Sorindeioxylon gorongosense, Bamford & Pickford, 2021

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 : 203-205

Sorindeioxylon gorongosense sp. nov.

Text-fig. 15

H o l o t y p e. BP/16/1738 – three slides and the remaining piece of wood ( Text-fig. 15 ).

P l a n t F o s s i l N a m e s R e g i s t r y N u m b e r.

PFN002686 (for new species).

R e p o s i t o r y. Curated at the Palaeobotany Herbarium, Evolutionary Studies Institute (previously the Bernard Price Institute), University of the Witwatersrand, Johannesburg, South Africa.

E t y m o l o g y. The species name refers to Gorongosa

National Park with the Latin suffix ense meaning “from”.

T y p e l o c a l i t y. Muaredzi Site 5 (18°56′02.0″S, 34°36′50.6″E) fossil wood associated with palaeopan, northwest side. Mazamba Formation ( Text-fig. 9 ), probably late Eocene .

D i a g n o s i s. Diffuse porous dicotyledonous wood with indistinct to absent growth rings. Vessels round, solitary or in pairs, tangential diameter 50–100 µm and less than 5 vessels per square mm. Perforation plates are simple. Intervessel and vessel-ray pits are the same, alternately arranged and 6–8 µm in diameter. Apotracheal parenchyma in narrow terminal or initial bands. Paratracheal parenchyma is vasicentric to rarely aliform. Rays are low, uni- to triseriate with the uniseriate sections as wide as the triseriate sections, heterocellular with mixed procumbent, square and upright cells. Prismatic crystals occur in the upright and square ray cells.

C l o s e s t e x t a n t t a x o n. Sorindeia madagascariensis

THOUARS ex DC.

D e s c r i p t i o n. Growth rings are not visible in the thin section and the wood is diffuse porous. There are narrow bands of parenchyma, up to three cells wide, and randomly distributed. Vessels are round and 1, rarely 2 cells in short radial multiples. The average radial and tangential vessel diameters are 70 µm (range 65–80 µm) with 3–4 vessels per square mm. Inter-vessel and vessel-parenchyma pits are alternate and 6–8 µm, and well spaced in case of the vessel-ray pits ( Text-fig. 15a, b ). Perforation plates are simple and oblique. Parenchyma is not abundant, mostly vasicentric to rarely aliform, and there are narrow bands of 1–3 cells wide with an irregular frequency ( Text-fig. 15a, b ). Prismatic crystals occur in the upright and square ray cells ( Text-fig. 15d ). Rays are 1–3 seriate and the width of the uniseriate part is the same as the multi-seriate part. They are heterocellular with mixed procumbent, square and upright cells. Rays are low with an average of 16 cells high (260 µm) but up to twice that height.

I d e n t i f i c a t i o n. One of the key features of this fossil wood is the presence of heterocellular rays with the uniseriate portions having the same width as the triseriate portions. This feature occurs in a few members of the Anacardiaceae , Apocynaceae , Euphorbiaceae , Rubiaceae and Sapotaceae and the combination of wood features narrows down to Sorindeia madagascariensis and Sorindeia spp. (InsideWood). The Gorongosa fossil wood features are a combination of these two entries. Since the modern database does not include all Sorindeia species and it is not certain that modern species were present (or recognisable) by the middle Tertiary, the fossil wood has been placed in a new fossil wood taxon, Sorindeioxylon .

Although the family is diverse in Africa today, the fossil wood record of the Anacardiaceae in the continent is very sparse. Only three examples from the Miocene to Pliocene of Ethiopia have been described and published (inventories of Dupéron-Laudoueneix and Dupéron 1995, Gregory et al. 2009; InsideWood database, accessed Oct 2021). Gregory et al. (2009) mentions four other fossil woods from Africa but they are either undescribed ( Lannea sp. , Lannea welwitschi ; Dechamps 1976) and not mentioned in the InsideWood database, or are so poorly assigned (Unnamed,? Anacardiaceae / Moraceae / Lauraceae ; Kamal El-Din et al. 2006) that it is difficult to follow. The fourth example is the only one to be added to our list, Ozoroa insignis ( Dechamps 1983, Dechamps and Maes 1985) but the rays are different from the Mozambican wood.

Anacardioxylon sorindeoides is similar to the Gorongosa wood except that the vessels are much larger and it has no banded parenchyma ( Lemoigne 1978). From the same site in the Omo Valley, Glutoxylon symphonioides has even larger vessels and wider and taller rays, but it does have some banded parenchyma ( Lemoigne 1978). The Gorongosa wood does not have the features of the other genera of fossil Anacardiaceae from other continents, so a new genus is proposed for this wood, Sorindeioxylon , and in recognition of the region, the type species is named gorongosense . Ozoroa insignis ( Dechamps 1983, Dechamps and Maes 1985) from the Omo valley Pliocene deposits, has much wider rays than the Gorongosa wood together with 1–4 or more rows of marginal upright cells, so they are not comparable.

Anacardiaceae are cosmopolitan and occupy a wide variety of habitats. In southern Africa today there are ten genera of trees and shrubs: Abrahamia (formerly Protorhus ), Harpephyllum , Lannea, Loxostylus , Ozoroa , Pseudospondias , Searsia (formerly Rhus ), Sclerocarya , Sorindeia and Trichoscypha . The past diversity is unknown. The fossil wood anatomy shares some similarities with these genera but there are too many differences. Sorindeia madagascariensis occurs in northern Mozambique today in riverine forests or in open woodland, and is a small to medium height tree ( Burroughs et al. 2018). It is the only member of this genus in Mozambique. The fossil wood Sorindeioxylon gorongosense gen. et sp. nov., therefore, indicates a local environment with sufficient sustained water for the growth of medium sized trees. Northern Mozambique, wedged between the huge rift floor lake, Lake Niassa (or Lake Malawi) and the east tropical coast, experiences summer rainfall and high temperatures.

Figures

Text-fig. 15. Photomicrographs of thin sections of holotype BP/16/1738, Sorindeioxylon gorongosense gen. et sp. nov. from Muaredzi site 5, Gorongosa, Mozambique. a: TS, note the irregularly spaced and very narrow bands of parenchyma and mostly solitary vessel elements; b: TS at higher magnification with narrow rays; c: radial longitudinal section (RLS), rather oblique but shows the alternate, small-to-medium inter-vessel pits; d: tangential longitudinal section (TLS), rays are 1–3 cells wide but maintain the same width. Small arrow towards the right hand ray indicates a prismatic crystal in the ray cell; e: TLS rays with fibres in between; f: RLS showing mixed ray cells (upright, square and procumbent) poorly preserved.

Text-fig. 9. Fossil wood localities 4 and 5 close to the palaeopan on the north flank of the Muaredzi Gorge. Image modified from Google Earth.