Fossil wood is one of the crucial proxies for understanding terrestrial vegetation composition and development in the Earth’s history. Before the advent and radiation of angiosperms, Mesozoic forests were dominated by flourishing gymnosperms, including bennettitaleans, cycads, ginkgos, and conifers. The well-defined fossil wood genus Xenoxylon is a significant member of the Mesozoic gymnosperm flora. This genus was established by the German scientist Gothan in 1905, nearly 120 years ago. However, its exact classification within the gymnosperms has been a puzzle for the paleobotanical community for a long time. Hence, clarifying its systematic classification position is very important and has always been a focus of paleobotanists.
Recently, the international journalJournal of Systematics and Evolution(JSE ) published a new result completed by a collaborative team of paleobotanists from China, Germany, and the Czech Republic, providing new scientific evidence to solve the century-long mystery of the systematic classification of Xenoxylon fossils. This study innovatively integrated datasets of gene, wood anatomy, and biomolecular characteristics for the first time to perform a phylogenetic analysis of Xenoxylon, which proposes that this genus has a close phylogenetic relationship with the extant conifer family Podocarpaceae.
Xenoxylon fossils are characterized by the strongly compressed radial tracheid pits and window-like cross-field pits in the secondary xylem (Figure 1). They are widely distributed across the Laurasia, with 22 verified species of Xenoxylon described. To understand its systematic classification position, the research team conducted interdisciplinary innovative studies. Three integrated datasets on genetic characteristics, wood anatomical structure, and biomolecular composition characteristics (such as biomarkers), from Xenoxylon and five extant conifers families were generated. These families include Araucariaceae, Cupressaceae, Pinaceae, Podocarpaceae, and Sciadopityaceae. Due to the lack of DNA in Xenoxylon fossils, the genetic sequences of Xenoxylon were coded as missing data in this analysis as commonly used for extinct taxa. Using maximum parsimony in Tree Analysis using New Technology (TNT), Xenoxylon appeared basal to Araucariaceae in the data set combining genes and xylological characters, whereas Xenoxylon is placed next to Podocarpaceae in the data set combining genes and biomolecular characters. To find a reliable systematic placement of Xenoxylon, a combined data set of genes, xylological and biomolecular characters is analyzed. The results and interpretations indicate that Xenoxylon is closely related to Podocarpaceae (Figure 2). This represents the first phylogenetic analysis of Xenoxylon, and fills the knowledge gap of the systematic relationship of this taxon, and contributes to a better understanding of the evolution of extant conifer Podocarpaceae.
Prof. Dr. Yongdong Wang from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences and Dr. Aowei Xie at the Senckenberg Forschungsinstitut und Naturmuseum Frankfurt, are the co-corresponding authors for this study. Dr. Shook Ling Low (Institute of Botany, Czech Academy of Sciences), Prof. Dr. Ning Tian (College of Palaeontology, Shenyang Normal University), and Prof. Dr. Dieter Uhl (Senckenberg Forschungsinstitut und Naturmuseum Frankfurt) jointly completed the research. This work was co-sponsored by the National Natural Science Foundation of China, Strategic Priority Research Program (B) of the Chinese Academy of Sciences, and the State Key Laboratory of Palaeobiology and Stratigraphy. Open Access funding enabled and organized by Projekt DEAL.
Reference: Xie A.*, Low S.L., Wang Y.*, Tian N., Uhl D., 2024. Novel phylogenetic analysis of the Mesozoic common gymnosperm Xenoxylon Gothan reveals close affinity with extant Podocarpaceae (Coniferales). Journal of Systematics and Evolution. https://doi.org/10.1111/jse.13132
Figure 1. The xylological characters of radial tracheid pits and cross-field pits in species of Xenoxylon. A–C, Xenoxylon kazuoense Xie, Wang, Tian et D.Uhl in the Lower Cretaceous Jiufotang Formation of western Liaoning, China (Xie et al., 2024). D–F, Xenoxylon utahense Xie et Gee in the Upper Jurassic Morrison Formation of northeastern Utah, USA (Xie et al., 2021). G–I, Xenoxylon guangyuanense Tian, Wang et Philippe in the Upper Triassic Xujiahe Formation of northern Sichuan, China (Tian et al., 2016). Red arrows show the window-like cross-field pitting. Scale bar =100μm.
Figure 2. Majority‐rule consensus tree for phylogenetic analysis of Xenoxyloninferred from combined xylological, biomolecular, and genetic characters analysis.
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