A odd “cycad” is an unexpected angiosperm from the Early Cretaceous of Patagonia

Recently a new chapter of my reinvestigation of material from the Early Cretaceous of Argentina has been published in the journal New Phytologist.

This is the second work stemming from my visit to the Natural History Museum in London (funded by the marvelous SYNTHESYS scheme), where I had the opportunity give a new look at the many cycad taxa described by Sergio Archangelsky from the Baqueró group in Patagonia (for the first work, see here).

Among these fossils, there was a very peculiar taxon named Mesodescolea plicata. This fossil leaf presents deep incisions in its margin (lobes), with each section having multiple teeth. The vein pattern is unlike many other plants, with veins ‘fusing’ in a very irregular net. This fossil leaf was first tentatively assigned to the cycads, and later reinvestigation seemed to indicate a relationship with the South African genus Stangeria.

1
Mesodescolea plicata, specimen LPPB 5126, showing the unusual lobed morphology of this leaf. Scale bar = 1 cm.  Picture by Leandro Martinez.

During my doctoral work, I had the opportunity to look at the anatomy of Stangeria in great detail, and after observing the cuticle of Mesodescolea using both light and confocal microscopy I realized that this taxon had only superficial similarities with the South African cycad. On the other hand, many characters of Mesodescolea would suggest an angiosperm affinity.

f
Cuticle of Mesodescolea plicata observed using Confocal Microscopy, showing the presence of laterocytic stomatal complexes with guard cells with raised poles. Specimen v44684. Scale bar = 50 μm.

Together with Leandro Martinez (who observed and photographed specimens in Argentinian collections and investigated the ultrastructure of the cuticle), Garland Upchurch and James Doyle (who helped with their knowledge of cuticular anatomy of fossil and extant angiosperms and their early evolution), we decided to reinvestigate this taxon more in detail. In the resulting article, we discuss the angiosperm interpretation and we test the placement of Mesodescolea within the angiosperms in three different backbones topologies, using exploratory methods (such as RoguePlots) and formal Bayesian hypothesis testing to understand the uncertainty in the analyses. The strongest placements appear to be either in Austrobaileyales or Chloranthales.

rogue
RoguePlots showing the potential placements of Mesodescolea on a backbone tree of the angiosperms.

So why is Mesodescolea so interesting?

First, it presents an unexpected combination of traits for a member of the ANITA grade. Extant Austrobaileyales and Chloranthales have entire leaves or leaves with toothed margins. This indicates that the morphology of the extant members of these groups is not entirely predictive of the morphology of their fossil relatives, and that extinction and other processes have removed many unique morphological combinations.

Second, it shares many traits with coeval leaves that had been previously assigned to the Eudicots. These leaves could have indeed been produced by members of a related group, a completely extinct lineage of angiosperms that originated during the early radiation of the flowering plants.

Third, it is a locally abundant angiosperm leaf in a rich flora mostly dominated by gymnosperms. Though the Anfiteatro de Ticó presents a relatively high diversity of gymnosperms and ferns, angiosperms are still quite rare. Mesodescolea would be the most abundant angiosperm in the flora, and one of the most abundant in South America during the Aptian.

I hope that this finding will help to stimulate further research in the unknowns of the Early Cretaceous radiation of the angiosperms, and reignite the study of problematic fossils with an uncertain placement that could illuminate the patterns and processes of the origin of the flowering plants.

References

Coiro, M., Martínez, L.C., Upchurch, G.R. and Doyle, J.A., 2020. Evidence for an extinct lineage of angiosperms from the Early Cretaceous of Patagonia and implications for the early radiation of flowering plants. New Phytologist, 228(1), pp. 344-360.
Archangelsky, S., 1963. A new Mesozoic flora from Ticó, Santa Cruz Province, Argentina (Vol. 8, No. 2). British Museum (Natural History).
Artabe, A.E. and Archangelsky, S., 1992. Las Cycadales Mesodescolea Archangelsky emend. Archangelsky y Petriella 1971 (Cretácico) y Stangeria Moore (Actual). Ameghiniana, 29(2), pp.115-123.
Passalia, M., Archangelsky, S., Romero, E. and Cladera, G., 2014. A new early angiosperm leaf from the anfiteatro de Tico Formation (Aptian), Santa Cruz Province, Argentina. Revista del Museo Argentino de Ciencias Naturales nueva serie, 5(2), pp.245-252.
Romero, E.J., Archangelsky, S. and Passalia, M.G., 2016. Two new angiosperm leaf morphotypes from the Anfiteatro de Ticó Formation (mid-Aptian) Santa Cruz Province, Argentina. Review of Palaeobotany and Palynology, 235, pp.148-156.

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