Revisting the Senga 5A Fish Fauna, Upper Semliki River, Democratic Republic of Congo
Total Page：16
File Type：pdf, Size：1020Kb
Archaeofauna 13 (2004): 145-154 Revisting the Senga 5a Fish Fauna, Upper Semliki River, Democratic Republic of Congo KATHLYN M. STEWART Canadian Museum Of Nature, P.O. Box 3443, Stn. D, Ottawa, Canada Tel. 613-364-4051; Fax 613-364-4027; e-mail: [email protected] ABSTRACT: In the mid-1980s, excavations were conducted at the archaeological site of Senga 5a, located along the Semliki River in the eastern Democratic Republic of Congo (formerly Zaire). The site had attracted interest after initial survey and excavation because of its Oldowan- appearing lithics and Plio-Pleistocene-aged fauna. In the final season of the Senga 5a excava- tions, the site geologists concluded that Senga 5a was not formed in situ in the Pliocene, but had been redeposited by the Holocene-aged Semliki River. However, conflicting with this interpre- tation was a large vertebrate fauna, primarily fish, in the eastern section of the site, which appeared to be a coherent Pliocene-aged fauna, associated with Pliocene artifacts. In this paper, the author compares the taxonomic composition of the Senga 5a fauna with that of other fish faunas from the Upper Semliki River and Lake Edward, ranging in age from Pliocene to Holocene. The resulting analysis indicates the Senga 5a fish fauna is a cohesive Middle to Late Pliocene-aged fauna, with no later intrusive fish. Further investigation of the geology and the invertebrate and vertebrate faunas as well as the artifacts is needed to develop a site formation history of Senga 5a that is consistent with all of the evidence. KEYWORDS: FOSSIL FISH, EARLY HOMINIDS, WESTERN RIFT VALLEY, LATE CENOZOIC, AFRICAN ARCHAEOLOGY RESUMEN: A mediados de la década de 1980 se realizaron excavaciones en el yacimiento arqueológico de Senga 5a, ubicado a orillas del Río Semliki, en la parte este de la Republica Democrática del Congo (antiguo Zaire). El yacimiento llamó la atención después de que en un sondeo y excavación preliminares aparecieran líticas con apariencia Oldowaense y restos fau- nísticos de época Plio-Pleistocénica. En la campaña final de excavaciones, los geólogos con- cluyeron que Senga 5a no se formó in situ durante el Pleistoceno, sino que fue el resultado de una redeposición del río Semliki surgido en el Holoceno. Tal interpretación fue posteriormente cuestionada por la presencia de grandes vértebras, principalmente de peces, en la sección este del yacimiento, aparentemente pliocénica y asociada con artefactos de dicha época. En este tra- bajo se compara la ictiofauna de Senga 5a con la de otras faunas de peces recuperadas en el tramo superior del río Semliki y en el lago Edward que abarcan desde el Plioceno hasta el Holo- ceno. El resultado de estos análisis indica que la fauna de peces encontrada en Senga 5a corres- ponde a una asociación depositada entre el Plioceno Medio y el Tardío, sin que se detecten res- tos intrusivos posteriores. En cualquier caso, se concluye destacando la necesidad de realizar más investigaciones sobre la geología, las faunas de vertebrados e invertebrados, así como sobre los artefactos del yacimiento para reconstruir una historia geomorfológica que sea consistente con toda la evidencia actualmente disponible. PALABRAS CLAVE: PECES FÓSILES, PRIMEROS HOMÍNIDOS, VALLE OCCIDENTAL DEL RIFT, CENOZOICO TARDÍO, ARQUEOLOGÍA AFRICANA 146 KATHLYN M. STEWART INTRODUCTION vai Gorge were located. Bordering the tropical Ituri forest, the Western Rift Miocene-Pleistocene In the mid-1980s excavations were conducted deposits contain evidence of both extreme pluvial- at the Senga 5a site, located along the Semliki ity and dessication, as well as drastic tectonic River (Figure 1) in the eastern Democratic Repub- movements, which repeatedly altered the land- lic of Congo (DRC) (formerly Zaire), by a team scape, subsequently followed by Holocene vol- led by Dr. J.W.K. Harris, Rutgers University. canic eruptions. As a result of these events, the for- Senga 5a was one of several Plio-Pleistocene sites mation of the Senga 5a site was complex, and surveyed and/or excavated in 1985, 1986, 1988, geological reconstruction is problematic, with and 1990 by a multinational team led by Dr. Har- consequent conflicting interpretations of the site ris, Dr. A. Brooks (George Washington Universi- formation history. ty), and Dr. N. Boaz (Virginia Museum of Natural By the end of the 1986 excavations, 435 arti- History) (Boaz, 1990). The site was first excavat- facts and 4,400 faunal elements were recovered. In ed in 1985, and the recovery of Oldowan-appear- 1988 the excavations continued, but were halted ing artifacts and Plio-Pleistocene-aged fauna led when the site geologists concluded, on the basis of the team to return in 1986 for further excavation. a variety of geomorphological evidence, that Senga 5a attracted much interest, as it was at the Senga 5a was not formed in situ in a Pliocene iron- time the earliest reported early hominid site with stone bed, but had been redeposited by a much tools. It was also unusual because of its location in younger Semliki River. The age of the artifacts and the African Western Rift Valley - a wetter, more fauna was therefore thrown into question. Howev- vegetated, and therefore less explored area than er, researchers noted at the time that the vertebrate the better known and drier, less vegetated Eastern fossils from the eastern section of the site, in par- Rift Valley, where well-known sites such as Oldu- ticular the numerous fish elements, seemed to be a FIGURE 1 Map of northeastern Zaire, with Lakes Edward and Albert at the right; Semliki River is shown as a thin line from Lake Edward to Lake Albert. REVISTING THE SENGA 5A FISH FAUNA, UPPER SEMLIKI RIVER... 147 coherent assemblage in terms of both age expeditions document an area with considerable (Pliocene) and preservation. Subsequent reports tectonic movements and climate change through- on Senga 5a have stated that the site was rede- out the Plio-Pleistocene, with parallel events often posited in the Holocene (Boaz et al., 1992; de occurring in both the Lake Albert and Lake Heinzelin & Verniers, 1996), and the latest report Edward regions. Numerous fossil and some (de Heinzelin & Verniers, 1996) has questioned archaeological sites have been documented in this the Pliocene age of even the fauna and artifacts. whole Albert-Edward region. In this paper I reexamine the fossil fish fauna, Field work was conducted by the Harris- which was by far the largest component of the fos- Brooks-Boaz team, or some subset thereof, in the sil assemblage from the Senga 5a site, in compar- Lake Edward and Upper Semliki River regions in ison with other Plio-Holocene Upper Semliki 1985, 1986, 1988, and 1990, with the intention of River and Lake Edward fish assemblages that have better documenting the geology, paleontology, and been excavated and/or analyzed since the Senga 5a archaeology of the region (e.g., Boaz 1990; field seasons. My findings here strongly reinforce Brooks et al., 1995). Work was primarily focused the Pliocene age and cohesiveness of the extensive on the Pliocene-aged Lusso Formation sites, the Senga 5a fish fauna; further, there is no evidence Pleistocene-aged Katanda Formation sites, and the of later intrusives. These conclusions suggest that Late Pleistocene-aged Ishango sites. the late derivation of Senga 5a is not so clearcut as reported in the geological reports, and that the site context needs to be reinvestigated. LUSSO FORMATION SITES As well as examining the context of Senga 5a, this study indicates that in tropical or near-tropical regions, where climatic and environmental vari- The Lusso Formation sites are scattered along ables result in complex site formation, faunal the Semliki River and the Lake Edward margin; assemblages can be useful in reconstruction. In though direct dating is not possible, the Lusso For- this paper, fossil fish assemblages are used as mation probably spans most of the Pliocene (de chronostratigraphic markers to make interpreta- Heinzelin & Verniers, 1996). The Formation tions about the formation of the Senga 5a site. deposits were initially intensively studied in the 1950s by the late Jean de Heinzelin (1955) and again in the 1980s by Jacques Verniers and de Heinzelin (Harris et al., 1987; Boaz et al., 1992; de GEOLOGICAL CONTEXT OF THE WESTERN Heinzelin & Verniers, 1996; Verniers & de RIFT VALLEY Heinzelin, 2000). Lusso Point, also known as Cape Lusso, on the shores of Lake Edward, is the type Senga 5a is one of several Plio-Holocene sites locality for the Lusso Formation. Lusso Formation located in the Western Rift Valley, along the Upper sediments there are at least 60 m thick, represent- Semliki River, as it flows north from present-day ing a lower clay facies and an upper sand and silt Lake Edward (Figure 1). The Western Rift Valley facies. These facies are lacustrine, deposited under is the western margin of the African Rift, and is a mainly permanently aerobic littoral conditions. The mountainous region in central Africa, bordered to Lusso 1 site, at Lusso Point on Lake Edward, dates the west by the tropical Ituri Forest, to the east by to the Middle Pliocene and contained over 4,000 four of the African Great Lakes, and to the south fossil fish teeth. Its sediments represent deeper lake by a large zone of tropical deciduous and bush deposition, while the Upper Lusso Formation sites vegetation. The Western Rift deposits date from along the Semliki River represent near-shore depo- the Miocene to Holocene, and these have been sition (de Heinzelin & Verniers, 1996). explored by several geological and paleontological There are several ironstone beds throughout the teams in the 20th century (e.g., Wayland, 1926, Lusso Formation sequence, interpreted as regres- 1934; Fuchs, 1936; Damas, 1940; Lepersonne, sive events. These ironstone beds are distinctive 1949; de Heinzelin, 1957, 1961; Bishop, 1969; and can be traced for kilometers along the Semliki Boaz, 1990); work has focused primarily on the River and Lake Edward; two distinctive marker Upper Semliki River and the northwestern Lake beds are the Middle Pliocene Senga 1 marker bed Edward shores, the Lower Semliki valley and the and the Late Pliocene “Three Meter Ironstone” Lake Albert area.