Burseroxylon 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 : 890-892

Burseroxylon sp.

( Fig. 19 A-J)

MATERIAL. — MNHN.F.50196 (field number: 17FN05) , MNHN.F.50197 (field number: 17FN11) , MNHN.F.50198 (field number: NAT17-01). Estimated minimal diameter:> 30 cm, as rays are almost parallel.

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 (75%) or in radial groups of 1-4 (25%) ( Fig. 19A ), oval to round, 2-13 per mm² (average: 7); tangential diameter 90-280 µm (average: 190 µm). Tyloses present in most vessels ( Fig. 19A, D ), budding from ray parenchyma cells. Vessel elements 130-540 µm long (average: 310 µm). Perforation plates simple. Intervessel pits alternate, polygonal shaped, 7-10 µm wide (average 8.5 µm). Vesselray pits apparently simple with different shapes and sizes, mostly horizontal (5-20 µm, 13 µm in average) ( Fig. 19E ). Parenchyma exclusively paratracheal, scanty and vasicentric forming a 1-cell sheath around vessels ( Fig. 19A, D ); parenchyma cells 45-90 µm long (average: 60 µm), 25-45 µm wide (average: 35 µm) in tangential section; some crystals are present in chambered parenchyma cells in the specimen MNHN.F.50197 Rays 1- to 4-seriate, mostly 3- or 2-seriate, always more than 50% of the rays are 3-seriate, rarely uniseriate, ( Fig. 19B ), non-storied, 3-12 rays per mm (average: 7), 150-640 µm (average: 380 µm) or 4-23 cells high, heterocellular made of procumbent cells with 1(-3) square or upright cells at both ends ( Fig. 19C, F, G ); crystals sometimes present in upright or enlarged marginal cells ( Fig. 19F ), maybe in procumbent cells as well ( Fig. 19G ). Fibres thin-to-thick to thin-walled (lumina 2.5 times the double wall thickness in average), almost all fibres are septate ( Fig. 19B ), tangential diameter 10-30 µm (average: 18 µm). Traumatic canals present (specimens MNHN.F.50197 and MNHN.F.50198) as holes of different size and shape surrounded by clusters of undifferentiated cells full of black content ( Fig. 19 H-J). The specimen MNHN.F.50198 displays an enlarged radial line of undifferentiated tissue embedding some rays that recalls a healed crack filled with tissue during the growth of the wood ( Fig. 19J ) that ends with a line of traumatic canals ( Fig. 19I ). Radial canals absent.

DISCUSSION

These specimens are characterized by: 1) diffuse-porous wood with isolated and tylosed vessels; 2) exclusively simple perforation plates; 3) septate fibres which although are not always very well preserved are clearly present throughout the whole sample; 4) exclusively vasicentric parenchyma; 5) growth rings indistinct or absent; and (6) 2- to 4-seriate heterocellular rays. These features suggest affinities with Burseraceae as well as some Anacardiaceae ( Metcalfe & Chalk 1950; InsideWood 2004 -onward; Ogata et al. 2008). Both families share several similar anatomical features. However, the presence of septate fibres is more consistent in Burseraceae , as well as a greater homogeneity in rays and parenchyma patterns, and the presence of crystals in enlarged ray cells. Moreover, traumatic canals are not reported in Anacardiacae, whereas they are reported in Burseraceae ( Metcalfe & Chalk 1950) . One species of Anacardiaceae resembles our specimens: Lannea coromandelica (Houtt.) Merr. , which has similar vessel, ray and parenchyma patterns, crystals in normal and enlarged marginal cells, in parenchyma and in procumbent cells; however, silica bodies are present in its ray cells and it is always described with radial canals ( InsideWood 2004 -onward; Gupta & Agarwal 2008), which are absent in our fossils. The three present specimens show slight differences in vessel and ray density, ray height and width, and crystal presence (the specimens MNHN.F.50197 has slightly shorter rays, more frequent 2-seriate rays, crystals in non-enlarged marginal cells, in parenchyma cells and possibly in procumbent ray cells) but the range of these variations is within the inter- and intraspecific diversity of Burseraceae ( Metcalfe & Chalk 1950; InsideWood 2004 -onward; Ogata et al. 2008). The wood anatomy of Burseraceae is very homogenous, both among extant and fossil species ( Awasthi & Srivastava 1989; Prasad 1993; Ogata et al. 2008). Among Burseraceae , our fossils resemble the genera Canarium L. and Protium Burm. f. which display homogeneous 1-3-(up to 4-)seriate rays and sometimes traumatic canals ( Metcalfe & Chalk 1950). The examination of Burseraceae from the Xylarium of the MNHN highlighted a resemblance between the fossil specimens MNHN.F.50196 and MNHN.F.50198 with Canarium bengalense Roxb. (specimen MNHN-P-P00395579), because of crystals only present in enlarged marginal ray cells, similar vessel size and frequency (sometimes <5 / mm² and over 100 µm of diameter) and ray pattern (1-3-seriate, 3-16 cells high). Ogata et al. (2008) mention that the genus Canarium can have rays up to 4-seriate. The fossil specimen MNHN.F.50197 is closer to Protium serratum (Wall. ex Colebr.) Engl. (specimens MNHN-P- P00396706 and MNHN-P-P00396663) as it displays crystals in normal marginal cells as well as sometimes in procumbent cells and in parenchyma cells, smaller and more frequent rays than Canarium (sometimes> 10/mm 2 and under 100 µm of diameter). Protium serratum is described with and without radial canals ( InsideWood 2004 -onward).

Burseraceae are not numerous in the fossil record ( Gregory et al. 2009) and include two dominant fossil genera: 1) Burseroxylon which includes fossils related to Bursera Jacq. ex L. (and by extension Protium ) and some Garuga Roxb. ( Lakhanpal et al. 1981) ; and 2) Canarioxylon Prakash, Březinová & Awasthi which includes fossils related to Canarium . Some fossils are also described under the name Canarium . The diagnoses of Burseroxylon and Canarioxylon are very similar; Burseroxylon is different in having (only sometimes) visible growth rings, crystalliferous rays, vasicentric parenchyma in addition to scanty paratracheal, sometimes radial canals, and rays 5-seriate or more (instead of 1-4 seriate in Canarioxylon ). The genus Garugoxylon Vozenin-Serra & Privé-Gill (1991) is not taken into consideration as the diagnosis can be included in the one of Burseroxylon ( Lakhanpal et al. 1981) . Only one fossil is described with crystals in parenchyma cells: Canarioxylon noduliforme Yang, Guo & Wei, 1995 , but it displays radial canals and larger rays among other incompatible features. If we strictly follow the diagnoses of the genera, our fossils are attributed to Burseroxylon and not Canarioxylon , because of the presence of vasicentric parenchyma and crystals in ray cells. Awasthi & Srivastava (1989) underline that Burseraceae species share overlapping descriptions and diagnoses; because of this overlap, we do not attribute our specimens to any Burseroxylon species and attribute our fossils to Burseroxylon sp. while awaiting a taxonomic clarification of the genus and the family.

Protium is a genus of trees growing in America, Madagascar and Asia in evergreen to semi-deciduous forests up to 800 m altitude ( Sosef et al. 1998). Protium serratum lives in humid areas of India and tropical forests of Myanmar ( Prasad 1993), and in moist evergreen and deciduous forests of Thailand ( Tropical Plants Database 2014 -onward), in riparian areas ( Gamble 1902). Up to 600-1000 m altitude ( Wu et al. 2008). Canarium species live in Africa, Asia and Australia in wet evergreen rainforests, mainly in lowlands, but also in monsoonal climate and open forests ( Lemmens et al. 1995). Canarium bengalense is a tree living in evergreen, moist and mixed dipterocarp forests below 1300 m altitude ( Leenhouts 1959; Wu et al. 2008; Ayyappan & Kokilavani 2020).

Figures

FIG. 19. — Burseroxylon sp., MNHN.F.50196 (C), MNHN.F.50197 (A, B, D-E, G, H), MNHN.F.50198 (F, I, J): A, D, Ts, vessels with tyloses (white arrow) and thin sheath of vasicentric parenchyma (black arrow); B, Tls, (1)-4-seriate rays, septate fibres (arrow); C, Rls, heterocellular rays with upright marginal cells; E, Rls, simple vessel-ray pits, oval in shape;F, Rls, heterocellular rays with upright marginal cells, some of them are enlarged and contain a single crystal (arrow); G, Rls, crystals in upright marginal cells (white arrow), maybe also in procumbent cells sometimes (black arrow); H, I, Ts, traumatic canals (arrows); J, Tls, tangential view of undifferentiated tissue surrounding a traumatic canal. Scale bars: A, 1 mm; B-D, F-H, J, 200 µm; E, 50 µm; I, 500 µm. Abbreviations: see Fig. 2.