Arthropitys bistriata (Cotta) Goeppert, 1864

Galtier, Jean, Ronchi, Ausonio & Broutin, Jean, 2011, Early Permian silicified floras from the Perdasdefogu Basin (SE Sardinia): comparison and bio-chronostratigraphic correlation with the floras of the Autun Basin (Massif central, France), Geodiversitas 33 (1), pp. 43-69 : 50-52

Arthropitys bistriata (Cotta) Goeppert, 1864

( Fig. 4 B-D, F, G)

We attribute a number of stems showing a wide range of sizes and of secondary xylem thickness to this species. The small stem ( Fig. 4B ) has a diameter of 19 × 5 mm including the cortex; it shows a small development of secondary xylem (0.5 mm thick, i.e. about 15 tracheids); 62 primary xylem strands with carinal canals (80 µm in diameter) are surrounding the pith at 0.7 mm intervals ( Fig. 4F ). Secondary xylem wedges are separated by interfascicular rays that are 5-6 cells wide. The outer sclerenchymatous cortex is well preserved as well as groups of large cells in the inner cortex (arrow, Fig. 4F ). This young stem shows the beginning of development of secondary xylem. In its general organization, small amount of secondary xylem and cortex anatomy, it is very similar to specimens of A. bistriata var. augustodunensis from Autun illustrated by Renault (1893: pl. 46, fig. 7; pl. 47, figs 1, 2).

Other observed specimens belonged to older parts of plants, with a wood thickness ranging from 1 to 60 mm, but generally the cortex is missing. For example the stem ( Fig. 4C ) has a wood of 10 mm thick; it represents probably one third of the original woody cylinder.About 30 primary xylem strands are visible; carinal canals are 100 to 250 µm in diameter, their preservation may be excellent ( Fig. 4G ) and sometimes there is no lacuna (arrow, Fig. 4C ). The average distance between carinal canals is 1 mm, it may be up to 2 mm in stems with wood several centimetres thick. In transverse section, interfascicular rays are less than 1 mm wide, made of 5 to 8 parenchymatous cells, and they are generally visible through the whole secondary xylem. The secondary xylem wedges consist of rays and tracheids showing an initial quick increase of their diameter from 30 to 70 µm ( Figs 4F, G ). Some specimens consist of a single fragment of wood including a portion of the pith and a few primary xylem strands. The observed maximum wood thickness of 60 mm suggests an

(Cotta) Goeppert; B, incomplete transverse section of a young stem showing a little development of secondary xylem (SAR40C1T03); C, partial transverse section of an older stem with thick secondary xylem and well-marked interfasciculary rays (SAR58C1B01); D, radial longitudinal section in the secondary xylem showing scalariform pitting of tracheids and high ray cells on the left (SAR51BL01); F, detail of the stem (B) showing two primary xylem strands with circular carinal canals, one interfascicular ray, small development of secondary xylem, cluster of large cells (arrow) in inner cortex and sclerenchymatous outer cortex; G, detail of one primary xylem strand and secondary xylem wedge of a stem with thick wood (SAR51A01). Scale bars: A-C, 1 mm; E-G, 0.5 mm; D, 100 μm.

original stem diameter of at least 15 cm if we include the pith and cortex. This corresponds to some specimens of A. bistriata illustrated by Renault (1893: pl. 44, figs 1, 2) as well as original specimens of the species described by Cotta (1832). In all the observed stems the secondary xylem tracheids show scalariform thickening to pitting on their radial walls ( Fig. 4D ). All the features recognized in these specimens suggest that they belong to Arthropitys bistriata . However,the original material of this species was decorticated and Cotta’s diagnosis did not include cortical features or information on young stems such as that presented by Renault (1893 -1896) for A. bistriata var. augustodunensis . There is also a strong presumption that the very small stems attributed to A. renaultii belong to the same plant.Detailed comparison of this material is out of the scope of the present paper.

Figures

FIG. 4. — Silicified plants from the Permian of Sardinia: A, E, Arthropitys cf. renaultii Boureau; A, transverse section of a young twig (SAR01B1B01); E, detail of the same showing the pith, primary xylem strands with carinal canals and well preserved cortex with large cells (C) in inner cortex and alternating bands of sclerenchyma and parenchyma in outer cortex (arrow); B-D, F, G, Arthropitys bistriata