Fig. S1) is a well-known laminated limestone deposit that yields exceptionally preserved vertebrates, arthropods, and plants from the ...

Reinvestigating the fossil leaf Welwitschiophyllum brasiliense Dilcher et al. (2005), from the Lower Cretaceous Crato Formation of Brazil

“…When analysed using FTIR and ATR they show a gum signal remarkably similar to that of extant Welwitschia gum (Fig. 5) (Roberts et al, 2020). In specimen UOP-PAL-MC0003 there are no remains of gum present, however, regular voids infilled with crystals towards the adaxial surface may represent ducts where the gum has been dissolved and infilled (Fig.…”

“…The spectra generated for the two gums are almost indistinguishable from each other and share key features in their spectra (a very large hydroxyl peak at ~3400 cm -1 , peak absence at 1516 cm -1 and a very strong peak at 1077 cm -1 (Fig. 5, Roberts et al, 2020). Further characterization of the gum is planned.…”

“…Recently the first example of gum found preserved in the fossil record was discovered in the leaves of Welwitschiophyllum (Roberts et al, 2020). FTIR analysis of this gum revealed it to be remarkably similar to that present in leaves of Welwitschia (Roberts et al, 2020).…”

“…Recently the first example of gum found preserved in the fossil record was discovered in the leaves of Welwitschiophyllum (Roberts et al, 2020). FTIR analysis of this gum revealed it to be remarkably similar to that present in leaves of Welwitschia (Roberts et al, 2020). The spectra generated for the two gums are almost indistinguishable from each other and share key features in their spectra (a very large hydroxyl peak at ~3400 cm -1 , peak absence at 1516 cm -1 and a very strong peak at 1077 cm -1 (Fig.…”

“…they occur regularly, and even have a principle and a secondary duct arrangement (Fig. 4D and Roberts et al, 2020).…”