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

Abstract

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.

Reconciling fossils with phylogenies reveals the origin and macroevolutionary processes explaining the global cycad biodiversity

“…The relationships of the main Mesozoic taxa are still poorly understood (Hermsen et al ., 2006 ; Coiro & Pott, 2017 ), and it remains unclear whether many genera represent biological rather than purely taxonomic units (Pott et al ., 2007 ). Fossil cycads representing reproductive structures are rare and have been often overlooked or have little or unclear phylogenetic value, though better‐preserved specimens have led to fruitful insights (Spencer et al ., 2017 ; Rothwell et al ., 2022 ; Elgorriaga & Aktinson, 2023 ). Although the leaf record is much more abundant, it is also poorly understood.…”

Zirabia cylindrica comb. nov. provides evidence of Doyleales in the Jurassic

“…Recent paleobotanical studies have characterized a striking diversity of exceptionally well‐preserved gymnosperm reproductive structures from Early Cretaceous deposits (e.g., Rothwell and Stockey, 2016; Herrera et al, 2017; Friis et al, 2019; Matsunaga et al, 2021; Klymiuk et al, 2022; Elgorriaga and Atkinson, 2023). These studies are ushering in a renaissance in the research of Mesozoic gymnosperms, shedding much light on their diversity and evolution through time, including plants with curved seeds that are enclosed within “cupules” (Rothwell and Stockey, 2016; Shi et al, 2021, 2022).…”

Sucrose promotes cone enlargement via the TgNGA1‐TgWRKY47‐TgEXPA2 module in Torreya grandis

“…After fertilization, the cone either enters a dormant state or undergoes rapid enlargement, which is crucial for seed production, plant reproduction, and seed yield in gymnosperms (Friedman, 2015;Kuhn et al, 2023). Despite its importance, our knowledge of cone development over the past century has largely been confined to gametophyte development and changes in anatomical structures observed in various species, including Ginkgo biloba, Araucaria angustifolia, and others (Florin, 1951;Owens & Moulder, 1977;Owens, 1984;Friedman, 2015;D'Apice et al, 2022;Elgorriaga & Atkinson, 2023;Kuhn et al, 2023). Over the past few years, the roles of several proteins potentially involved in cone development have been identified, including MADS-box, AGAMOUS-LIKE6, and CONSTANS-like proteins (Carlsbecker et al, 2013;Wang et al, 2020Wang et al, , 2022Yang et al, 2024).…”