Abstract
Previously unrecognized anatomical features of the cone scales of the enigmatic Early Cretaceous conifer Krassilovia mongolica include the presence of transversely oriented paracytic stomata, which is unusual for all other extinct and extant conifers. Identical stomata are present on co-occurring broad, linear, multiveined leaves assigned to Podozamites harrisii, providing evidence that K. mongolica and P. harrisii are the seed cones and leaves of the same extinct plant. Phylogenetic analyses of the relationships of the reconstructed Krassilovia plant place it in an informal clade that we name the Krassilovia Clade, which also includes Swedenborgia cryptomerioides–Podozamites schenkii, and Cycadocarpidium erdmanni–Podozamites schenkii. All three of these plants have linear leaves that are relatively broad compared to most living conifers, and that are also multiveined with transversely oriented paracytic stomata. We propose that these may be general features of the Krassilovia Clade. Paracytic stomata, and other features of this new group, recall features of extant and fossil Gnetales, raising questions about the phylogenetic homogeneity of the conifer clade similar to those raised by phylogenetic analyses of molecular data.
Highlights
Excluding angiosperms, conifers are the most diverse group of living seed plants, with approximately 638 species [1]
We present new evidence on the anatomy of the seed cone scales of Krassilovia, including the presence of distinctive transversely oriented paracytic stoma that is unusual among conifers, and that help to identify Podozamites harrisii as the leaves of the Krassilovia plant
We review the characteristics of this interesting group of putative conifers and assess the potential systematic implications, especially in regard to the hypothesized close relationship between extant Pinaceae and Gnetales based on molecular data
Summary
Conifers are the most diverse group of living seed plants, with approximately 638 species [1]. Since the taxonomic separation of Ginkgo more than a hundred years ago, e.g., [2], conifers have been regarded as a single higher taxonomic unit, e.g., [3, 4] and as a monophyletic group in phylogenetic analyses based on morphological data, e.g., [5–11]. Krassilovia mongolica for the Promotion of Science to M.T. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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