Palmoxylon dutoitii KAUL , 1945

Bamford, Marion & Pickford, Martin, 2021, Stratigraphy, Chronology And Palaeontology Of The Tertiary Rocks Of The Cheringoma Plateau, Mozambique, Fossil Imprint 77 (1), pp. 187-213 : 200-203

Palmoxylon dutoitii KAUL, 1945

Text-figs 13 , 14

S p e c i m e n. BP/16/1732 (slides and stem samples housed in the Palaeobotany Herbarium, Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg).

L o c a l i t y. Site 3, lower slopes on northwest side of Mhengere Hill, Gorongosa National Park, Mozambique ( Text-fig. 8 ).

D e s c r i p t i o n a n d c o m p a r i s o n s. There is one large trunk, about 90 cm in diameter lying on the side of the hill ( Text-fig. 13a ) accompanied by scattered fragments close by and farther down the hillside. Natural sections in broken specimens are easy to recognise by the regular arrangement of the large vascular bundles. Photomicrographs of the palm stem and an annotated drawing of a fibrovascular bundle (fvb) are presented in Text-fig. 14 .

The distribution of the fibre vascular bundles in the parenchymatous cortex is random and the bundles are more or less the same size throughout the cortex, the socalled Coccos - type ( Dransfield et al. 2005). Parenchyma is poorly preserved but appears to be homocellular with uniform and compact parenchyma cells ( Text-fig. 13c, d ). A few parenchyma cells appear to have thicker walls so are interpreted to be lignified ( Thomas and De Franceschi 2013). Diameters of the fibre vascular bundles range from 1,000 µm to 1,160 µm (1.0– 1.16 mm) and they are round to sub-round in cross-sectional shape ( Text-fig. 14a–f ).

Each fibre vascular bundle is composed of a fibrous part and a vascular part, with the reniform fibrous part surrounded by a thin layer of tabular parenchyma. The much smaller vascular part has an inner metaphloem region that tends to break down and leaves an empty patch. Within the paravascular parenchyma are mostly two large metaxylem elements (sometimes three) adjacent to each other with an average diameter of 116 µm, and external to these are one to two small protoxylem elements. The vascular section is surrounded by a narrow sheath of parenchyma that appears to merge with the parafibrous sheath.

Parenchyma between the fibre vascular bundles is poorly preserved but appears to be compact and homogeneous ( Text-fig. 14a ). In longitudinal section ( Text-fig. 14g, h ) the metaxylem elements have horizontal thickenings. The fibres are poorly preserved. There are only a few patches of intact parenchyma cells and they are rounded to slightly elongated. Parenchyma cells with darker and thicker cell walls that may be lignified occur throughout the stem. Scattered throughout the section are globular echinate phytoliths ( Text-fig. 14 ).

I d e n t i f i c a t i o n. The simple and collateral bundles distributed throughout the stem with a parenchymatous ground tissue are typical of the Arecaceae . The even distribution, or Coccos - type, of fibre vascular bundles throughout the cortex occurs in the Coryphoideae and a few of the Arecoideae, and reniform fibre vascular bundles are typical of the Borasseae ( Thomas and de Franceschi 2013). The two subtribes of the Borasseae (Lataniinae and Hyphaeninae) can be separated by the occurrence of two (to three) metaxylem elements and spheroid, compact ground tissue parenchyma in the Hyphaeninae, while the Lataniinae have only one metaxylem element per fibre vascular bundle and the ground tissue is composed of an irregular network of rhombohedral to lobed cells. The fossil palm, therefore, is a member of the Hyphaeninae, in which there are two extant genera, Borassus and Hyphaene .

Although Nypa and Sabal have the same type of vascular bundles, they are unlike the Mhengere Hill specimen. Nypa fruticans has a prostrate stem, up to 45 cm in diameter (Burkhill 1997) whereas the fossil stem described herein is 90 cm in diameter and from a tall, straight trunk. Sabal does not occur in Africa and it has no fossil record ( Pan et al. 2006); the extant species of the genus occur in the Americas.

There are several classifications of the palms (Palmae JUSS. / Arecaceae BERCHT. et J.PRESL ) (see review in Baker and Dransfield 2016), but we have selected to follow one that takes stem anatomy into account. According to the classification of the Arecaceae ( Baker and Dransfield 2016) there are five recognised subfamilies, one of which is the Coryphoideae that encompasses eight tribes. Compared with other tropical regions, the diversity of palms in Africa is low ( Uhl and Dransfield 1987) and is meagre in southern Africa with only Phoeniceae and Borasseae present. In this region there are eight native species ( Klopper et al. 2006, van Wyk and van Wyk 2013, Burroughs et al. 2018) in three genera: Phoenix , Hyphaene and Borassus .

The Mhengere Hill fossil stem is most similar to Hyphaeninae based on the structure and distribution of the fibre vascular bundles but without well-preserved ground tissue, it is not possible to determine which extant genus is the most similar.

Fossil Arecaceae in Africa are represented from the Aptian to the Pleistocene by fruits, leaves, petioles, pollen and stems (Vaudois-Miéja and Lejal-Nicole 1987, SalardCheboldaeff and Dejax 1991; see reviews in Harley 2006, Pan et al. 2006, Nour-El-Deen et al. 2018). Fossil stems are usually ascribed to Palmoxylon and about 23 species have been described to date, the majority from Egypt, some from other countries in North Africa and one species from South Africa ( Kaul 1945, Nour-El-Deen et al. 2018). Slides of Palmoxylon dutoitii ( Kaul 1945) were described based on material collected by Alex du Toit from river gravels of the Vaal River near Barkly West. According to du Toit (1929) the deposit is probably Tertiary in age. P. dutoitii has the Coccos - type regular distribution of reniform fibre vascular bundles. Each bundle has 2(–3) metaxylem elements and a dense fibrous cap. The ground tissue is composed of wellpreserved, spherical, compact parenchyma cells. Without any additional features, the Gorongosa fossil stem is considered to represent the same species as the Barkly West stem from South Africa.

Palms are restricted to tropical areas and most species do not tolerate frost ( Tomlinson 2006). Hyphaene and Borassus are mainly African taxa and all require a high water table, so they occur along water courses or in floodplains, in more arid areas in the case of Hyphaene thebaica and Hyphaene compressa , or in moister but not wet areas in the case of Hyphaene coriacea , Hyphaene petersiana , Hyphaene crinita and Borassus aethiopum ( Dransfield 1986, Beentje 1994, Arbonnier 2002, van Wyk and van Wyk 2013). Palmoxylon dutoitii , therefore, indicates a local environment with a high water table, frost-free climate suitable for the sustained growth of large trees (based on the large diameter of the trunk).

Family Anacardiaceae R.BR., 1818 nom. cons.

Figures

Text-fig. 13. Macrophotographs of the fossil stems from Mhengere. a: large (90 cm diameter) palm tree trunk in situ; b: external view of the outer roots at the base of the trunk; average diameter of single root is 7 mm; c, d: cross-sections of a fragment of trunk showing the random distribution of equal-sized fibre vascular bundles throughout the trunk, the so-called Coccos-type.

Text-fig. 14. Photomicrographs of thin sections of specimen BP/16/1732 Palmoxylon dutoitii from Mhengere Hill, Gorongosa, Mozambique. a: transverse section (TS) with four fibre vascular bundles (fvb) and poorly preserved parenchyma between; b: diagram of one of the fvbs in (a) of the reniform type (f – fibres, mx – metaxylum, p – phloem, px – protoxylum); c: close up of the vascular part of a fvb with 2 metaxylem elements and the collapsed cells to the lower left represents the phloem; d: fvb with 2 metaxylem elements, fibrous part to the left and parencymarous ground tissue to the right; e: lower magnification of fvb in (c); f: fvb with three metaxylem elements and phloem patch below; g: longitudinal section (LS) showing the vascular sections alternating with the fibrous sections; h: LS showing the horizontal thickening on the walls of the metaxylem vessels and a patch of parenchyma to the right (darker cells); i: spheroid echinate phytoliths that are typical of Hyphaene and Borassus.

Text-fig. 8. Mhengere Hill fossiliferous localities (1–3). Silicified tree trunks and fragments of wood are abundant in the poorly indurated basal deposits (marly sand and conglomerate) as well as in the silicified lime-rich sandstones that form the hill, which is interpreted to be the remains of a palaeopan. Image modified from Google Earth.