In this article we describe the microscopic structures of a silicified piece of wood collected in the Middle Awash Valley (Ethiopia). The fossil wood was extracted from sediment precisely dated 4.4 Ma. Its attribution to the
Cet article décrit les structures microscopiques d’un fragment de bois fossile, silicifié, récolté dans la vallée moyenne de l’Awash (Éthiopie). Le bois fossilisé a été trouvé dans des dépôts datés avec précision de 4,4 Ma. Son attribution en tant que
The Pliocene fossil wood sample (SIMA 98-39), was collected in 1998 from the Pliocene deposit in the Aramis area of the Middle Awash study area of Ethiopia. It was collected at Aramis Vertebrate Paleontology Locality 6, the holotype site for
The sample measures 10 cm in length, 6 × 7 cm in width. It is blackish grey in color and strongly silicified. Nine distinct thin sections along the transverse, tangential and radial planes were made from this sample at the Laboratory of Palaeobotany, MNHN Paris. They revealed only partially preserved structures. Analysis of structural characters and measurements along each plane could only be made on surfaces no greater than 5 mm2, therefore too small to allow statistical comparisons of the measurements.
Descriptions of the microscopic structures and characteristics of the Middle Awash fossil specimen conform to the hardwood feature list of diagnostic characters as recommended by the International Association of Wood Anatomists (
Growth rings are absent or indistinct. Vessels: diffuse, mostly solitary. There are five per square millimeter. Tangential diameter ranges from 138 μm to 299 μm, with a mean of 220 μm. Radial diameter ranges from 195.5 μm to 368 μm, the mean is 240 μm. Because of the poor preservation of the sample the mean was calculated only from 11 measurements. Tyloses are abundant ( Parenchyma: Two types of parenchyma can be distinguished. One vasicentric, surrounding the vessels, with one or two rows of elongated cells greater in length (30.8 μm) than in width (14 μm), ( Rays: the tangential section shows rays one to six cells wide (27 μm to 115 μm) ( Fibres: they are also not well preserved, and the thickness of their wall not appreciable, with no visible pits and septae. Fibres are generally circular, and the diameter measures between 13.5 μm and 18 μm in the tangential section (
The structural characteristics described for the Middle Awash fossil wood sample: medium size of the vessels (150 μm -300 μm), the pore density (5 pores/mm2), large bands of abundant axial parenchyma, heterocellular, irregular and long rays, occurrences of crystals inside the parenchyma and terminal ray cells indicate a clear affinity with modern wood of trees from the Moraceae family, in particular with the genus
In Ethiopia, attribution to the genus
Attribution of the fossil wood to
Many fossil wood structures close to
Some authors have discussed the relationships between climate variables and wood anatomical characters. (
However, before going further in this investigation we need a more precise calibration between extant wood anatomy and climatic variability, notably for Ethiopian species. For interpreting environmental variables, other wood anatomical characters (wood ring porous, vessels mean tangential diameter less than 100 μm, vessels with multiple perforations, spiral thickenings present in the vessels, fibre septae, rays more than 10 cells wide, rays heterocellular with 4 or more upright cells, rays storied, presence or not of axial parenchyma) have been used to estimate mean annual temperature (MAT), cold monthly mean temperature (CMMT), mean annual precipitation (MAP) and length of dry season (DRY) (
This study was supported by grants from Berkeley University, we thank Pr. T. White. We thank the Middle Awash research project and its primary sponsors, the National Science Foundation (BCS 99-10344) and IGPP Los Alamos, and to the Revealing Hominid Origins Initiative (NSF-HOMINID-RHOI BCS-0321893). We are grateful to Mr Lemoine (CNRS, MNHN-Paléontologie. Paris) for preparing the thin sections. Mr Nicaise (UMR 5143 Paléodiversité & Paléoenvironnements, MNHN Paris) for helpful photographs. We thank the reviewers for their corrections.
1. Transverse section, general view, axial parenchyma in bands more than three cells (scale bar 160 μm). 2. Transverse section, general view, axial parenchyma in band more than three cells (scale bar 145 μm). 3. Transverse section, detail of vessel with tyloses and parenchyma band, (scale bar 190 μm). 4. Transverse section, detail of vessel with tyloses, (scale bar 35 μm). 5. Tangential section, disposition of rays, general view (scale bar 108 μm). 6. Tangential section, detail of ray, (scale bar 62 μm) 7. Tangential section, detail of rays and prismatic crystal in axial parenchyma (scale bar 45 μm). 8. Tangential section, detail of heterocellular ray structure and prismatic crystal in parenchyma cell (scale bar 84 μm). 9. Radial section, ray structure, ray composed mainly of procumbent cells with upright marginal cells (scale bar 53 μm).
1. Section transversale, vue générale, vestige de vaisseaux et parenchyme en bandes (échelle 160 μm). 2. Section tranversale, vue générale, vestige de vaisseaux et parenchyme en bandes (échelle 145 μm). 3. Section transversale, détail d’un vaisseau et du parenchyme en bande (échelle 190 μm). 4. Section transversale, détail d’un vaisseau avec présence de thyllose (échelle 35 μm). 5. Section tangentielle, vue générale, disposition des rayons (échelle 108 μm). 6. Section tangentielle, détail d’un rayon (échelle 62 μm). 7. Section tangentielle, détail d’un rayon et parenchyme cristallifère (échelle 45 μm). 8. Section tangentielle, détail de la structure hétérocellulaire des rayons, et parenchyme cristallifère (échelle 84 μm). 9. Section radiale, structure des rayons composés de cellules couches, avec cellules dressées terminales (échelle 53 μm).