Dichrostachyoxylon cf. zirkelii (Felix) Müller-Stoll & Mädel

Sakala, Jakub, 2000, Silicified angiosperm wood from the Dangu locality (Ypresian of the Gisors region, Eure, France) - final part: the problem of palaeoclimate reconstruction based on fossil wood, Geodiversitas 22 (4), pp. 493-507 : 497-498

Dichrostachyoxylon cf. zirkelii (Felix) Müller-Stoll & Mädel ( Figs 3 ; 5A, C )

DESCRIPTION

The growth rings are slightly distinct to the naked eye (emphasized by secondary mineralization), but they are hardly noticeable under the microscope. The wood is diffuse porous.

The vessels are solitary (60%) and in radial multiples of 2 (23%), 3 (10%) or 4 (7%), 9 in number (13-14 pores) per mm 2, rounded to elliptical in section, very often deformed (radially elongated). The tangential diameter varies from 36 to 153 µm, with a mean of 83 µm; the radial diameter varies from 70 to 190 µm. The wall thickness varies from 7 to 12 µm. The vessel elements are 237-328 µm long, with simple perforation plates. The intervascular pits are bordered, alternate, quite dense, circular to elliptical, horizontally elongate, from 2.5 to 12 µm in diameter according to the longest axis, 2.5-4.5 µm in diameter according to the shortest axis and with an aperture from 1 to 8 µm in diameter. They appear to be vestured.

The fibres are quadrangular in cross section. They form regular radial lines of two to six fibres between two rays. Their mean tangential and radial diameters vary from 4.5 to 14 µm. Fibre pits are not visible. Two fibres types can be distinguished: 1) thin-walled (walls 1-2.5 µm thick) in which the lumen in cross section represents 75% of the fibre diameter; 2) thickwalled (walls about 4.5 µm thick) in which the lumen in cross section represents 20% of the diameter. The arrangement of both fibre types is irregular.

The axial parenchyma is apotracheal, forming tangential circummedullar bands and paratracheal (vasicentric, slightly aliform, very rarely confluent). The cell dimensions (tangential diameter × radial diameter × cell height) are: 7- 21 µm × 6-46 µm × 35-51 µm.

The rays seem to be homogeneous (such as they appear in the tangential section, the radial section is not available) or slightly heterogeneous. They are uni- and multiseriate (type 1 or slightly 3 of Metcalfe & Chalk 1989: 23), one to four cells wide (mostly bi- to triseriate), with 12-19 rays per one horizontal mm in tangential section. The uniseriate rays are made up of weakly vertically elongate cells. These cells are also present at the extremities of the multiseriate rays, forming unicellular endings, while the body of the multiseriate rays is constituted of rather flattened cells. The height of the uniseriate rays varies from two to ten cells (48-231 µm), their width varies from 7.5- 18 µm. The multiseriate rays are 15-40 µm wide, their height varies from 4 to 18 cells (66-333 µm). In tangential section, the cell dimensions (height × width) are 7.5-60 µm × 5.5-18 µm.

DISCUSSION

The most characteristic features of the studied wood are the following: wood diffuse porous; vessels solitary (60%) or in radial multiples of 2, 3 or 4 (very rarely of 5), composed of very short elements; small vessel pits (from 2.5 to 11.5 µm) which seem vestured; axial parenchyma vasicentric, slightly aliform, and very rarely confluent, or in tangential bands; rays homogeneous, 1-4 seriate, very short and thin. These features point to the Fabales , especially to the families Caesalpiniaceae and Mimosaceae (Louvet 1976) . Storied rays, which would have placed the fossil wood nearer to the family Fabaceae or some Caesalpiniaceae (Metcalfe & Chalk 1950) , are absent.

The first report of leguminous wood of Dangu appears in Koeniguer et al. (1985), although its affinity is not discussed. Based on the work done by Müller-Stoll & Mädel (1967), our fossil can be assigned to the genus Dichrostachyoxylon Müller-Stoll & Mädel (Müller-Stoll & Mädel 1967: 138). Among the species described within this genus (see the comparative table in Privé 1970: 200), two are very close: D. zirkelii (Felix) Müller-Stoll & Mädel and D. royaderum Privé. Unlike our fossil, D. zirkelii has more porous

R

wood with larger vessels, whereas D. royaderum , described by Privé (1970), has fewer vessels, which are less frequently grouped. Another fossil species, described as Leguminoxylon sahabiense by Louvet (1976), is similar to both species of Dichrostachyoxylon mentioned above, and is in fact a species of the genus Dichrostachyoxylon . It is also similar to our fossil, except for its lower ray density and markedly bigger vessels.

Therefore we propose: 1) to transfer the species Leguminoxylon sahabiense Louvet to Dichrostachyoxylon , because its species diagnose (Louvet 1976: 82) fits entirely that of Dichrostachyoxylon (Müller-Stoll & Mädel 1967: 138) : Dichrostachyoxylon sahabiense (Louvet, 1976) n. comb. (1976 – Leguminoxylon sahabiense Louvet, Actes 97 e Cong. nat. Soc. sav., Nantes, 1972, sci. 4: 82, pl. 1, figs 1; 2, text-fig. 1 [basionym]); and 2) to call our fossil wood Dichrostachyoxylon cf. zirkelii (Felix) Müller-Stoll & Mädel , pointing to the species D. zirkelii as the nearest and most similar, but not identical species to the Dangu fossil.

Family COMBRETACEAE R. Brown

Figures

FIG. 3. — Dichrostachyoxylon cf. zirkelii (SY7), cross-section. Abbreviations: F, fibres; PCM, circummedullar parenchym; PV, vasicentric parenchyma; R, rays; V, vessels. Scale bar: 200 µm.

FIG. 5. — A, Dichrostachyoxylon cf. zirkelii (SY7), cross-section; B, Anogeissus sp. (W4), cross-section; C, Dichrostachyoxylon cf. zirkelii (SY7), tangential section; D, Anogeissus sp. (W4), tangential section, detail of a septate fibre (septum indicated by the arrow); E, Anogeissus sp. (W4), tangential section, general view. Scale bars: A, B, 500 µm; C, 100 µm; D, 50 µm; E, 200 µm.