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.2004 Aug;94(2):281-8.
doi: 10.1093/aob/mch138. Epub 2004 Jun 30.

A new Permian gnetalean cone as fossil evidence for supporting current molecular phylogeny

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A new Permian gnetalean cone as fossil evidence for supporting current molecular phylogeny

Zi-Qiang Wang. Ann Bot.2004 Aug.

Abstract

Background and aims: The order Gnetales has been the central focus of controversy in seed plant phylogeny. Traditional treatment of morphology supports the anthophyte hypothesis with Gnetales sister to angiosperms but current molecular data reject this hypothesis. A new fossil gnetalean cone, Palaeognetaleana auspicia gen. et sp. nov., is reported from the Upper Permian in North China, and its phylogenic implications are considered.

Methods: Samples of cones from the upper part of the Upper Permian redbeds of Baode section, northwestern Shanxi Province, China, were examined.

Key results: The cone is characterized by its unusual nature of reproduction that combines features of post-Triassic gnetaleans and some of the Palaeozoic conifers. It is made up of a number of imbricate axillary units, each simply formed by an ovule and a subtending bract, which may be comparable with the axillary seed-scale complex of some of the Palaeozoic conifer cones. The cone exhibits at least a partially bisexual character that appears to have pollen sacs with monosulcate ribbed pollen grains and sessile, asymmetric, and radiospermic ovules. The ovule has an integument of three envelopes: an outer one of pointed scales; a middle sclerified one; and an inner cuticle that extends upward into a micropyle with an oblique tip.

Conclusions: The new Permian cone has unequivocal affinity with the Gnetales. The fossil has considerably extended the divergence time of the Gnetales from 140 (210?) back to 270 myr ago and, therefore, provides the first significant fossil evidence to support the current conclusion based on molecular data of seed plants, i.e. monophyletic gymnosperms, comprising the Gnetales are closely related to conifers.

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Figures

F<sc>ig</sc>. 1.
Fig. 1.
Map of the fossil locality.
F<sc>ig</sc>. 2.
Fig. 2.
Drawings ofPalaeognetaleana auspicia gen. and sp. nov.: (A) reconstruction of aPalaeognetaleana auspicia cone (holotype); (B) diagram of an axillary unit, showing structures of ovular integument: mm (dashed line), megaspore membrane (see Fig. 6A); tp, tapetal tissue (see Fig. 6A); ic, cuticle of inner envelope (see Fig. 5C, only developing at micropyle end); sc, sclerotic envelope (see Fig. 5B); os, outer scales or fibres (see Fig. 5A); ms, a presumed pollen sac; br, bract; ch, chalazal end (see suture of the ovule in Fig. 5B).
F<sc>ig</sc>. 3.
Fig. 3.
Palaeognetaleana auspicia gen. and sp. nov. cone and its bracts. (A) A cone borne terminally on a short peduncle that possibly extends from the primary axis (lower right), showing attached rounded ovules near to the cone axis and dentate margin of the bract (lower left), 9107–1, ×2. (B) Some isolated bracts preserved in a transverse plan, each showing a long lateral ridge and an expansive base on which one ovule overlies, 9705–45, ×2. (C) Basal part of a bract showing one ovule lying on it, 9705–41, ×8. (D) An isolated bract showing the long lateral ridge of its margin and dentate anterior margin (arrow b) at expansive base. Note the ovule having become detached but having left delicate fibres (arrow a), 9805–048, ×5. All scale bars = 5 mm.
F<sc>ig</sc>. 4.
Fig. 4.
Pollen sacs andin situ pollen grains. (A) Part of a degraded pollen sac enclosed by pubescence of tapetal tissue, 9705–37, ×42; scale bar = 240 µm. (B) Local magnification of A showing ribbed pollen mass, ×600. (C) Another pollen mass, 9705–39, ×600. (D) Showing an entire ribbed grain of C, ×1200. Scale bars in B and C = 25 µm.
F<sc>ig</sc>. 5.
Fig. 5.
Ovules without or with little maceration. (A) A small ovule (upper left) enclosed by fine fibres, and a large ovule (lower right) enclosed by narrow and pointed scales, both from same cone, 9805–045, ×20. (B) An ovule after incomplete maceration, showing the solid middle layer of the integument and a suture ridge, 9705–39a, ×20. (C) An ovule after complete maceration, exhibiting the thin cuticle of the inner integument that extends upwards into an asymmetric micropyle and rare remnants of fibres at the base, 9107–8 (counterpart of 9107–1), ×8. All scale bars = 0·5 mm.
F<sc>ig</sc>. 6.
Fig. 6.
Ovules through maceration: (A) an ovule showing a long, unbifid micropylar tube at apex, 9705–0; ×40; (B) upper part of an ovule showing a prominent micropyle with an oblique tip, 9107–8, ×40. Both scale bars = 0·25 mm.
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References

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