 |
| Krassiloviaceae Herrera, Shi, Mays, Ichinnorov, Takahashi, et al., 2020.
Krassilovia mongolica
Herrera, Shi, Leslie, Knopf, Ichinnorov, et al.,, 2015 a branch of Krassilovia mongolica bearing terminal seed cones and alternately arranged leafy shoots of Podozamites harrisii. Mature and maturing cones are depicted distally showing the ultimate disarticulation of the bract-scale complexes and the dispersal of the winged seeds.
in Herrera, Shi, Mays, Ichinnorov, Takahashi, et al., 2020.
Illustration: Pollyanna von Knorring.
|
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.
 |
| Fig 1. Seed cones, cone axis, bract-scale complexes, and winged seeds of Krassilovia mongolica and associated leaves of Podozamites harrisii. (A–C) Articulated seed cones showing tightly imbricate interlocking bract-scale complexes (A: PP55848; B: PP59064; C: PP59065). (D) Isolated cone axis; note conspicuous spirally arranged abscission scars (PP59066). (E) Incomplete leafy shoot showing a cluster of three attached leaves (one represented only by the leaf base); the fourth leaf (left) was attached to the axis when discovered (PP56218). (F) Three detached strap-shaped leaves; note variation in leaf size and shape, and conspicuous parallel venation (PP56226; PP56223; PP56222). (G) Detail of A showing tightly imbricate interlocking bract-scale complexes. (H) Detail of leaf apex showing converging veins (left; PP56228); leaf base showing the absence of a clearly differentiated petiole (right; PP56230). (I) Three isolated bract-scale complexes showing abaxial (top) and adaxial (bottom) surfaces; note three prominent, spiny, distal lobes and two prominent, spiny, proximal lobes (PP59067; PP59068; PP59069). (J) Two isolated seeds showing narrow wings and variation from more or less symmetrical (top; PP59070), to strongly asymmetrical (bottom; PP59071).
Scale bars: E, F = 1 cm; A–C, G = 5 mm; D, H, I = 2 mm; J = 1 mm.
|
 |
| Fig 6. Reconstruction of Krassilovia mongolica. (A) Complete mature seed cone showing the strongly imbricate spiny bract-scale complexes. (B) Isolated bract-scale complex in adaxial view showing five seed scars (top left), isolated bract-scale complex in adaxial view with five seeds (top middle), isolated bract-scale complex in abaxial view showing the inconspicuous leafy bract (top right); isolated bract-scale complexes in lateral view showing seed scars and leafy bract (bottom). (C) Isolated asymmetrical (top) more or less symmetrical (bottom) winged seeds. (D) Isolated seed cone axes showing prominent abscission scars
. Drawings not to scale. Illustration: Pollyanna von Knorring.
|
 |
| Fig 7. Reconstruction of a branch of Krassilovia mongolica bearing terminal seed cones and alternately arranged leafy shoots of Podozamites harrisii. Mature and maturing cones are depicted distally showing the ultimate disarticulation of the bract-scale complexes and the dispersal of the winged seeds.
Illustration: Pollyanna von Knorring.
|
Systematics and nomenclature
Class: Coniferopsida
Order: Voltziales
Family: Krassiloviaceae Herrera et al. fam. nov.
Type: Krassilovia Herrera, Shi, Leslie, Knopf, Ichinnorov, Takahashi, Crane et Herendeen. Int. J. Plant Sci. 176:793, 2015.
Krassilovia mongolica Herrera, Shi, Leslie, Knopf, Ichinnorov, Takahashi, Crane et Herendeen. (Figs 1–4 and 6–7).
Familial diagnosis: Leaves distichously arranged on slender deciduous shoots, borne helically on small persistent leaf cushions, but flattened into a single plane by twisting of their bases. Leaves narrowly oblong to strap-shaped, with multiple conspicuous veins. Seed cone with helically arranged, imbricated, and tightly interlocked bract-scale complexes on a slender central axis. Each bract-scale complex consisting of an inconspicuous bract partially fused to the stalk of an ovuliferous scale. Ovuliferous scale with five conspicuous spine-tipped lobes; three distal (always pointing away from the cone base), the other two proximal (always pointing toward the cone base). Bract scale complexes bearing three to five winged seeds. Leaves and bract-scale complexes with thin, delicate cuticles. Outlines of epidermal cells frequently arranged in two pairs, sometimes forming quartets. Stomatal complexes of both organs transversely oriented, paracytic and monocyclic. Outlines of the guard cells not sunken, flanked by the outlines of two lateral subsidiary cells.
Note: The family includes the seed cone genus Krassilovia and the leaf species Podozamites harrisii Shi, Herrera, Herendeen, Leslie, Ichinnorov, Takahashi et Crane.
Conclusions:
Krassilovia and the Krassilovia Clade suggest the need to reevaluate current models of conifer evolution and reassess the significance of unusual morphological traits in living and fossil conifers. Current concepts of “conifers” as an evolutionary meaningful group may have been unduly influenced by their simple leaves. Furthermore, their other potential unifying feature, the compound ovulate shoot, is not diagnostic and occurs in other groups of living and fossil plants. In our cladistic analyses, the Krassilovia Clade appears to be close to the conifer crown group but it likely evolved from a paraphyletic and diverse assemblage of ancient conifers or conifer-like plants. Morphological differences among the seed cones of Krassilovia Clade, from elongated and lax in the Cycadocarpidium-Podozamites and Swedenborgia-Podozamites plants, to compact and tightly interlocked in the Early Cretaceous Krassilovia-Podozamites plant, highlight the diversity within the group. However, their conifer-like features, combined with their potential similarities to Gnetales, suggest new lines of investigation to further examine the close gnetalean-conifer relationship inferred from DNA data.
The analysis presented here provides only an initial assessment of the potential relationship of Gnetales, given the few alternative phylogenetic positions that were possible for the Dechellyia-Masculostrobus plant with such limited taxonomic sampling of other potentially relevant seed plants. Nevertheless, it is interesting that the Dechellyia-Masculostrobus plant is resolved close to the Krassilovia Clade even when most extant and fossil placeholders for extant families of conifers are excluded (S5 Appendix). Also, recognition of the Krassilovia Clade, which combines conifer-like cones with leaves that have transversely oriented paracytic stomata, highlights similarities to both conifers and Gnetales, as also do features of the Dechellyia-Masculostrobus plant. Ultimately, conifer monophyly may or may not be supported, but a more definitive understanding will require incorporating more fossil material into morphology-based phylogenetic analyses, not only putative conifers, but also other Gnetales, Bennettitales and Erdtmanithecales, as well as corystosperms and similar plants.
Fabiany Herrera, Gongle Shi, Chris Mays, Niiden Ichinnorov, Masamichi Takahashi, Joseph J. Bevitt, Patrick S. Herendeen and Peter R. Crane. 2020. Reconstructing Krassilovia mongolica supports Recognition of A New and Unusual Group of Mesozoic Conifers. PLoS ONE. 15(1): e0226779. DOI: 10.1371/journal.pone.0226779
