Shoreoxylon cf. deomaliense Prakash & Awasthi

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 : 882-884

Shoreoxylon cf. deomaliense Prakash & Awasthi ( Fig. 15 )

Shoreoxylon deomaliense Prakash & Awasthi, 1971: 219 , pl. 1, figs 3-4.

HOLOTYPE. — Birbal Sahni Institute of Palaeosciences Museum , India, specimen no. 34050.

MATERIAL. — MNHN.F.50192 (field number: NAT17-4). Estimated minimal diameter: 25-37 cm.

LOCALITY. — Kalewa Township, Sagaing Region, Myanmar.

AGE. — Upper lower to lowermost middle Miocene.

DESCRIPTION

Wood diffuse-porous, showing lateral compression. Growth limits marked by tangential canal lines. Vessels about 77-90% solitary ( Fig. 15A ) and in radial groups of 2-4, oval due to lateral compression, 4-13 per mm² (average: 8; likely overestimated due to compression); tangential diameter 120-200 µm (average: 150 µm; likely underestimated due to compression). Tyloses present ( Fig. 15F ). Vessel elements 140-460 µm (average: 330 µm) long. Perforation plates simple. Intervessel pits alternate. Vessel-ray pits not preserved. Vasicentric tracheids present ( Fig. 15F ). Parenchyma mostly vasicentric and aliform with short wings, sometimes confluent ( Fig. 15B ) or rarely forming thin bands from several rays to rays; diffuse parenchyma with cells larger than fibres ones ( Fig. 15K ), sometimes gathered in small groups; thin bands of parenchyma are tangentially crossing the section at regular intervals (5-7 mm). They can contain secretory canals but these ones are not always visible ( Fig. 15H ). When no canal is present, the bands are only 1-4 cells wide. Parenchyma cells 50-90 µm long (average: 70 µm), 15-35 µm wide (average: 25 µm) in tangential section; sometimes crystals in chambered cells (up to 8 crystals per strand seen) ( Fig. 15E ). Parenchyma cells can be more or less enlarged in the form of idioblasts. Rays 1- to 5-(6-)seriate (mainly 4) ( Fig. 15C ), uniseriate about 15% of the rays, non-storied, 5-10 rays per mm (average: 8), 270-1600 µm (average: 650 µm) or up to 30-40 (even 70) cells high, heterocellular made of procumbent cells with 1-4 or more upright cells at the ends ( Fig. 15G ), end-to-end fusion possible resulting in very high rays ( Fig. 15D ). Fibres non-septate, 5-19 µm (average: 12 µm) wide, thin-to-thick walled (lumina 1 time the double wall thickness in average) ( Fig. 15J ). Secretory canals in long tangential lines surrounded by parenchyma ( Fig. 15A, H, I ), but also in seemingly short lines probably due to compression in concentric parenchyma bands, or very rarely scattered by 2 in the section, 30-100 µm in tangential diameter (average 60 µm).

DISCUSSION

This specimen is characterized by: 1) diffuse-porous wood; 2) mostly solitary vessels as well as in radial groups; 3) crystalliferous and mostly aliform parenchyma, as well as diffuse; 4) 1-6-seriate heterocellular rays; 5) long tangential lines of secretory canals; and 6) vasicentric tracheids. As for our previous specimens (start p. 878), these features are diagnostic of the Dipterocarpaceae family. According to the identification key of Schweitzer (1958), long tangential lines of canals are found in the genera Shorea , Dryobalanops , Hopea , and Parashorea Kurz but the latter three can be dismissed: the genus Dryobalanops has exclusively solitary vessels and visible fibretracheids, Hopea has smaller and more frequent vessels (less than 200 µm in average diameter for 10-20 or more vessels per mm²), while Parashorea has less vessels and larger rays (up to 7-seriate) ( Metcalfe & Chalk 1950; Gottwald & Parameswaran 1966; Soerianegara & Lemmens 1993; Richter & Dallwitz 2000 -onward; Ogata et al. 2008). The genus Shorea is divided into several sections that are more or less phylogenetically supported and roughly characterized by few features: section ‘Pentacme’ by big vessels, section ‘Richetioides’ (or ‘Richetia’) by the presence of radial canals, section ‘Anthoshorea’ by the presence of silica bodies in ray cells as well as short rays, thin-walled fibres and rare crystals in parenchyma, sections ‘Rubroshorea’ by solitary crystals or in short chain of non-chambered (or chambered) parenchyma cells and idioblasts, section ‘Shorea’ by short rays, few marginal ray cells and crystals in long chains of chambered parenchyma cells and idioblasts. The present wood would thus be close to the section ‘ Shorea ”.

Among Shorea extant species, Shorea laevis Ridl. shares many features of our fossil including the aliform parenchyma (but without crystals), the vessel size and density, the similar rays (mostly 3-5-seriate) with few marginal cells and few uniseriate rays. The same arrangement of crystalliferous parenchyma and ray size is found in S. parvifolia Dyer , S. pauciflora King , S. atrinervosa Symington. , and with a lesser extent in S. maxwelliana King and S. almon Foxw.

Shoreoxylon groups the fossil specimens close to all Shorea and Parashorea . The genus Hopenium was instituted ( Awasthi 1980) for woods resembling Hopea , with upright ray cells in the middle of the rays. Species descriptions in Shoreoxylon are often overlapping and rarely consider inter- and intraspecific variations. In addition, they sometimes lack diagnostic characters or qualitative illustrations. Consequently, it is difficult to identify a unique species that could be attributed to our specimen. Some species display features that are close to our fossil ( Appendix 1): Shoreoxylon burmense Prakash ( Prakash 1965a, 1973; Licht et al. 2014) share the same type of rays, the parenchyma is also quite similar, but the secretory canals are grouped in very close lines, from 2 to 4, which is not the case in our fossil, and it has no crystal in parenchyma. Shoreoxylon indicum Awasthi (1974) has the same vessel and ray distribution, crystals in parenchyma cells, but its apotracheal and confluent parenchyma are more developed whith only 5 crystals per parenchyma strands (up to 8 in our fossil). Shoreoxylon posthumi Schweitzer (1958) has crystals in parenchyma as well as enlarged parenchyma cells, but it has much developed apotracheal parenchyma and its canal lines are irregularly distributed or superimposed. Shoreoxylon tipamense Prakash & Awasthi (1970) has similar vessel, parenchyma and ray arrangement, but it also has bigger vessels, larger canals, thinner fibres cell walls and sheath cells. Shoreoxylon deomaliense is the closest fossil species to our specimen ( Prakash & Awasthi 1971; Licht et al. 2014), though the present fossil has a lesser frequency of vessels, more aliform parenchyma and slightly thinner rays (up to 6-seriate, compared to 7-seriate for S. deomaliense ) with shorter rows of marginal ray cells. Considering its preservation, we attribute our fossil to Shoreoxylon cf. deomaliense .

Shorea is a genus of tropical Asian trees growing in humid lowland areas, on podzols and peat swamps, mostly below 1000 m altitude ( Ashton 1982; Soerianegara & Lemmens 1993). Shorea laevis mostly grows on well-drained to dry soils, on ridges or hillsides up to 1000 m. It is also found in lowland mixed dipterocarp forests and on alluvial sites ( Ashton 1982; Soerianegara & Lemmens 1993; Pooma et al. 2017). All the other species cited above are found in mixed dipterocarp forests in lowlands or on rolling hills, on well-drained soils at up to 1000 m altitude ( Ashton 1982, 2004).

Figures

FIG. 15. — Shoreoxylon cf. deomaliense Prakash & Awasthi, MNHN.F.50192: A, Ts, mostly solitary vessels and long tangential line of secretory canals embedded in parenchyma bands (arrow); B, Ts, zoom in on solitary vessels, mostly aliform parenchyma as well as some diffuse parenchyma (arrow); C, D, Tls, long 1-6-seriate rays with end-to-end fusion (arrow); E, Rls, crystals in chambered parenchyma cells; F, Tls, vasicentric tracheids (black arrow) and tylose in vessel (white arrow); G, Rls, heterocellular ray with upright marginal cells; H, Ts, synthetic drawing of the transversal section (with only some vessels and rays displayed); visible bands of parenchyma are shown as grey lines and secretory canals as red dots (orange when their identification is less clear), arranged mostly in long tangential lines; I, Ts, zoom in on secretory canals embedded in parenchyma; J, Ts, thin-to-thick walled fibres; K, Ts, diffuse parenchyma (arrows). Scale bars: H, 1 cm; A, 1 mm; B, D, I, 250 µm; C, G, 200 µm; F, K, 100 µm; E, J, 50 µm. Abbreviations: see Fig. 2.

FIG. 2. — Cupressinoxylon sp. MNHN.F.50171: A, transverse section (Ts), growth limits (white arrows), diffuse parenchyma (grey arrows) and zonate parenchyma (black arrows), large black areas are cells filled with deposit material; B, Ts, growth limit with gradual transition from earlywood to latewood; C, tangential longitudinal section (Tls), 1- sometimes 2-seriate rays and parenchyma lines (arrow); D, Tls, detail of 1- sometimes 2-seriate rays; E, Tls, smooth longitudinal parenchyma walls (arrows); F, Tls, 1-2-seriate rays and tangential pits (arrows); G, radial longitudinal section (Rls), possibly cupressoid and/or podocarpoid crossfield pits (arrow); H, Rls, uniseriate radial pits; I, Rls, smooth parenchyma cell walls.Scale bars: A, C, 1 mm; B, 200 µm; E-F, H, 100 µm; G, 50 µm; I, 20 µm.