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Sclerosperma fossils from the late Oligocene of Chilga, north-western Ethiopia

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The palm family, Arecaceae, is notoriously depauperate in Africa today, and its evolutionary, paleobiogeographic, and extinction history there are not well documented by fossils. In this article we report the pollen of two new extinct species of the
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  Sclerosperma  fossils from the late Oligocene of Chilga, north-westernEthiopia FRIÐGEIR GRÍMSSON 1 , BONNIE F. JACOBS 2 , JOHAN L. C. H. VAN VALKENBURG 3 , JAN J. WIERINGA 3 , ALEXANDROS XAFIS 1 , NEIL TABOR  2 , AARON D. PAN 4 &REINHARD ZETTER   1 1 Department of Palaeontology, University of Vienna, Vienna, Austria,  2 Roy M. Huf   fi ngton Department of Earth Sciences,Southern Methodist University, Dallas, TX, USA,  3  Naturalis Biodiversity Center, National Herbarium of The Netherlands,Leiden, The Netherlands,  4 Don Harrington Discovery Center, Streit Drive, TX, USA Abstract The palm family, Arecaceae, is notoriously depauperate in Africa today, and its evolutionary, paleobiogeographic, andextinction history there are not well documented by fossils. In this article we report the pollen of two new extinct species of the small genus,  Sclerosperma  (Arecoideae), from a late Oligocene (27  –  28 Ma) stratum exposed along the Guang River inChilga Wereda of north-western Ethiopia. The pollen are triporate, and the two taxa can be distinguished from each otherand from modern species using a combination of light and scanning electron microscopy, which reveals variations in the fi ner details of their reticulate to perforate exine sculpture. We also report a palm leaf fragment from a stratum higher in thesame section that is in the Arecoideae subfamily, and most likely belongs to  Sclerosperma . The implications of thesediscoveries for the evolutionary history of this clade of African arecoid palms is that their diversi fi cation was well underwayby the middle to late Oligocene, and they were much more widespread in Africa at that time than they are now, limited toWest and Central Africa.  Sclerosperma  exhibits ecological conservatism, as today it occurs primarily in swamps and  fl oodedforests, and the sedimentology of the Guang River deposits at Chilga indicate a heterogeneous landscape with a high watertable. The matrix containing the fossil pollen is lignite, which itself indicates standing water, and a variety of plantmacrofossils from higher in the section have been interpreted as representing moist tropical forest or seasonally inundatedforest communities. Keywords:  Arecaceae, palm leaf, light microscopy, palaeovegetation, palms, pollen morphology, rainforest, scanning electron microscopy, swamp element Sclerosperma  G. Mann et H. Wendl. is a small genusin the palm family, Arecaceae (Arecoideae, Scleros-permeae; Drans fi eld et al. 2005), found in tropicalWest and Central Africa where three species areknown;  Sclerosperma mannii   H. Wendl.,  S. pro  fi zia-num  Valk. et Sunderl. and  S. walkeri   A. Chev. (vanValkenburg et al. 2008). The genus was  fi rstdescribed by Gustav Mann and Hermann Wendlandin 1864 based on material belonging to  S. mannii  collected by Mann in an inundated forest near theGaboon (now the Ogooué) River upstream fromPoint Clara (Mann & Wendland 1864). But,  Scler-osperma  remained rather enigmatic over time, andalthough new species were described, they were sorarely collected that the circumscription of thesetaxa was clari fi ed only recently (van Valkenburget al. 2008). All three species of   Sclerosperma  aresmall, clustering understorey palms, ranging inheight from 2 to 6 (to 12) m. They are vulnerable,pleonanthic, monoecious palms that grow in tropicallowland rainforests (from sea level to  c . 1400 m), onswampy sites as well as, less commonly, on terra fi rma.  Sclerosperma  is present in old secondary for-ests dominated by okoumé (  Aucoumea klaineana Pierre) and persists in secondary growth nearhuman habitations where the leaves are often col- Correspondence: Friðgeir Grímsson, Department of Palaeontology, University of Vienna, Vienna, Austria. E-mail:  fridgeir.grimsson@univie.ac.at (Received 12 June 2018; accepted 8 August 2018) Grana , 2019Vol. 58, No. 2, 81  –  98, https://doi.org/10.1080/00173134.2018.1510977 © 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), whichpermits unrestricted use, distribution, and reproduction in any medium, provided the srcinal work is properly cited.  lected for roof thatch. The stem in  Sclerosperma  isusually absent above-ground, is stout with closely-spaced ringed leaf scars, and if above-ground mayextend horizontally and produce suckers. Occasion-ally,  S. pro  fi ziana  can produce a stout stem up to9 cm in diameter and 2 m in height. The crown Figure 1. Locality map.  A.  Location of Ethiopia in Africa.  B.  The study area, Gonder, and the current capital city, Addis Ababa. Majorrivers are shown, including the Blue Nile and its source, Lake Tana, to the immediate south of the study area.  C.  Location of the Guangand Hauga Rivers, and measured section shown in Figure 2. 82  F. Grímsson et al.  Figure 2. Stratigraphic section, measured along the Guang River. Stars indicate stratum that produced the  Sclerosperma  fossil pollen, andthe fossil leaf locality, CH41, is labelled and marked by a leaf icon. Radioisotopic dates are shown near the base and top of the section. Sclerosperma  fossils from Ethiopia  83  Figure 3. Light microscopy (LM) ( A ) and scanning electron microscopy (SEM) ( B  –  I ) micrographs of   Sclerosperma protomannii   sp. nov.(holotype: IPUW 7513/223).  A.  Pollen grain in polar view (upper in high focus and middle in optical cross-section) and equatorial view(lower).  B.  Pollen grain in polar view, distal side.  C.  Pollen grain in polar view, proximal side.  D.  Close-up of apex with aperture, distalside.  E.  Close-up of apex, proximal side.  F.  Close-up of central polar area, distal side.  G.  Close-up of central polar area, proximal side.  H. Close-up of interapertural area, distal side.  G.  Close-up of interapertural area, proximal side. Scale bars  –   10 µm (A  –  C), 1 µm (D  –  I). 84  F. Grímsson et al.  Figure 4. Light microscopy (LM) ( A ) and scanning electron microscopy (SEM) ( B  –  E ) micrographs of   Sclerosperma protomannii   sp. nov.(paratype: IPUW 7513/224).  A.  Pollen grain in polar view (optical cross-section).  B.  Pollen grain in polar view, distal side.  C.  Pollen grainin polar view, proximal side.  D.  Close-up of central polar area, distal side.  E.  Close-up of interapertural area, proximal side. LM ( F ) andSEM ( G  –   J ) micrographs of   S  .  protomannii   sp. nov. (paratype: IPUW 7513/225).  F.  Pollen grain in polar view (high focus).  G.  Pollen grainin polar view, distal side.  H.  Pollen grain in polar view, proximal side.  I.  Close-up of apex with aperture, distal side.  J.  Close-up of apex,proximal side. Scale bars  –   10 µm (A  –  C, F  –  H), 1 µm (D, E, I, J). Sclerosperma  fossils from Ethiopia  85
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