Diaphorodendron rhombicum sp. nov., a New Anatomically Preserved Lepidodendralean Stem from the Taiyuan Formation in Southern Shandong Province, North China

摘要

In this paper a new species of anatomically-preserved lepidodendralean stem is described from coal balls in the No.16 coal seam of the Taiyuan Formation in Shanjialin Coal Mine, Tao-Zao Coal Field, southern Shandong Province. The stem has a well preserved stele, outer cortex, periderm and leaf cushions preserved. Its organization is protostelic and it lacks secondary xylem. Outer cortex consists of alternatively arranged radial thick-walled cell bands and damaged thin-walled cell areas. Periderm is well developed, 4-5 mm wide, and is divided into phellem and phelloderm. Phelloderm is wider than the phellem and is generally uniform but in some places is differentiated into alternatively arranged zones of tangentially thicker- and thinner-walled cell bands. Leaf cushions are approximately equidimensional or rhomboidal in tangential section, whereas leaf scars are slightly elevated and are probably lenticular or helmet-like in shape, located at the upper part of the leaf cushion. Leaf cushions are flat and lack a well developed keel or plications. Leaf traces extend through a horizontal course in the cushion and turn upwards slightly and then horizontally into the leaf scars, resulting in the outermost parts of the course being somewhat "S" shaped. In radial section ligule pits are obliquely orientated with a broad basic part and its aperture is directly located at the top of the leaf scar. The ligule is large, ovoid in shape, and approximately 1.2 mm long and 1.0 mm wide. Compared with the stems of other genera in Lepidodendrales, the stem under discussion is most comparable to the stems of Diaphorodendron to which it is here assigned. Each of the three previously known species of Diaphorodendron occur in the middle Late Carboniferous of the Euramerican Flora and are distinct from the stem under discussion in the form of the leaf cushions. These differences support the erection of a new species, D. rhombicum, representing the first discovery of the genus from late Late Carboniferous and Permian ages worldwide, and also representing the first occurrence of the genus within the Late Palaeozoic Cathaysian Flora. It is very interesting that the new species bears equidimensional leaf cushions with that probably represents a derived feature within the genus, while the leaf cushions with greater vertical than horizontal dimension occupied by the Euramerican species of Diaphorodendron are here considered ancestral in morphology. This hypothesis is supported by the younger geological age (late Late Carboniferous or early Early Permian) of the new species than that (middle Late Carboniferous) of the previously known Euramerican species of Diaphorodendron.