Shoreoxylon cf. sumatraense Du

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 : 884-886

Shoreoxylon cf. sumatraense Du

( Fig. 16 )

Shoreoxylon sumatraense Du, 1988b: 342 , pl. 1, figs 1-4, pl. 2, figs 1-4, pl. 5, fig. 4.

HOLOTYPE. — National Museum of Geology and Mineralogy, Leiden, specimen no. RGM B ( RGM 383446 ).

MATERIAL. — MNHN.F.50193 (field number: 17FN12). Estimated minimal diameter: 10-14 cm.

LOCALITY. — Kalewa Township, Sagaing Region, Myanmar.

AGE. — Upper lower to lowermost middle Miocene. DESCRIPTION

Wood diffuse-porous. Growth rings marked by tangential canal lines. Vessels 45-70% solitary, otherwise in radial multiple of 2-4 and clusters of different size with sometimes many small vessels surrounding bigger ones ( Fig. 16A ), round to oval, 8-20 per mm² (average: 15); tangential diameter 80-240 µm (average: 160 µm); walls of the vessel cells are thick compared to other cells (about 6-11 µm).Tyloses present, common and visible ( Fig. 16 A-C). Vessel elements 150-570 µm (average: 350 µm) long. Perforation plates simple. Intervessel pits alternate, 3-6 µm of diameter (average: 4 µm). Vessel-ray pits apparently irregular in size and shape, simple, 4-10 µm ( Fig. 16F ). Vasicentric tracheids present ( Fig. 16H ). In cross section, parenchyma indistinguishable from fibres due to poorly preserved cell walls and no obvious difference of size. At least vasicentric and around canals appearing as little flattened cells; parenchyma easily recognizable in tangential section and abundant in places, 4-8 cells per strand ( Fig. 16E ), it seems as abundant as fibres; parenchyma cells 40-120 µm long (average: 90 µm) 10-30 µm wide (average: 20 µm) in tangential section; no crystal. Rays usually 1- to 4-seriate (mainly 3-, very rarely 5-seriate) ( Fig. 16C, D ), uniseriate for about 20% of the rays, non-storied, 4-8 rays per mm (average: 6), 340- 1040 µm (average 620 µm) or 10-45 cells high; multiseriate rays are heterocellular, made of procumbent cells mostly with 1-4 but sometimes more (9) square or upright cells at the ends ( Fig. 16E, I ); no mineral inclusion seen. Fibres non-septate, 8-20 µm (average: 13 µm) wide, apparently thin-walled (lumina 1.9 times the double wall thickness in average where the walls are best preserved; or individual cell wall 3-4.4 µm thick), fibres cell walls are poorly preserved; clearly aligned in radial rows. Secretory canals in long tangential bands surrounded by parenchyma, frequent and closely spaced (every 0.4-3 mm) ( Fig. 16B, G ), 40-75 µm of diameter (average: 60 µm). Black spots and long lines crossing tangentially the section (well visible in low magnification) are interpreted as potential lines of canals, even though their origin remains to be confirmed ( Fig. 16G ).

DISCUSSION

This specimen is characterized by: 1) diffuse-porous wood; 2) exclusively simple perforation plates; 3) closely and frequently spaced long tangential lines of secretory canals; 4) vasicentric tracheids; 5) presence of sheath cells; 6) vessels mostly in groups and sometime forming clusters; and 7) 1-4-seriate rays, mostly less than 1mm high. The poor preservation of the specimen makes our observations on ray and parenchyma arrangement uncertain. Nonetheless, its features indicate an affinity with modern and fossil Dipterocarpaceae . As discussed p. 882, long tangential bands of canals, grouped vessels and no fibre-tracheids are features compatible with extant Shorea . Among this genus, the sections ‘Shorea’ and ‘Rubroshorea’ are the more compatible with our fossil: ‘Shorea’ for the frequency of vessels (4-10[16] per mm²), the frequency of solitary vessels (55-85%), common tylose, mostly 3-4-seriate rays; ‘Rubroshorea’ for the thickness of fibres, the composition of the rays (with 1-4 row of marginal cells and sometimes more), the propencity to have lines of canals spaced by less than 1 mm and small canals (40-80 µm). An examination of different species of Shorea shows that this fossil is similar to Shorea negrosensis Foxw. (‘Rubroshorea’ section). This species has closely spaced canals that are sometimes small (surrounded by only four parenchyma cells), thin-to-thick fibres walls, parenchyma cells mostly of the same diameter as fibres and flattened around canals, frequent groups of vessels, as well as similar vessel groups and clusters, and aliform to aliform-confluent parenchyma. The modern specimen in our possession (no. CTFT25647) has mostly 3-4-seriate rays with 1-4 marginal rows of cells, sometimes more for thinner rays.

As for the previous Shorea -like fossil, we compared this one with the genus Shoreoxylon . Our specimen is among the few to have a high frequency of vessels, closely spaced canal lines and thin rays with not so rare uniseriate ones. Four species of Shoreoxylon share many similarities with our fossil ( Appendix 1; Awasthi 1974; Sukiman 1977; Trivedi & Ahuja 1979; Bande & Prakash 1980; Du 1988b): S. arcotense Awasthi has a high density of vessels and small canals, but exclusively solitary and smaller vessels; S. pachitanensis Sukiman has similar bands of canals, but higher rays and wider canals; S. ornatum (Trivedi & Ahuja) Bande & Prakash also has similar bands of canals, but less frequent and more solitary vessels, rays up to 5-seriate and higher (up to 1870 µm) and sheath cells are present; S. sumatraense from the Quaternary of Sumatra shares most of the main features of our specimen. It has frequent vessels (9-14 per mm²), multiples up to 5 vessels, sometimes clusters, thick walls (15 µm, 6-11 µm in our fossil), parenchyma abundant, rays 1-4-seriate with similar composition and height, radially aligned and thin-walled fibres (3 µm, compared to 3-4.4 µm in our fossil), small canals (40-90 µm compared to 40-75 µm in our fossil).However, it displays wider vessels (200-360 µm), a storied tendency in the parenchyma that cannot be observed in our specimen, and no visible vasicentric tracheids (but they seem present in the figures provided) and sometimes crystals in ray cells. The higher density and smaller vessels in our specimen could be explained by compression or environmental constraints. These differences are minor and can be related to intraspecific variations and the poor preservation of our specimen; we thus attribute our specimen to Shoreoxylon cf. sumatraense . Du (1988b) indicates that S. sumatraense shares most features with extant Shorea negrosensis , which is in adequacy with our own observations.

Shorea negrosensis grows today in the Philippines in evergreen, semi-evergreen and seasonal dipterocarp forests at low elevation ( Ashton 1982; Soerianegara & Lemmens 1993; EDC 2020).

Figures

FIG. 16. — Shoreoxylon cf. sumatraense Du, MNHN.F.50193: A, Ts, vessel often in groups, sometimes in clusters (arrows); B, Ts, long tangential lines of secretory canals (arrows) close to each other; C, D, Tls, 1-4-seriate rays; E, Tls, 3-seriate rays with a uniseriate row of six marginal cells (white arrow), abundant parenchyma (black arrow), 4-8 cells per strand; F, Rls, vessel-ray pits irregular in shape, simple with reduced borders (arrows); G, Ts, synthetic drawing of the transversal section (with only some vessels and rays displayed), visible bands of parenchyma as grey lines and recognized secretory canals as red dots,arranged mostly in long and closely spaced tangential lines; H, Rls, vasicentric tracheids (arrows); I, Rls, heterocellular ray with square or upright marginal cells (arrows). Scale bars: G, 1 cm; A, 1 mm; C, 500 µm; B, 330 µm; D, I, 200 µm; E, H, 100 µm; F, 50 µm. Abbreviations: see Fig. 2.