科研之友
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摘要
A single cylindrical, abraded cone specimen has been found associated with ammonites of the Lower Cretaceous Budden Canyon Formation near Ono, California. The specimen was embedded in bioplastic and sectioned using the cellulose acetate peel technique. The pith is composed of parenchyma and scattered sclereids and is bounded by a ring of resin canals. The secondary xylem of the cone axis, with a ring of small-diameter resin canals, is continuous. The cortex is composed primarily of parenchyma with an outer sclerotic zone 10-16 cells thick and has 27-30 dilating resin canals. Vascular traces to the ovuliferous scale and bract diverge separately from the vascular cylinder and are accompanied by a single resin canal from the pith that is positioned between the arms of the horseshoe-shaped ovuliferous scale trace. The bract trace is terete, accompanied by two lateral resin canals from the cortical axial system, and enters the free part of the bract. The ovuliferous scale is almost completely sclerotic, with resin canals both abaxial and adaxial to the vascular strands. A large interseminal ridge is present between the two winged seeds on each scale. This cone represents a new species of Pityostrobus and is described as Pityostrobus californiensis sp. nov. A phylogenetic analysis using morphological data from the ovulate cones of all the extant and fossil taxa of Pinaceae, as well as those of Cryptomeria japonica, Sciadopitys verticillata, and Pararaucaria patagonica, was done. Relationships of the fossil species of Pityostrobus, Obirastrobus, and Pseudoaraucaria to the living taxa of Pinaceae were assessed. The idea that Pityostrobus represents an artificial assemblage of pinaceous taxa is supported by the analyses. The genus Obirastrobus cannot be distinguished from Pityostrobus species, while Pseudoaraucaria species appear as a clade. The difficulty of basing fossil taxa purely on cone characteristics is discussed in light of the analyses.
- 出版日期2001-5
