indet. 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 : 892-894

indet. sp . 1

( Fig. 20 A-F)

MATERIAL. — MNHN.F.50199 (field number: 17FN08). Estimated minimal diameter: 33 cm.

LOCALITY. — Kalewa Township, Sagaing Region, Myanmar.

AGE. — Upper lower to lowermost middle Miocene.

DESCRIPTION

Wood diffuse-porous. Growth rings distinct, marked by marginal parenchyma ( Fig. 20A ). Vessels mostly solitary (80%) or in groups of 2, rarely 3 ( Fig. 20A ), oval in shape, 1-7 per mm² (average: 3); tangential diameter 120-200 µm (average 160 µm). Tyloses absent. Vessel elements 190-430 µm (average: 310 µm) long. Perforation plates simple. Intervessel pits alternate, 3-6 µm in diameter (average: 5 µm). Vessel-ray pits maybe of the same nature as intervessel pits. Parenchyma scanty paratracheal or vasicentric ( Fig. 20A ), in (2-6 cells wide?) marginal bands, also around canals; parenchyma cells 60-130 µm long (mean 90 µm), 20-50 µm wide (average: 35 µm) in tangential section ( Fig. 20D ), 2-5 cells per parenchyma strand; the epithelial parenchyma around the canals possibly contains crystals in chambered cells. Rays 1- to 4-seriate (mostly 3-seriate) ( Fig. 20B, C ), uniseriate rays about 10-20% of the rays, 2-12 cells high, 4-7 rays per mm (average:6), multiseriate rays 310-1260 µm (average: 720 µm) high or 11-44 cells (average: 24 cells), heterocellular made of procumbent cells with 1-8 (or more) upright cells at the ends ( Fig. 20B, C ), sheath cells sometimes present, some rays appear with mixed procumbent and upright cells in radial section ( Fig. 20F ), ray cells possibly containing prismatic crystals. Fibres non-septate, 11-26 µm in diameter (average: 20 µm). Canals diffuse or in short tangential lines, tangentially as big as vessels, radially longer up to 500 µm (average: 350 µm) ( Fig. 20A, E ).

DISCUSSION

The most diagnostic character of the fossil is the presence of wide pores (tangentially the same size as vessels but radially longer) that are often 2-6 tangentially grouped. In tangential section, these pores do not show any perforation plate and are surrounded by parenchyma, as expected for secretory canals. The presence of solitary secretory canals or in short tangential lines in diffuse-porous wood are only found in Dipterocarpaceae and Fabaceae (in the Detarioideae subfamily, more specifically in the Prioria, Detarieae and some of the Daniellia clades) ( InsideWood 2004 -onward; De la Estrella et al. 2018; Choo et al. 2020). The specimen displays the same ray arrangement as modern Dipterocarpaceae ; it also has marginal or seemingly marginal parenchyma bands without tyloses, as found within the Fabaceae family.We note a close affinity with extant Dipterocarpus , as the presence of short tangential lines is very characteristic of this genus ( Schweitzer 1958), but the attribution to Dipterocarpaceae is uncertain because we can not observe vasicentric tracheids or simple vessel-ray pits. In addition, vessels are not exclusively solitary as expected for Dipterocarpus . For the second family, short tangential lines of canals are mostly found in the genera: Prioria Griseb., Daniellia Benn. , Copaifera L., Detarium Juss. , Eperua Aubl. and Sindora Miq. ( Gasson 1994) . Canals at least as big as vessels are only reported in Prioria (synonyms: Kingiodendron Harms , Gossweilerodendron Harms , Oxystigma Harms, Pterogopodium Harms and Eriander H.J.P.Winkl. ) and especially in Prioria copaifera Griseb. , which is described with parenchyma scanty to aliform and in 3-4(6) cells wide bands; 2-4 cells per parenchyma strands, rays mostly 1-3-seriate up to 37 cells high with many uniseriate rays and upright marginal cells. Parenchyma can also contain crystals especially in the epithelial cells of canals ( Banks & Gasson 2000; Gasson et al. 2003). The former genus Kingiodendron share similar characteristics ( Banks & Gasson 2000; Gasson et al. 2003). However, Prioria is not reported with sheath cells and have vessel-ray pits similar to intervessel pits, as well as mostly diffuse canals.

Fossils described with diffuse and short lines of secretory canals belonging to Dipterocarpaceae or Fabaceae , with the particularity of bigger canals than vessels are relatively few. They are reported bigger or of the same size as vessels in Dipterocarpoxylon surangei Prakash (1981) , D. premacrocarpum Prakash (1975) and D. arcotense Awasthi (1980) , but their vessels are almost exclusively solitary, and their rays are broader (up to 5- or 7-seriate). Bancroft described fossil specimens from Africa twice ( Bancroft 1933, 1935) under the name D. africanum Bancroft. They share many features of our fossil, including 1-4-seriate rays (mostly 3-seriate) with many upright cells in rays with up to 6-cells in marginal rows (mostly for biseriate rays) and up to 30 cells high. Bancroft (1935) mentioned structures that looked like vasicentric tracheids which suggests an affinity to Dipterocarpaceae , though the author still mentions a possible affinity with Detarioideae , as vesselray pits are still not visible. Many fossil species related to Detarioideae have no canals. Among those with canals, Hopeoxylon Awasthi (1977) has often banded parenchyma and canals in short to often long lines, smaller than vessels. Several specimens of Kingiodendron ( Awasthi & Prakash 1987; Awasthi 1992; Guleria et al. 2002; Pérez-Lara et al. 2021) have the same size and arrangement of canals than the present fossil, but rays are much shorter and less heterocellular or canals are smaller than vessels. Ramos et al. (2017) described two fossil genera: Paraoxystigma Ramos et al. which has only diffuse canals and only one row of marginal ray cells; and Gossweilerodendroxylon Ramos et al. which has no uniseriate rays, only weakly heterocellular rays and canals much smaller than vessels. Two fossil wood specimens of Prioria are described by Rodríguez-Reyes et al. (2017): P. hodgesii Rodríguez-Reyes et al. which has smaller canals than vessels and P. canalensis Rodríguez-Reyes, Gasson, Falcon-Lang & Collinson , which shares most of the features of our fossil except that it displays long lines of canals, parenchyma is sometimes aliform and strands of parenchyma are composed of 3-8 cells (down to 2 cells for our fossil). As it is not possible to determine precisely the nature of vessel-ray pits nor if there are any vasicentric tracheids or common crystals in our specimen, the attribution to a given family is difficult. As a consequence, we consider this specimen as undetermined.

Modern Dipterocarpus are mainly tropical trees growing in evergreen, sometimes present in semi-evergreen forests or dry deciduous dipterocarp forests, mostly in lowlands and occasionally up to 1400 m. ( Ashton 1982; Soerianegara & Lemmens 1993; Ghazoul 2016). Prioria s.l. is a genus of large trees adapted to seasonally-flooded riparian environments (Rodríguez-Reyes et al. 2017). Prioria s.s. is an American tropical tree growing in lowlands often in coastal forests, sometimes swamps and along estuaries (Rodríguez-Reyes et al. 2017), whereas Kingiodendron is found in Asia in evergreen rainforests at low elevation, and flood-plains up to 800 m altitude ( Hou et al. 1996; Pascal et al. 2004).

indet. sp. 2

( Fig. 20 G-J)

MATERIAL. — MNHN. F.50200 (field number: NAT17-5). Estimated minimal diameter: 11 cm.

LOCALITY. — Kalewa Township, Sagaing Region, Myanmar.

AGE. — Upper lower to lowermost middle Miocene.

DESCRIPTION

Wood diffuse-porous. Growth rings indistinct. Vessels solitary (55%), often grouped by 2 but also up to 4 ( Fig. 20G ), round to oval, 4-12 per mm² (average: 7); tangential diameter 90-200 µm (average: 150 µm).Tyloses absent. Vessel elements 210-420 µm (average:330 µm) long. Perforation plates simple. Intervessel pits alternate, 3-6 µm in diameter (average: 5 µm). Parenchyma vasicentric possibly aliform; parenchyma cells 60-100 µm long (average: 80 µm), 12-30 µm wide (average: 20 µm) in tangential section. Rays 1- to 5-seriate (mostly 3-seriate) ( Fig. 20H, I ), 5-8 rays per mm (average: 6), 270- 1130 µm (average: 650 µm) or 6-36 cells high, heterocellular made of procumbent cells with 1-2 upright cells at the ends ( Fig. 20J ); some sheath cells present ( Fig. 20I ). Fibres nonseptate, 8-20 µm in diameter (mean 15 µm). Secretory canals possibly present as some ducts without perforation plates seem to be filled with an orange content ( Fig. 20G ), also coloring the surrounding cells, recalling a resin.

DISCUSSION

This fossil recalls Dipterocarpaceae because of: 1) ray heigth, strongly heterocellular; and 2) ducts filled with orange content and without any trace of perforation plates, which could be secretory canals, in tangential sections. Unfortunately, these potential canals are not observable in transverse section, nor any vasicentric tracheids.This specimen resembles Shoreoxylon sp. 1 or Shoreoxylon cf. deomaliense for ray size and arrangement, as well as vessel size. The state of preservation of this specimen is not good enough for a conclusive determination; it is impossible to clearly distinguish parenchyma, fibres and pore outlines, suggesting that the specimen has been degraded (see part 5.1).

Figures

FIG. 20. — Indet. sp. 1, MNHN.F.50199 (A-F): A, Ts, vessels and few vasicentric parenchyma, solitary secretory canals (arrows) or in short tangential lines, surrounded by parenchyma,remarkably bigger than vessels;B, C, Tls,1-4-seriate rays with sometimes long uniseriate row of marginal cells (arrow); D, Tls,parenchyma cells (white arrow) and secretory canal (black arrow); E, Tls, detail of secretory canals in short tangential line; F, Rls, heterocellular rays with procumbent cells (black arrow) and square or upright cells (white arrow). G-J, Indet. sp. 2, MNHN.F.50200: G, Ts, poorly preserved wood, vessel arrangement often in groups; H-I, Tls, 1-5-seriate rays, with maybe sheath cells (arrows); J, Rls, heterocellular rays with procumbent and square or upright cells.Scale bars: A, G, 1 mm; B, H, 500 µm; C-F, I-J, 200 µm. Abbreviations: see Fig. 2.