Dipterocarpoxylon cf. jammuense Guleria, Gupta & Srivastava

Gentis, Nicolas, Licht, Alexis, Boura, Anaïs, Aung, Dario De Franceschi Zaw Win Day Wa & Dupont-Nivet, Guillaume, 2022, Fossil wood from the lower Miocene of Myanmar (Natma Formation): palaeoenvironmental and biogeographic implications, Geodiversitas 44 (28), pp. 853-909 : 876

Dipterocarpoxylon cf. jammuense Guleria, Gupta & Srivastava ( Fig. 12 )

Dipterocarpoxylon jammuense Guleria, Gupta & Srivastava, 2002: 230 , pl. 1, figs 1-5.

MATERIAL. — MNHN.F.50189 (field number: 17FN01). Estimated minimal diameter: non-assessable due to compression, but seemingly very parallel rays.

LOCALITY. — Kalewa Township, Sagaing Region, Myanmar.

AGE. — Upper lower to lowermost middle Miocene.

DESCRIPTION

Wood diffuse-porous.Growth rings indistinct.Vessels exclusively solitary ( Fig. 12A ), rounded to oval, 4-9 per mm² (average: 7); tangential diameter 115-230 µm (average: 170 µm). Tyloses sometimes present ( Fig. 12E ). Vessel elements 90-300 µm (average: 170 µm) long. Perforation plates simple, horizontal. Intervessel pits not visible. Vasicentric tracheids present and abundant ( Fig. 12G ). Parenchyma mostly vasicentric and surrounding secretory canals in aliform-shaped short bands ( Fig. 12A, B, D, E ); parenchyma cells estimated 75 µm long in average, 21-31 µm wide (average:26 µm) in tangential section. Rays 1- to 6-(7)-seriate, numerous uniseriate rays (25-50%) made of procumbent, upright and square cells, multiseriate rays being mainly 4-6-seriate ( Fig. 12C ), leading to a tendency of 2 distinct sizes, 5-9 rays per mm (average: 6), 440-1230 µm (average: 780 µm) or up to 50 cells high or more, heterocellular made of procumbent cells with 1-19 upright or square cells at one or both ends ( Fig. 12F ); rays can appear with both upright and procumbent cells throughout the ray in radial section due to frequent uniseriate rays or sheath cells ( Fig. 12C ). Fibres non-septate, 13-32 µm in diameter (average: 20 µm), very thick-walled with sometimes almost no lumina ( Fig.12E ). Secretory canals isolated or in short or seemingly short lines, often crushed, but the parenchyma band is still visible ( Fig.12A, B, D, E ); also recognizable in tangential and radial section as tubes surrounded by dislocated parenchyma cells, 40-80 µm of tangential diameter, 60-100 µm in radial diameter ( Fig. 12D ).

DISCUSSION

Despite its poor state of preservation, this wood specimen displays several diagnostic features. It is characterized by: 1) solitary secretory canals or in short lines; 2) vasicentric tracheids; 3) vasicentric parenchyma; 4) mostly 1-6-seriate rays; 5) heterocellular rays with a row of up to 19 uniseriate cells; and 6) sheath cells. The first two features are diagnostic of most Dipterocarpaceae ( Metcalfe & Chalk 1950, Schweitzer 1958). Solitary secretory canals in short lines, as well as almost exclusively solitary vessels indicate an affinity with the genus Dipterocarpus ( Schweitzer 1958) . However, the compression and the preservation state of the wood make it complicated to precisely describe the pattern of the canals. Solitary or paired vessels and canals are also found in the genus Anisoptera and Upuna Symington , in addition to sheath cells ( Schweitzer 1958; Gottwald & Parameswaran 1966; InsideWood 2004 -onward; Ogata et al. 2008). The genus Anisoptera has frequently larger rays (up to 8-seriate or more) and more or less continuous sheath cells in rays with shorter row of marginal ray cells. Upuna has less pronounced sheath cell, more tylose and canals less distinguishable in size from vessels than Dipterocarpus ( Richter & Dallwitz 2000 -onward). In addition, Upuna can have diffuse and diffuse-in-aggregate parenchyma ( Soerianegara & Lemmens 1993) and is usually described with short lines of canals up to two canals maximum. However, the pictures on InsideWood (2004 -onward) show lines up to 4 canals and Soerianegara & Lemmens (1993) mention also short and long tangential lines. Dipterocarpus (and Anisoptera ) have silica bodies in ray cells in opposition to Upuna . Unfortunately, the preservation state of our fossil does not allow us to distinguish any mineral inclusion. Genus Dipterocarpus is the best modern analogue to our specimen because of the frequent short bands of parenchyma in transverse section, the great frequency of uniseriate rays, the length of the marginal part of the rays ( InsideWood 2004 -onward; wood specimens of extant species MNHN-P-P00400540, P00415879, P00402617 and CTFT19039), and numerous sheath cells ( Gottwald & Parameswaran 1966). Among Dipterocarpus , our specimen resembles modern species D. retusus Blume , D. alatus Roxb. ex G.Don , D. dyeri Pierre ex Laness. , D. cornutus Dyer , D. grandiflorus (Blanco) Blanco and D. kunstleri King because of their rays with long uniseriate row of marginal cells and sheath cells as well as few paratracheal parenchyma other than scanty / vasicentric.

Fossil wood related to Dipterocarpus are grouped under the genus Dipterocarpoxylon (Ghosh & Kazmi 1958; Prasad & Gautam 2016).To our knowledge, no fossil wood resembling Upuna has been described. It is yet difficult to attribute our specimen to an individual Dipterocarpoxylon species due to its poor preservation. Among the closets described Dipterocarpoxylon ( Appendix 1), D. jammuense shows the closest anatomy, with parenchyma restricted to vasicentric and very rarely diffuse, 1-6-seriate rays, uniseriate ones 2-11 cells high, marginal row of ray cells up to 12 cells, common sheath cells and small canals (up to 80 µm in diameter); D. jammuense differs in having mostly 3-5-seriate rays (compared to mostly 1- and then 5-seriate) and longer vessel elements (up to 600 µm, compared to up to 300 µm); other species display a similar ray composition, such as D. gracile Schweitzer , D. siwalicus Prakash (1975) , D. malavii Ghosh & Ghosh (1959) , D. sarapeense Vozenin-Serra & Privé-Gill (2001) , but they often differ by either higher rays, shorter lines of canals, bigger canals, mostly diffuse or diffuse-in-aggregate parenchyma or less sheath cells. Our specimen is thus attributed to Dipterocarpoxylon cf. jammuense .

Dipterocarpus is common in evergreen, sometimes present in semi-evergreen forests or dry deciduous dipterocarp forests, mostly in lowlands and occasionally up to 1400 m altitude. ( Ashton 1982; Soerianegara & Lemmens 1993; Ghazoul 2016). All extant Dipterocarpus species mentioned above grow in evergreen or semi-evergreen lowland tropical forests, in mixed dipterocarp forests and sometimes seasonal forests ( Ashton 1982; IUCN 2021). Dipterocarpus retusus is also found in montane forests ( Ashton 1982; Ly et al. 2017). Upuna is a genus endemic to Borneo that grows in lowland mixed dipterocarp forests and on coastal hills up to 350 m altitude ( Ashton 1982; Soerianegara & Lemmens 1993).

Figures

FIG. 12. — Dipterocarpoxylon cf. jammuense Guleria, Gupta & Srivastava, MNHN.F.50189: A, Ts, seemingly exclusively solitary vessels,vasicentric parenchyma, small pores interpreted as solitary secretory canals or in short lines (arrows); B, D, Ts, short lines of secretory canals (arrows); C, Tls,1-6-seriate rays, with rows of uniseriate marginal cells up to 19 cells, frequent uniseriate rays and sheath cells present (arrow); E, Ts, vessel puggled with tyloses, surrounded par vasicentric parenchyma (black arrow), secretory canals solitary or in short lines, surrounded by parenchyma (white arrow); F, Rls, heterocellular rays with square or upright cells (black arrow) and procumbent cells (white arrow) mixed throughout the rays because of the presence of sheath cells, uniseriate rays and long row of marginal cells; G, Tls, vasicentric tracheids (arrow). Scale bars: A, B, 1 mm; C-F, 200 µm; G, 100 µm. Abbreviations: see Fig. 2.