Shoreoxylon sp. 1

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 : 886-888

Shoreoxylon sp. 1

( Fig. 17 )

MATERIAL. — MNHN.F.50194 (field number: 19NAT03-1). Estimated minimal diameter: 13-17 cm.

LOCALITY. — Kalewa Township, Sagaing Region, Myanmar.

AGE. — Upper lower to lowermost middle Miocene. DESCRIPTION

Wood diffuse-porous. Growth rings indistinct. Vessels solitary (55-60%) or in groups of 2-4, sometimes in clusters made of a mix of small vessels and vasicentric tracheids ( Fig. 17A, D ), round to oval due to radial compression, 4-16 per mm² (average: 9); tangential diameter 70-270 µm (average: 180 µm). Tyloses present and common, especially in tangential section ( Fig. 17C ). Vessel elements 150-400 µm (average: 300 µm) long. Perforation plates simple ( Fig. 17E ). Intervessel pits alternate, 4-7 µm. Vasicentric tracheids present, interspaced with parenchyma ( Fig. 17F ). Parenchyma frequent, mostly aliform or aliform-confluent, crossing rays and joining vessels together but without any particular arrangement ( Fig. 17A ), diffuse-in-aggregate or drawing a network between rays in the form of small bands or groups; 4 cells sheath around canals ( Fig. 17D ); parenchyma cells 40-110 µm long (average: 80 µm), 13-26 µm wide (average: 20 µm) in tangential section, 4-5 cells (possibly more) per parenchyma strand, possibly subdivided ( Fig. 17F ). Rays 1- to 5-seriate (dominantly 4-seriate) ( Fig. 17B, C ), uniseriate few (<10%) and short (mainly less than 10 cells high), 4-8 rays per mm (average: 6), 130-800 µm (average: 460 µm) or 6-50 cells high. Heterocellular made of procumbent cells with 1-2 upright cells at the ends ( Fig. 17E ), rarely more (up to 7). Sometimes rays appear weakly heterocellular as the marginal rows are composed of enlarged procumbent cells ( Fig. 17E ); rays homogeneous in shape (fusiform) and size ( Fig. 17C ), all ray cells filled with dark content. Fibres non-septate, 12-20 µm in diameter (average: 17 µm), (lumina 1 times the double wall thickness on average). Secretory canals few and small ( Fig. 17A, D ), in irregularly spaced short (to long) tangential bands up to 4-6 canals, embedded in parenchyma bands ( Fig. 17G ), canals possibly crushed, 30-50 µm in tangential diameter (average: 40 µm).

DISCUSSION

This specimen is characterized by: 1) diffuse-porous wood; 2) high density of vessels, often in groups and sometimes in clusters; 3) short fusiform rays; 4) short lines of very small secretory canals; 5) abundant parenchyma, mostly aliformconfluent and diffuse-in-aggregate; and 6) vasicentric tracheids. Like for previous specimens (from p. 873), secretory canals and vasicentric tracheids are diagnostic of the Dipterocarpaceae family. Few canals are distinguishable, and it is hard to determine if they are arranged in short or long lines. Our specimen is only compatible with genera Hopea , Parashorea and Shorea as they have vessels smaller than 150 µm of average diameter, not exclusively solitary vessels, no fibre-tracheids, and canals in short or long lines. A closer affinity to Shorea genus is indicated because of its homogeneous rays, common tyloses, vessel density sometimes more than 10/mm². Following the discussion of the page 882, the sections ‘Pentacme’ and ‘Shorea’ are the closest to this fossil although ‘Shorea’ is the best option with short rays sometimes weakly heterocellular, with frequent tyloses, a variable frequency of groups of vessels, sometimes short lines of canals and frequent apotracheal parenchyma. This specimen recalls: Shorea robusta C.F.Gaertn. for its abundant parenchyma, 4-5-seriate and short fusiform rays, and vessels often in groups and clusters; Shorea obtusa Wall. (ex Blume) for abundant parenchyma, rays with only one upright marginal ray cell, vessels often in group and in radially aligned clusters; Shorea siamensis Miq. for the mostly aliform parenchyma, yet without a defined arrangement, abundant diffuse to diffuse-in-aggregate parenchyma, short (yet shorter than 50 cells high) and fusiform rays. All of them have few and short uniseriate rays (often less than 10 cells high). In a lesser extend, we can find some similarities with Shorea parvifolia for aliform parenchyma forming very thin lateral lines, rays up to 5-seriate, quite short and fusiform with few marginal cells.

The specimen shares all diagnostic features of fossil genus Shoreoxylon . Three fossil species with 1-5-seriate rays display similar features to the ones of our fossil ( Appendix 1; Schweitzer 1958; Prakash 1965a; Ramanujam & Rao 1967; Sukiman 1977; Prakash & Bande 1980; Gurusamy & Kumarasamy 2007): S. indicum for parenchyma, rays and crystals but with bigger canals, less frequent but more commonly solitary vessels; S. posthumi has short and fusiform rays but broader (mainly 5-seriate), enlarged parenchyma cells are frequent (idioblasts) and clearly visible in figures and secretory canals are wider; S. burmense has similar rays although slightly broader but its vessels are more solitary and canal lines are often grouped by 2-5. In a lesser extent: S. irrawaddiensis Pakash & Bande is described with larger vessels (up to 6-(7)-seriate) although mostly 3-5-seriate and solitary vessels, however the rays in the figures seem to be mainly 4-5-seriate. Even though it is probable that our fossil might belong to one of these spe- cies, their descriptions are overlapping, often incomplete or lack clear figures; they do not allow us to attribute a clear identification to our specimen with confidence, nor to create a new species. As a consequence, we attribute this fossil to Shoreoxylon sp. with a noted resemblance with S. indicum , S. posthumi or S. burmense .

Shorea parvifolia is common in dipterocarp forests up to 1100 m altitude ( Soerianegara & Lemmens 1993; Tropical Plants Database 2014 -onward; Barstow 2018b); Shorea obtusa lives in dry lowland deciduous dipterocarp forests, in savannas and in monsoonal forests with a marked dry season and waterlogged periods, up to 1000 m altitude ( Soerianegara & Lemmens 1993; Ghazoul 2016); Shorea robusta is a common semi-deciduous tree in South Asia in areas with a dry season lasting 4 to 8 months (a monsoon climate). Thus, it is mainly found in dry deciduous forests and savannas, but also in evergreen moist forests on well-drained soil and riverbanks. It is usually found below 800 m altitude ( Wu et al. 2007; Timilsina et al. 2007; Orwa et al. 2009).

Figures

FIG. 17. — Shoreoxylon sp. 1, MNHN.F.50194: A, Ts, vessels often grouped, sometimes in clusters (white arrows), abundant parenchyma, aliform to aliformconfluent, short lines and diffuse secretory canals (black arrows); B, C, Tls, 1-5-seriate rays, tyloses (arrow); D, Ts, zoom in on secretory canals in short lines and vessels in clusters with vasicentric tracheids or small vessels; E, Rls, weakly heterocellular rays with one row of larger procumbent cells (white arrow) or square to upright cells (black arrow); F, Tls, vasicentric tracheid (black arrow) and vasicentric parenchyma (white arrow); G, Ts synthetic drawing of the transverse section (only a few vessels and rays are displayed); secretory canals are displayed as red dots, very few are preserved. Scale bars: A, 1 mm; B, F, 200 µm; C, E, 500 µm; D, 250 µm; G, 1 cm. Abbreviations: see Fig. 2.