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Lacustrine Mollusc Radiations in the Lake Malawi Basin

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Lacustrine Mollusc Radiations in the Lake Malawi Basin EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Open Access Biogeosciences, 10, 5767–5778, 2013 Open Access www.biogeosciences.net/10/5767/2013/ Biogeosciences doi:10.5194/bg-10-5767-2013 Biogeosciences Discussions © Author(s) 2013. CC Attribution 3.0 License. Open Access Open Access Climate Climate of the Past of the Past Discussions Lacustrine mollusc radiations in the Lake Malawi Basin: Open Access Open Access experiments in a natural laboratory for evolutionEarth System Earth System Dynamics Dynamics D. Van Damme and A. Gautier Discussions Paleontological Research Unit, Ghent University, Krijgslaan 281, 9000, Gent, Belgium Open Access Correspondence to: D. Van Damme ([email protected]) Geoscientific Geoscientific Open Access Instrumentation Instrumentation Received: 17 July 2012 – Published in Biogeosciences Discuss.: 18 December 2012 Revised: 25 July 2013 – Accepted: 25 July 2013 – Published: 3 September 2013 Methods and Methods and Data Systems Data Systems Discussions Open Access Abstract. In terminal Pliocene–early Pleistocene times, part other past and present rift lakes, molluscan in-lakeOpen Access diver- Geoscientific of the Malawi Basin was occupied by paleo-lake Chiwondo. gence is surprisingly modestGeoscientific and young geologically speak- Molluscan biostratigraphy situates this freshwater lake either ing. This view is partly applicable even to the unique mala- Model Development in the East African wet phase between 2.7–2.4 Ma or that of cofauna of LakeModel Tanganyika. Development The present study links the late Discussions 2.0–1.8 Ma. In-lake divergent evolution remained restricted origin of the malacofaunas of the present rift lakes to the in- to a few molluscan taxa and was very modest. The lacustrine creasing climate destabilisation in East Africa since the late Open Access Chiwondo fauna went extinct at the beginning of the Pleis- Pliocene. Open Access tocene. The modern Lake Malawi malacofauna is depauper- Hydrology and Hydrology and ate and descends from ubiquistic southeast African taxa and Earth System Earth System some Malawi basin endemics that invaded the present lake 2 The fossil record after the Late Pleistocene mega-droughts. The Pleistocene Sciences Sciences aridity crises caused dramatic changes, affecting the malaco- This study is based on the collections of terminal Pliocene– Discussions Open Access fauna of all East African lakes. All lacustrine endemic faunas early Pleistocene molluscs collected in the ChiwondoOpen Access region that had evolved in the Pliocene rift lakes, such as paleo-lake (NW margin of Lake Malawi), respectively, in the 1960s dur- Ocean Science Chiwondo, became extinct. In Lake Tanganyika, the fresh- ing the Desmond ClarkOcean Palaeo-Antropological Science Investigation Discussions water ecosystem did not crash as in other lakes, but the en- and in 1980–1990s during the Hominid Corridor Research vironmental changes were sufficiently important to trigger Project led by Timothy Bromage and Friedemann Schrenk. a vast radiation. All African endemic lacustrine molluscan Albrecht Gorthner, the HCRP malacologist, also sampled Open Access Open Access clades that are the result of in-lake divergence are hence ge- the Holocene assemblages near the Shire River outlet and ologically young, including the vast Lavigeria clade in Lake this preliminary investigation was recently continued (Van Solid Earth Tanganyika (ca. 43 species). Bocxlaer, 2004; Van BocxlaerSolid et al., 2012).Earth All relevant fos- sil material collected in the Malawi Basin is provisionally Discussions stored at the Paleontological Research Unit, Ghent Univer- sity, awaiting formal taxonomic description. The taxonomy Open Access Open Access 1 Introduction of the Desmond Clark collection was studied by Gautier (A. Gautier, unpublished data, 1968) and taxonomy and bios- The Cryosphere The hypothesis that the large African lakes are “natural lab- tratigraphy of the HCRP-collectionThe Cryosphere by A. Gorthner (unpub- Discussions oratories of evolution” and that their diversified molluscan lished data, 1995). Copies of both manuscripts are kept at fauna are prime examples of intense and ancient processes of the Paleontological Research Unit Ghent as part of the col- in-lake evolution (Michel et al., 1991) has become generally lection. accepted. The recent and fossil malacofauna of Lake Malawi The present paper is essentially a critical review of the does not corroborate this theory. In fact, all actual molecular fossil material cited above and the published literature on and paleontological investigations provide evidence pointing the modern and fossil Lake Malawi malacofauna in the to the contrary, namely that in Lake Malawi, as well as in light of our vastly improved knowledge on phylogeny and Published by Copernicus Publications on behalf of the European Geosciences Union. 5768 D. Van Damme and A. Gautier: Lacustrine mollusc radiations in the Lake Malawi Basin paleontology of the molluscs of the African Great Lakes and of the paleolimnological evolution of these lake basins. The published literature in which provisional species lists of the Chiwondo fossil molluscs or part of them are provided is quite extensive and widely scattered. It includes Pain (1966), Gautier (1966), Gautier (1970), Van Damme (1984, 1988), Van Damme and Pickford (1999, 2003), Gorthner (1994), Gorthner et al. (1992), Schrenk et al. (1995), Schultheiß et al. (2009) and Van Bocxlaer (2010). Fig. 1. Example of taxonomic hyper-splitting in modern Lake Malawi molluscs. Representatives of the four thiarid genera created 3 The evolution of taxonomic concepts about the by Bourguignat (1889), endemic to Lake Malawi: 1 - Nyassia mag- modern Malawi malacofauna: from speciose nifica; 2 - Nyassella pulchra; 3 - Nyassomelania truncatelliformis 19th Century Lake Nyasa to species-poor 21st and 4 - Micronyassia singularis. All ca. 40 thiarid species recog- Century Lake Malawi Fig.nized 1. by Bourguignat have been shown by molecular research to be parthenogenetic clones belonging to the “Melanoides polymorpha- The spectacular thalassoid or marine-like fauna of Lake complex”. Some (e.g., Melanoides truncatelliformis) are still “offi- Tanganyika (area: 32 900 km2) greatly puzzled 19th Cen- cially” considered as distinct endemic species (Scale bar: 10 mm). tury scientists and led to heated debate as to their origin. The comparatively unspectacular molluscs of Lake Nyasa, as L. Malawi formerly was known, saddled them with the revealed that both groups consist not exclusively of in-lake subsidiary question why in this equally vast lake (area: endemics, as was formerly assumed, but of in-lake endemics 29 600 km2) no such a diversified thalassoid fauna was sensu stricto plus one or two paludal/fluvial species endemic found, for according to Bourguignat (1889) only the Nyasan to the whole of the Malawi Basin, i.e., basin endemics (Sen- thiarids did possess thalassoid characters. This author, rec- gupta et al., 2009; Schultheiß et al., 2009, 2011). Both these ognizing about 40 thiarid species, divided them in five gen- Malawi species groups are monophyletic, young and the spe- era: Melania (= Melanoides) represented only by the ubiquis- ciation processes still likely are going on in the viviparid tic M. tuberculata and the rest belonging to the endemic flock (Schultheiß et al., 2011). In addition, the possibility that genera Nyassia, Nyassella, Micronyassia and Nyassomelania the clonal Melanoides lineages endemic to lake Malawi are (Fig. 1). He believed that the relatively low species richness not the result of an in-lake radiation either, but that they are was due to insufficient sampling in the at the time virtually allopatric paleo-endemics that colonized Lake Malawi at dif- unexplored lake. ferent times, is considered a possible scenario in view of their In the middle of the 20th Century, during what Michel et para-/polyphyly (Genner et al., 2007b). al. (2003) call “the dawn of Mayrian optimism for the prac- Dudley’s (2000) remark that “Malawi gastropod classifi- ticality of a single ‘biological’ species concept”, the num- cation is in a continuing state of revision and that it will be ber of molluscan species in all African lakes was taxonomi- some time before a system comes to be generally agreed for cally decimated. In Lake Malawi none of the endemic thiarid the groups of higher rank” is still painfully actual. Certain genera survived this taxonomic lumping event and only 16 is that the recent genetic research on Lake Malawi molluscs gastropods are presently considered to live in the lake sensu does not lend support to the ingrained concept of “spectacu- stricto of which eight are endemic Melanoides (Brown, 1994; lar” species diversification/radiation in this supposedly long- Darwall et al., 2005), but the number of the Melanoides en- lived “natural laboratory of evolution”. The largest Malawi demics is still unresolved (see Table 1). Eldblom and Kris- group, that of Melanoides, can only partially result from an tensen (2003) retain three endemics only in the last revision in-lake radiation and the two other “clades” are small and based on morphology.
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