Volume 238, Issue 4 p. 1695-1710
Full paper

Cretaceous pollen cone with three-dimensional preservation sheds light on the morphological evolution of cycads in deep time

Andres Elgorriaga

Corresponding Author

Andres Elgorriaga

Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS, 66045 USA

Biodiversity Institute, University of Kansas, Lawrence, KS, 66045 USA

Author for correspondence:

Andres Elgorriaga

Email: [email protected]

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Brian A. Atkinson

Brian A. Atkinson

Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS, 66045 USA

Biodiversity Institute, University of Kansas, Lawrence, KS, 66045 USA

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First published: 21 March 2023
Citations: 3

Summary

  • The Cycadales are an ancient and charismatic group of seed plants. However, their morphological evolution in deep time is poorly understood. While molecular divergence time analyses estimate a Cretaceous origin for most major living cycad clades, much of the extant diversity is inferred to be a result of Neogene diversifications. This leads to long branches throughout the cycadalean phylogeny that, with few exceptions, have yet to be rectified by unequivocal fossil cycads.
  • We report a permineralized pollen cone from the Campanian Holz Shale located in Silverado Canyon, CA, USA (c. 80 million yr ago). This fossil was studied via serial sectioning, SEM, 3D reconstruction and phylogenetic analyses.
  • Microsporophyll and pollen morphology indicate this cone is assignable to Skyttegaardia, a recently described genus based on disarticulated lignitized microsporophylls from the Early Cretaceous of Denmark. Data from this new species, including a simple cone architecture, anatomical details and vasculature organization, indicate cycadalean affinities for Skyttegaardia. Phylogenetic analyses support this assignment and recover Skyttegaardia as crown-group Cycadales, nested within Zamiaceae.
  • Our findings support a Cretaceous diversification for crown-group Zamiaceae, which included the evolution of morphological divergent extinct taxa with unique traits that have yet to be widely identified in the fossil record.

Data availability

All S. nagalingumiae fossils are deposited at the University of Kansas Paleobotanical Collections (Biodiversity Institute) in Lawrence, KS, USA. Raw image data and segmentations of S. nagalingumiae are archived in Morphosource (Boyer et al., 2016) under the project title ‘Late Cretaceous cycad pollen cone’. The two morphological matrices (.nex files) can be found both in Supporting Information and Morphobank (Project 4378). Combined (morphology + DNA) matrix can be found in Supporting Information (.tnt file). Pollen cone measurements of extant cycads can be found in Supporting Information.