Anisopteroxylon sp.

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 : 873-875

Anisopteroxylon sp.

( Fig. 11 )

MATERIAL. — MNHN.F.50188 (field number: 17FN07). Estimated minimal diameter: non-assessable due to compression.

LOCALITY. — Kalewa Township, Sagaing Region, Myanmar.

AGE. — Upper lower to lowermost middle Miocene.

DESCRIPTION

Wood diffuse-porous. Growth rings indistinct or absent. Vessels mostly solitary (>90%) oval, 2-13 per mm² (aver- age: 6) ( Fig. 11A ); tangential diameter 80-230 µm (average: 150 µm; measured on less compressed vessels, n=30). Tyloses present.Vessel elements 130-290 µm long (average: 220 µm). Intervessel and vessel-ray pits not found due to compression and rare occurrence of vessel multiples. Perforation plates simple ( Fig. 11B ). Vasicentric tracheids present ( Fig. 11B ). Axial parenchyma not well defined due to compression, probably vasicentric and/or diffuse; without mineral inclusions. Rays 1- to 7-seriate, mostly 6- to 7-seriate ( Fig. 11C, D ), rare uni- or biseriate made of upright or square cells, non-storied, 4-7 rays per mm (average: 5), 510-1500 µm (average: 1050 µm) or up to 60 cells high, heterocellular made of procumbent cells and continuous 1-seriate margin of sheath cells ( Fig. 11C, D ) which may appear as a mix of procumbent and upright cells in radial section, 2- to 3-seriate rays with over 4 rows of upright or square marginal cells ( Fig. 11E ), otherwise, only one row; silica bodies (?) present in some ray cells ( Fig. 11F ). Fibres non-septate, tangential diameter 9-25 µm (average: 16 µm). Secretory canals cannot be identified but we observed some scattered, very small pores ( Fig. 11A ); no signs of tangential bands of any kind that could indicate a presence of canal lines.

DISCUSSION

This specimen is characterized by: 1) diffuse-porous wood; 2) exclusively simple perforation plates; 3) solitary vessels; 4) heterocellular with mostly 5-6-seriate rays; 5) continuous sheath cells; and 6) vasicentric tracheids. These features suggest an affinity with modern Malvaceae and Dipterocarpaceae ( Metcalfe & Chalk 1950) . Most Malvaceae have vasicentric to confluent parenchyma, apotracheal parenchyma, rays of two distinct sizes (1- and 4- to 9-seriate; Metcalfe & Chalk 1950), and no vasicentric tracheid, which contrast with our specimens. Dipterocarpaceae share all these features, but are additionally characterized by vertical canals, except for non-Asian genera Monotes De Candolle , Marquesia Gilg and Pakaraimaea Maguire & P.S. Ashton ( Chowdhury & Ghosh 1958; Ogata et al. 2008). The character of secretory canals apart, and searching for diffuse-porous woods with vasicentric tracheids, exclusively solitary vessels, rays more than 4-seriate, heterocellular and with sheath cells on InsideWood (2004 -onward) (code: 5p 9p 60p 96a 98p 104a 105a 110p 118a) gives us back multiple results. But adding the features 159 and 160 (silica bodies present in ray cells), only the Dipterocarpaceae family matches with our specimen. Even without silica bodies in ray cells, the affinity with Dipterocarpaceae is the most likely. Among them, Cotylelobium Pierre and Dryobalanops C.F.Gaertn. genera are ruled out because of smaller vessels (<150 µm) and secretory canals in long lines respectively ( Schweitzer 1958; Ogata et al. 2008). Although the preservation state of our fossil is not good enough to see individual secretory canals or parenchyma surrounding them, their long lines or seemingly bands should have been observable if they were present, as mentioned by Schweitzer (1958).

Following the identification key of the main genera of Dipterocarpaceae provided by Schweitzer (1958), we narrow down the identification to two genera: Anisoptera or Dipterocarpus . They are not always distinguishable from each other when the information about the secretory canals is not available. However, Anisoptera has solitary secretory canals or in short tangential lines (rarely more than 2-3 canals), and sheath cells are frequent in rays; Dipterocarpus has comparatively longer tangential lines (2-8 canals), more frequent uniseriate rays and uniseriate portions in rays and less sheath cells, although they can sometimes be present in the whole ray ( Chowdhury & Ghosh 1958; Gottwald & Parameswaran 1966; Prakash & Tripathi 1970; Ogata et al. 2008). We also observed these trends on Anisoptera and Dipterocarpus specimens available at the MNHN (CTFT19039, CTFT18446, CTFT11366, MNHN-P- P00402617, P00400540, P00415879, P00406060) and on InsideWood (2004 -onward). Our specimen displays features closer to those of genus Anisoptera , and in particular with species Anisoptera costata Korth. and Anisoptera scaphula (Roxb.) Kurz. Both display the same wide rays (up to 7-8-seriate) as well as uniform and unfrequent uniseriate rays and solitary vessels. These two species are hardly distinguishable from each other based on xylotomy ( Chowdhury & Ghosh 1958).

Seven fossil genera of Dipterocarpaceae are described ( Gregory et al. 2009). The genus Anisopteroxylon has been instituted by Ghosh & Kazmi (1958) and emended by Poole (1993) for fossil resembling Anisoptera , leaving the genus Dipterocarpoxylon (Holden) Den Berger for fossil resembling Dipterocarpus only (see Prasad & Gautam 2016, p. 263). The differences between Anisopteroxylon and Dipterocarpoxylon are mostly based on secretory canals (solitary for Anisopteroxylon and in short tangential bands for Dipterocarpoxylon ). The quantity of sheath cells is not mentioned in the diagnosis, although Ghosh & Kazmi (1958) describe them as “rather common”, and uniseriate rays as “not very common” (Ghosh & Kazmi 1958: 486). Even though our fossil has no visible canals, which is possibly due to poor preservation and compression, its solitary vessels, vasicentric tracheids, abundant sheath cells, broad rays with silica bodies in some cells, make it compatible with the genus Anisopteroxylon . Some specimens of this genus are close to our fossil ( Appendix 1): A. garoense (Chowdhury) Prakash & Tripathi ( Prakash & Tripathi 1970) from the upper Miocene of India, for its 7-9-seriate rays, 4-5 vessels/mm², and the presence of identical continuous sheath cells; A. oblongoides Yadav ( Yadav 1989) and A. surmaensis Prasad, Agarwal & Mandaokar ( Prasad et al. 2009) but the former has long tangential lines of canals of equal size as vessels and the latter has narrower rays (mostly 4-5-seriate); Anisopteroxylon jawalamukhi Ghosh & Ghosh ( Ghosh & Ghosh 1958) has longer rays (up to 1950 µm). Considering the lack of several diagnostic characters at the species level in our specimen, we only assign it to Anisopteroxylon sp.

Anisoptera is a genus of south-east Asian trees, absent in India, living in evergreen or semi-evergreen forests; in mixed dipterocarp, mixed swamp or heath forests; on well-drained soils or in peat swamps, rarely above 1000 m altitude ( Ashton 1982; Soerianegara & Lemmens 1993). Anisoptera costata is a tropical tree that can be found in Myanmar, living in semievergreen dipterocarps forests and evergreen forests of seasonal areas. Mostly in moist or slightly dry areas. It rarely occurs in everwet forests but grows well along rivers and streams, at up to 700 m altitude ( Ashton 1982; Smitinand et al. 1990; Soerianegara & Lemmens 1993; Sam et al. 2004; Nguyen et al. 2017). Anisoptera scaphula grows in semi-evergreen and evergreen dipterocarp forests on foothills ( Ashton 1982; Soerianegara & Lemmens 1993; Ly et al. 2020).

Figures

FIG. 11. — Anisopteroxylon sp., MNHN.F.50188:A, Ts, strongly compressed wood,poorly distinguishable vessels,seemingly exclusively solitary,small pores that could be solitary secretory canals (arrows); B, Tls, simple perforation plates, vasicentric tracheids (arrows); C, D, Tls, 1-7-seriate rays with continuous sheath cells all around multiseriate rays (arrow); E, Rls, heterocellular rays with procumbent cells in median portion (white arrow) and square or upright marginal cells (black arrow), sometimes appearing with both types mixed due to sheath cells; F, Rls, silica bodies (?) in ray cells. Scale bars: A, C, 500 µm; B, D-E, 200 µm; F, 100 µm. Abbreviations: see Fig. 2.