Abstract: A new genus and species Pinnatiramosus qianensis is described from the Xiushan Formation of Middle Silurian (Wenlockian)in the north of Guizhou, China. This plant consists of a prostrate axis and erect pinnate branching system. Conductin gbundle is comprised of tubular cells with scalariform or alternate simple pits. Pinnatiramosus gen. nov. Diagnosis: Plant cylindrical, differentiated into erect branches and prostrate axes; erect branches much branched, consisting of regular or irregular pinnate lateral branches; laterals opposite or alternate, with basal parts constricted and acute or blunt apices. Conducting bundle comprised of longitudinally aligned tubes with secondary thickening. Reproductive structures nor seen. Derivation of name: Pinnatus (Latin)=pinnate and ramosus (Latin)=ramose, referred to the external morphology of the plant. Pinnatiramosus qianensis sp. nov. Diagnosis: Characters described as the genus. Prostrate axes at least 400 mm in length and 3–6 mm in diameter, produced erect longer branches and downward shorter branches; erect branches up to 200 mm long and vary from 1.0 to 2.5mm in diameter, branched at least six orders, with lateral branches unequally developed, long laterals scattered among short ones; lateral branches oppositely or alternately arranged on main axis, 1–3 mm: apart; ultimate bran- ches 0.4–0.8 mm in diameter, with acute or blunt apex; borne at angles of 40–70˚ with main axis. Conducting bundle ca. 1/4 of the axis diameter, composed of tubular elements with secondary thickening; tubes range from 15–54μm in diameter, with blunt end wall; secondary wall averages 4.86 μm in thickness, ranging 2.5–6.6 μm, with scalariform and alternate pits. Derivation of name: Qian is the abbreviation for Guizhou province from where the plants are found. In this paper, is made the comparison of this new plant with fossil and living algae as well as early vascular plants on the basis of its external morphology and internal structure According to the comparison, Pinnatiramosus belongs to one of specialized plants with tracheophyte- like conducting tissue. Some interesting opinions are here provided as follows: (i) Silurian is, an important age of plant evolution. During this period certainly coexist not only various algae and undoubted vascular plants but also a group of interesting plants that were just immigrating from aquatic to terrestrial environments. (ii) The transmigration of algae from water to land occurred along several different evolutionary lines. Accompanying changes of these algae on morphology and anatomy were adapted to terrestrial environments. (iii) Pinnatiramosus represents a group of new plants with remarkably nontracheophyte morphology but tracheophyte- like anatomy. (iv) The tracheid-like tubes, macerated out of Silurian rocks, derived more probably from non-vascular plants such as Pinnatiramosus, Prototaxitesor Nematothallus than from vascular plants.

Key words: Middle Silurian, Xiushan Formation (Wenlockian), Pinnatiramosus gen. nov., Pitted tube