Xenoxylon hopeiense Chang, 1929

Akkemik, Ünal, Kandemir, Raif, Philippe, Marc, Güngör, Yildirim & Köroğlu, Fatih, 2022, Palaeobiogeographical implications of the first fossil wood flora from the Jurassic of Turkey, Acta Palaeontologica Polonica 67 (3), pp. 745-766 : 757-761

Xenoxylon hopeiense Chang, 1929

Figs. 9–11 .

Material.—One silicified wood fragment coded as ISTO- FW-00243 from Jurassic of Turkey, locality Yeşilbağlar of the city of Oltu in the province of Erzurum. It is deposited at the Department of Forest Botany , Faculty of Forestry, Istanbul University-Cerrahpasa, Istanbul, Turkey .

Description.—Growth ring boundary faint but distinct with only 1–2 rows of flattened latewood tracheids. Axial resin canals absent. Transition from earlywood to latewood abrupt. Growth rings generally less than 0.5 mm wide generally. Tracheid cross-section hexagonal, rounded or rectangular ( Fig. 9A, B ). Tangential and radial widths of tracheids are 46.7 (30.2–57.6) µm and 56.9 (32.5–111.7) µm in the earlywood, and 27.9 (10.0–44.3) µm and 21.8 (14.2–32.5) µm in the latewood, respectively. Double wall thicknesses are 10.7 (7.7–14.2) µm in earlywood and 14.1 (9.1–20.8) µm in latewood, respectively.

Rays exclusively uniseriate, rarely partly biseriate and ray heights 5–15 (2–47) cells (314.8 [119.1–1218] µm, and their average width is 18.1 [12.9–27.0] µm) ( Figs. 9C , 10A– C ). Intertracheary pits present, sparse and uniseriate on tangential walls and their diameters are 11.0 (8.8–12.8) µm ( Fig. 10D ). Rays are 4–5 (3–7) per mm. Axial parenchyma not observed.

Rays homogenous. Intertracheary pits on radial walls uniseriate or in some cases biseriate opposite. Intertracheary pits locally xenoxylean, mostly strongly flattened, pit rows locally interrupted. Rims of Sanio commonly present between both uniseriate and biseriate pits. Heights of intertracheary pits are 17.2 (12.3–23.7) µm, their widths are 27.1 (20.0–33.3) µm, and the ratio of height to the width of pits is 0.64. Cross-fields feature predominantly one large fenestri- form oopore in the earlywood, and the ray cells thin-walled. Very rarely two oopores may be seen in a single cross-field. Heights of oopores are 14.1 (9.3–19.2) µm, and their widths are 31.6 (14.4–46.6) µm, and the ratio of height of oopores to the width is 0.45. Axial parenchyma absent ( Figs. 10E, F , 11A–F ).

Remarks.— Xenoxylon was established by Gothan in 1905 based on the unusual features of some fossil gymnosperm woods from the Mesozoic, which have large rectangular earlywood cross-fields and strongly flattened more than twice as wide as high, contiguous intertracheary pits on radial walls of tracheids. This type of intertracheary pitting is called “xenoxylean radial pitting” ( Müller-Stoll 1951; Suzuki and Terada 1992; Philippe and Bamford 2008; Boura et al. 2021). In similar fossil genera, cross-field oopores are more phyl- locladoid (pointed and oblique, as in Protophyllocladoxylon Kräusel, 1939 [nomen conservandum]), or round and nar- rowly bordered all around (as in Circoporoxylon Kräusel, 1949 ). Due to its large and single oopores per cross-field and its locally xenoxylean pitting on radial walls of tracheids, our wood is assigned to Xenoxylon Gothan,1905 .

Philippe et al. (2013) proposed an identification key for the Xenoxylon Gothan, 1905 , species, and divided them into three informal groups. One of the groups is the Xenoxylon phyllocladoides Gothan, 1906 , which is characterized by less strictly xenoxylean pitting, with some spaced, round or elliptic, intertracheary pits on the radial tracheid walls and rare pits on tangential walls. Our fossil clearly belongs to this group. In this group, there are: Xenoxylon hopeiense Chang, 1929 ; Xenoxylon huttonianum (Witham, 1833) Philippe and Hayes, 2010 ; Xenoxylon jakutiense Shilkina, 1986 , and Xenoxylon phyllocladoides Gothan, 1906 . Having Sanio’s rims, no axial parenchyma, and both uni- to biseriate radial intetracheary pitting, the present specimen differs from all other fossil species within the Xenoxylon phyllocladoides group. As axial parenchyma could have been overlooked, and as Xenoxylon jakutiense Shilkina, 1986 , intertracheary radial pitting is always uniseriate, we con- sider that greatest similarities are with Xenoxylon hopeiense Chang, 1929 .

Stratigraphic and geographic range.—Upper Jurassic for Olurdere Formation. As all Xenoxylon this is a boreal species. It has a wide distribution, ranging from Asia ( China and Korea) northward to the Arctic.

Figures

Fig. 9. Wood sections of Xenoxylon hopeiense Chang, 1929 (ISTO-FW-00243) from Jurassic of the Yesilbaglar locality of the province of Erzurum, Turkey.A1–A2, transversal sections with distinct growth ring boundary;A3, predominantly uniseriate rays in tangential section.

Fig. 10. Wood sections of Xenoxylon hopeiense Chang, 1929 (ISTO-FW-00243) from Jurassic of the Yesilbaglar locality of the province of Erzurum, Turkey.A1–A3, tangential sections with predominantly uniseriate and rarely partly biseriate rays; A4, intertracheary pits on the tangential walls of trache- ids; A5,A6, mostly flattened xenoxylean type intertracheary pits on radial walls of tracheids.

Fig. 11. Wood sections of Xenoxylon hopeiense Chang, 1929 (ISTO-FW-00243) from Jurassic of the Yesilbaglar locality of the province of Erzurum, Turkey. A1–A3, intertracheary pits on radial walls uniseriate or in some cases biseriate opposite, intertracheary pits locally xenoxylean, mostly strongly flattened, pit rows locally interrupted, rims of Sanio commonly present between both uniseriate and biseriate pits;A4–A6, window-like cross-field pits.