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A morphotype catalog and paleoenvironmental interpretations of early Miocene fossil leaves from the Hiwegi Formation, Rusinga Island, Lake Victoria, Kenya

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A morphotype catalog and paleoenvironmental interpretations of early Miocene fossil leaves from the Hiwegi Formation, Rusinga Island, Lake Victoria, Kenya
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   Palaeontologia Electronica palaeo-electronica.org PE Article Number: 16.3.28ACopyright: Palaeontological Association November 2013Submission: 10 August 2013. Acceptance: 5 November 2013Maxbauer, Daniel P., Peppe, Daniel J., Bamford, Marion, McNulty, Kieran P., Harcourt-Smith, William E.H., and Davis, Larry E. 2013.  A morphotype catalog and paleoenvironmental interpretations of early Miocene fossil leaves from the Hiwegi Formation, Rusinga Island, Lake Victoria, Kenya, Palaeontologia Electronica  Vol. 16, Issue 3; 28A; 19p; palaeo-electronica.org/content/2013/547-rusinga-island-flora  A morphotype catalog and paleoenvironmental interpretations of early Miocene fossil leaves from the Hiwegi Formation, Rusinga Island, Lake Victoria, Kenya Daniel P. Maxbauer, Daniel J. Peppe, Marion Bamford, Kieran P. McNulty, William E.H. Harcourt-Smith, and Larry E. Davis  ABSTRACT Early Miocene deposits on Rusinga Island (Lake Victoria, Kenya) contain anabundance of faunal and floral remains. Despite the attention that has historically beengiven to the early Miocene fauna from Rusinga Island, little attention has been given tothe early Miocene fossil floras and to date no studies have described fossil leaf mor-photypes from Rusinga Island. Here, we present a morphotype catalog of fossil leavescollected from the Grit Member of the Hiwegi Formation on Rusinga Island. Wedescribe 14 morphotypes, comprised of 12 dicotyledonous angiosperms and twomonocotyledonous angiosperms, as well as two distinct dicotyledonous angiospermleaf fragments. Characteristics of the flora and sedimentological evidence, coupledwith previous research, suggest that the local paleoenvironment was a riparian habitatwithin a patchwork of woodland and forested biomes in what was likely a warm climate.This work represents an important first step in understanding the early Miocene vege-tation of Rusinga Island, and highlights both the need and potential for future researchon these early Miocene floras.Daniel P. Maxbauer.   Department of Biology, Saint John’s University, Collegeville, Minnesota, USAand Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut, USA and Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota, USA maxba001@umn.eduDaniel J. Peppe.   Department of Geology, Baylor University, Waco, Texas, USA daniel_peppe@baylor.eduMarion Bamford. Bernard Price Institute for Palaeontology, University of the Witwatersrand, Johannesburg, South Africa Marion.Bamford@wits.ac.zaKieran P. McNulty. Evolutionary Anthropology Lab, Department of Anthropology, University of Minnesota, Minneapolis, Minnesota, USA kmcnulty@umn.eduWilliam E.H. Harcourt-Smith. Department of Anthropology, Lehman College CUNY, Bronx, New York, USAand Department of Anthropology, Graduate Center CUNY, New York, New York, USA willhs@amnh.organd Division of Paleontology, American Museum of Natural History, New York, New York, USALarry E. Davis. Department of Biology, Saint John’s University, Collegeville, Minnesota, USA ldavis@csbsju.edu  M  AXBAUER   ET    AL .: R USINGA  I SLAND  F LORA 2 Keywords:  early Miocene; Rusinga Island; megafloral paleobotany; paleoenvironment INTRODUCTION Fossil collections from the early Miocenedeposits on Rusinga Island, Lake Victoria, Kenya(Figure 1) provide some of the best evidence of East African paleocommunities immediately follow-ing the connection of Africa to Eurasia (e.g., Sav-age, 1965; Pickford, 1986, 2004; Schmidt-Kittler,1987; Cote et al., 2007; Drake et al., 1988; Peppeet al ., 2009; Peppe et al., 2011). Rusinga Island isparticularly well known for its abundant well-pre-served fossil catarrhine primates, Proconsul , Nyan-zapithecus, Limnopithecus, and Dendropithecus (e.g., MacInnes, 1943; Le Gros Clark and Leakey,1950; Andrews and Simons, 1977; Walker et al.,1993). However, these early Miocene deposits alsocontain an abundance of plant fossils. Despite this,only a few studies from Rusinga Island havefocused on fossil plant remains (e.g., Chesters,1957; Collinson, 1985; Collinson, et al., 2009), andnone has focused on fossil leaves. Since leavescannot be transported intact over great distances,fossil leaves are often excellent indicators of localenvironment. Historical collections on RusingaIsland have yielded mostly fragmentary or poorlypreserved megafloral material (e.g., Kent, 1994;Collinson et al., 2009), and this paucity of speci-mens and research highlights the need for further  0123km Kaswanga R5R117 R76R4R3AR3 & R3BR1 & R1AR118R119 Wakondo R126R107 GumbaRed Beds Kulu Fm.Hiwegi Fm.Kiahera Fm.Wayando Fm. 2 N R127R120R114R121R75R74   Kiahera R105 Fossil Localities on Rusinga    N   y   a   m  s   i   n  g    u   l  a MfanganoIsland Rusinga Island   M  f a n g a n o   F a u  l  t Lake Victoria 05km N Kaksingiri Bay Rangwa 1   K a  n  y a  m  w  i a   F a  u  l  t Rusinga Group 10 Kiahera Fm. Rusinga Agglomerate    H   i  w  e  g   i   F  m . Kulu Fm.Kiangata  Agglomerate Lunene LavasWayando Fm.    R  u  s   i  n  g  a   K   i  s   i  n  g   i  r   i 1000 m 3 FORMATION    G   R   O   U   P Kaswanga Point Mbr. Grit Mbr. Fossil Bed Mbr. KibangaMbr. StudyLocality FIGURE 1. 1.  A map showing Africa, star indicates approximate location of Lake Victoria, Rusinga Island and Mfan-gano Island. 2.  Generalized map of Rusinga Island including basic stratigraphic distributions and general site loca-tions. Star indicates the approximate location of this studies location. GPS coordinates for the study site: S 00°24.350’ E 034° 8.834’. 3. Generalized Miocene stratigraphy on Rusinga Island. Star indicates stratigraphic position of fossil leaf locality. Mbr. = member, Fm. = formation.  PALAEO - ELECTRONICA . ORG 3 studies into Rusinga Island’s early Miocene mega-flora and their associated terrestrial environments.Here, we document the first assemblage of fossil leaf morphotypes collected on RusingaIsland from a restricted stratigraphic interval withinthe Grit Member of the Hiwegi Formation (Figure1). We then present a paleoenvironmental interpre-tation based on the flora and the sedimentologywithin the collection area. Geological Setting and Previous Paleoecological WorkGeological history and stratigraphy.   Today, Rus-inga Island resides on what was once the flank of the large carbonatite-nephelinite Kisingiri Volcano,which formed in the early Miocene in associationwith the failed Nyanza Rift (Figure 1). These Mio-cene deposits pre-date the formation of Lake Victo-ria (see review of Lake Victoria’s history in Danleyet al., 2012). The stratigraphic nomenclature usedhere follows Peppe et al .  (2009) and Van Couver-ing (1972) (Figure 1). K-Ar dates published byDrake et al. (1988) suggested that the Hiwegi For-mation was deposited ~17.9 Ma, and that theentire fossiliferous Rusinga Group sequence (Fig-ure 1) was deposited in less than a half millionyears. More recent analyses using 40  Ar/ 39  Ar dates,magnetostratigraphy, and lithostratgiraphy demon-strate that the fossiliferous strata on Rusinga weredeposited over a much longer time interval,between ~17-20 Ma (Peppe et al., 2009; Peppe etal.,2011; McCollum et al., 2012). Previous Paleoecological and PaleobotanicalWork.  Paleoenvironmental reconstructions fromvarious proxies have yielded contradictory results,with interpretations ranging from tropical rain forestto woodland to a semi-arid climate (Chesters,1957; Andrews and Van Couvering, 1975; Evans etal .,  1981; Collinson, 1985; Thackray, 1994; Retal-lack et al .,  1995; Bestland and Krull, 1999; Forbeset al .,  2004; Collinson et al ., 2009; Ungar et al.,2012). Many studies have examined data from theentire Hiwegi Formation (e.g., Andrews and VanCouvering, 1975; Evans et al., 1981; Retallack etal., 1995; Forbes et al., 2004; Ungar et al., 2012),which may span >100 kyr (Peppe et al., 2011;McCollum et al., 2012). Hence, these studies likelysampled a mixture of environments from differenttime periods during the deposition of the HiwegiFormation. Alternatively, work by Collinson (1985),Collinson et al. (2009), and Thackray (1994) wasbased on restricted stratigraphic intervals andtherefore report estimates of paleoclimate andpaleoenvironments from narrow slices of time.These different types of datasets (stratigraphicallyrestricted vs. time-averaged) may help to explainthe range of paleoenvironmental interpretationsthat persist in the literature. To date, only three studies have focusedexclusively on plant fossils from Rusinga deposits.Chesters (1957) examined primarily fossil woodand seeds from Rusinga and Mfangano Islandsand suggested that the early Miocene paleoenvi-ronment of the region was a tropical rain forest or gallery forest near a river margin. However,because the fossil material used in the analyseswas derived from surface collections from multiplesites of different ages, these results may not bereliable. Collinson et al. (2009) and Collinson(1985) used nearest living relative (NLR) analyseson in situ  fruits, seeds, wood of dicotyledonousangiosperm trees, shrubs, herbaceous and woodyclimbers, and the fruit of a monocotyledonouspalm. In contrast to Chesters (1957), they con-cluded that the local paleoenvironment studied wasa woodland with limited forest present. This inter-pretation was based largely on the determinationthat the flora consisted of only 4.2% definitively for-est dwelling taxa, belonging to 3 of the 21 familiesrepresented by their assemblage (see Collinson etal., 2009 for complete taxon list). Unlike the Ches-ters (1957) study, these analyses were from a sin-gle stratigraphic unit in the Grit Member and aremore likely to reflect the local paleoenvironment. Study Area . For this study, the fossil leaves comefrom a site near the R5 vertebrate fossil locality atKaswanga Point (Figure 1), in close proximity tothe fossil site R117 described in Collinson (1985)and Collinson et al .  (2009) (Figure 1). Both our study area and locality R117 are within the GritMember, and probably are roughly contemporane-ous. However, the exact location of the R117 floraand its stratigraphic position in the Grit Member isuncertain, making a direct correlation between our study area and R117 impossible at this time.  At the study area, five distinct stratigraphiclayers of the Grit Member were exposed, mea-sured, and described (Figure 2, Table 1). Eachlayer was assigned a number (to indicate strati-graphic relationship) preceded by “GM”. Fossilleaves were collected from level GM-02. Many of the leaves are fragmentary and often conform tothe rippled bedforms in layer GM-02 preventing theleaves from being flat-lying. Ripple marks (Figure3.1) identified in GM-02 and GM-05 indicate thepresence of moving water, whereas salt hoppers(Figure 3.2) and desiccation cracks in bed GM-03indicate periodic aerial exposure, desiccation, and  M  AXBAUER   ET    AL .: R USINGA  I SLAND  F LORA 4 evaporitic conditions. The fragmentary nature of the leaves, their preservation on rippled bedforms,and the fluvial indication in layer GM-02, suggestthat the fossil leaves may have been transported ashort distance before being deposited. METHODS The fossil leaves were collected from a tuffa-ceous sandstone layer, GM-02, in the Grit Member (Figure 2, Table 1). All samples were collected inJuly 2010 and are housed at the National Muse-ums of Kenya, Nairobi (NMK). Specimens weregrouped according to morphological characteristicsand assigned to a morphotype. Morphotypes aremorphologically distinct groups of specimens thathave no formal taxonomic status but often reflectbiological species (see review of the morphotypingmethod in Ash et al., 1999 and Peppe et al.,   2008).The specimen that best represented the character-istics of each morphotype and/or showed the high-est level of preservation was chosen to be themorphotype exemplar. Specimen numbers listedhere coordinate with the catalog numbers of speci-mens housed at the NMK (Appendix 1). Thosespecimens that were not identifiable to an existingmorphotype or were too fragmented to be placed ina new category were marked as unidentifiable and GM-05GM-04GM-03GM-02GM-01 0 cm100200 Fine grained sandstoneVery fine grained sandstoneInterbedded sandstone and mudstoneSilty sandstoneRipplesSalt hoppersFossil leaves FIGURE 2.  Stratigraphic section of the Grit Member exposed at fossil leaf locality. Ripple marks were found in GM-05,salt hoppers in GM-03, and fossil leaves in GM-02. GM = Grit Member. TABLE 1. Descriptions of stratigraphic layers of the exposed section of the Grit Member (GM) at the study site. Layer Thickness(cm)Description GBM-0570 Bluish, greenish light grey. Fine to very fine sandstone. Ripple marks present. GBM-0425 Greenish light grey. Finely laminated, medium fine sandstone, mudstone. GBM-031 Dark grey. Silty sandstone that in areas shows signs of mud cracks. GBM-0240 Greenish light grey. Fine grained sandstone. Massively bedded. Organic material, including leaves, was found within this layer. GBM-0130 Dark grey. Fine grained silty sandstone.  PALAEO - ELECTRONICA . ORG 5 grouped together under one specimen and catalognumber. No taxonomic affinities have been deter-mined at this time for the dictolydenous angio-sperms. This morphotype catalog is intendedinstead to act as an important first step in docu-menting the poorly studied Rusinga Island mega-flora.Morphotypes were described following thewell-established protocols of Ellis et al. (2009).Each morphotype description adheres to the fol-lowing format: Description:  Blade attachment, laminar size,length:width (L:W) ratio, laminar shape, medialsymmetry, and basal symmetry. Margin type, apex 12 FIGURE 3. 1.  Ripple marks a top GM-05. 2.  Salt hoppers from GM-03. Scale bar = 1 cm.
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