Molecular signatures of fossil leaves

Leaves of Ptilophyllum mueller, from Emmaville, New South Wales. Scale bars=10 mm (From McLoughlin et al., 2011)

The first plants colonized land approximately 450 million years ago. The transition from an exclusively aquatic to a terrestrial life style implied the evolution of a new set of morphological and physiological features. The most critical adaptive trait for survival during terrestrialization was the ability to retain water in increasingly dehydrating habitats. Consequently, the capacity to maintain a hydrophobic surface layer, or cuticle, over the surfaces of aerial organs was arguably one of the most important innovations in the history of plant evolution.

Spores, pollen and leaf cuticles, are among the most resilient organic structures in the geological record. These components may retain some phylogenetically unique signals, not only in well-preserved fossils, but also in remains with a high level of diagenetic maturity.

Ginkgo biloba, Eocene fossil leaf from the Tranquille Shale of MacAbee, British Columbia, Canada (From Wikipedia Commons)

Generally, the cuticle is divided into two major structural constituents: cutin and cutan. The fatty acid polyesters which constitute cutin, gives the cuticle considerable resistance to biodegradation. Cutan is a non-ester and non-hydrolyzable matrix of aliphatic compounds linked by ether bonds, which remain after cutin hydrolysis. Additionally, the surface of the cuticle may be covered by various long-chain hydrophobic waxes. All these components  favours the survival of the cuticle in many fossil plants, and can be used to resolve the stratigraphic ranges and relationships of extinct plants.

Data from infrared spectroscopy of modern plant cuticles, have been used successfully to support and clarify the species-level taxonomy of extant plants, for example, in Camellia and angiosperm pollen. Using infrared spectroscopy and statistical analysis, researchers at Lund University, the Swedish Museum of Natural History in Stockholm, and Vilnius University, analysed a selection of fossil Cycadales, Ginkgoales and conifers. The team obtained two major groups in the dendrogram of infrared spectra. One branch unites podocarpacean and araucariacean conifers (excluding the Jurassic Allocladus). A relationship consistent with all modern phylogenetic analyses of gymnosperm. The second branch unites a broad range of gymnosperms. Within this branch, Bennettitales (Otozamites and Pterophyllum) form a well-defined group in association with Ptilozamites and Nilssoniales. This cluster is linked to a group incorporating Cycadales on one sub-branch, and Leptostrobales, Ginkgoales and the putative araucariacean Jurassic conifer Allocladus on a second sub-branch.

 

Dendrogram based on HCA of infrared absorption spectra of an expanded group of 13 fossil gymnosperm taxa (From Vajda et al., 2017)

Early palaeobotanical studies generally linked Bennettitales to Cycadales, but more recent anatomical studies and cladistic analyses have indicated that Bennettitales are not closely related to Cycadales. By contrast, Bennettitales, Nilssoniales and Ptilozamites are grouped closely. Additionally,  the systematic position of Allocladus within Araucariaceae should be reassessed based on its close association with Ginkgo in the cluster analysis of infrared spectra.

References:

Vivi Vajda, Milda Pucetaite, Stephen McLoughlin, Anders Engdahl, Jimmy Heimdal, Per Uvdal. Molecular signatures of fossil leaves provide unexpected new evidence for extinct plant relationships. Nature Ecology & Evolution, 2017; DOI: 10.1038/s41559-017-0224-5

Stephen McLoughlinRaymond J. Carpenter, and Christian Pott, Ptilophyllum muelleri (Ettingsh.) comb. nov. from the Oligocene of Australia: Last of the Bennettitales?, International Journal of Plant Sciences 2011 172:4574-585, DOI: 10.1086/658920

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