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A Multiproxy Approach to Studying Lake Ecosystems in the Mesozoic

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A Multiproxy Approach to Studying Lake Ecosystems in the Mesozoic A multiproxy approach to studying lake ecosystems in the Mesozoic der Naturwissenschaftlichen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades Dr. rer. nat. vorgelegt von Manja Hethke aus Greifswald A multiproxy approach to studying lake ecosystems in the Mesozoic Rekonstruktion mesozoischer Seeökosysteme: Ein Multiproxyansatz der Naturwissenschaftlichen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades Dr. rer. nat. vorgelegt von Manja Hethke aus Greifswald Als Dissertation genehmigt von der Naturwissenschaftlichen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg Tag der mündlichen Prüfung: 11.12.2014 Vorsitzender des Promotionsorgans: Prof. Dr. Jörn Wilms Gutachter: Prof. Dr. Franz T. Fürsich Prof. Dr. Alexander Nützel Abstract The lake sediments of the Barremian to Aptian posits. The redox state of the lake has been resolved Yixian Formation of western Liaoning, China, have using pyrite framboid size distributions. Phase 2 received worldwide attention for their outstanding was governed by dysoxic bottom waters with spells fossil preservation and evolutionary significance. of anoxia. The lake was therefore characterized by Previous work on this Mesozoic fossillagerstätte mainly holomictic conditions that episodically al- has centred on feathered dinosaurs, early birds, and ternated with meromictic intervals. Spatial varia- early angiosperms. However, the physico-chemical tions in redox state were pronounced. Conversely, conditions that led to its formation and the response Phase 3 was marked by oxic conditions and an en- of palaeocommunities to varying environmental tirely holomictic lake. conditions, necessary to establish it as an important Clam-shrimp taxonomy of eastern Asia suffers window on Mesozoic lake evolution, are poorly un- from extreme oversplitting as phenotypic and on- derstood. The state-of-the-art palaeoenvironmental togenetic variation has repeatedly been neglected. interpretation of the so-called Lake Sihetun is a We herein comprehensively revise the existing taxo- shallow, eutrophic setting governed by seasonal an- nomic framework for Lake Sihetun by integrating oxia and synsedimentary volcanism. all representatives of ontogenetic stages and sexes. Considering the proposed lake duration of 0.7 to During one study that investigates the taxonomic 1.5 Ma, it is hypothesized that the lake underwent validity of the ten alleged clam-shrimp species oc- several evolutionary phases in response to climate curring within the Yixian Formation of Western Li- change and ecological disturbances, each marked by aoning, it could be demonstrated that clam-shrimp distinct palaeoenvironmental conditions. The diffi- diversity was considerably overestimated. The num- culty of reconstructing the ecosystem evolution of ber of valid species has been reduced to five, poten- Lake Sihetun results from the general absence of tially even four. seasonally-responsive marker organisms, which is The key to pinpointing the clam-shrimp species characteristic of Mesozoic lake deposits in general. governing Lake Sihetun has been the identifica- The lake is, however, characterized by an abundance tion of size and shape diversity within the domi- of clam shrimps, branchiopod crustaceans of the nating species (Eosestheria middendorfii). This marks suborder Spinicaudata, which experienced a diversi- the first morphometric study on clam shrimps that fication during Mesozoic times and a characteristic takes allometric relationships among characters into decline during the Cenozoic. The main goal of this account. The three main objectives, which have been thesis is to establish this group as one of the most investigated in two interlinked studies, have been important proxies for the reconstruction of Meso- (1) to inspect possible sexual dimorphism in adult zoic lake ecosystems. This can only be achieved by carapaces, (2) to identify ontogenetic and pheno- a highly resolved study of the lake development, a typic variation within E. middendorfii, and (3) to comprehensive taxonomic revision, multiple mor- estimate the influence of environmental parameters phometric studies of its spinicaudatan fauna, and a on carapace shape. palaeocommunity analysis. (1) Clam shrimps rival many other animals re- Lake evolution has been subdivided into four garding the diversity of their sexual systems, which developmental phases. While phases 1 and 4 mark ranges from obligate sexual reproduction over self- the formation and the eventual siltation of the lake, fertilization with the occasional presence of males respectively, phases 2 and 3 represent the bulk of to unisexuality, rendering the Spinicaudata a model the time of its existence. The latter are in focus due clade for the study of reproductive system evolution. to their excellent fossil preservation. The sedimen- Obligate sexuality (“dioecy”) is the inferred repro- tological evidence points to a change in climate ductive system for Eosestheria middendorfii. Sexual from dry to humid between both phases, which is dimorphism accounts for about 10% of the adult accompanied by a deepening of lake waters and an shape variation. Carapace shape variation resulting increased fluvial influx at the onset of Phase 3. Rare from malformation and deformation is more pro- chrysophycean cyst accumulations in Phase-2 sedi- nounced than the underlying sexual dimorphism. A ments indicate annual sedimentation (varves), and discriminant function that uses linear measurements they represent the earliest unambiguous appearance has been proposed for the classification of adult in- of this seasonally-responsive algal group in lake de- dividuals of E. middendorfii as female or male. 1 Manja Hethke (2) Numerous horizons of Phase 3 yield very two assemblages) is dominated by a more special- small clam shrimps that have barely completed ized fauna that was restricted to the early Phase 3, their naupliar phase, rendering them unidentifiable which was marked by comparatively deep waters. with existing methods. The analysis of ontogenetic Carapace size distributions point to environmental- shape variation shows that Eosestheria middendorfii ly-induced mass mortality events of juvenile clam is a strongly allometric species. Importantly, all in- shrimps triggered by lethally high temperatures in dividuals (juveniles and adults) fit a single allometric shallower waters of the late Phase 3 (Association 1). model (no divergence in the ontogenetic trajectory), In contrast, clam shrimps of most other horizons supporting the presence of a single species within died due to senescence instead of ecological distur- Lake Sihetun. This identification of growth-related bance. variation has led to an emendation of the species In summary, the combination of sedimentary diagnosis of E. middendorfii. and biotic proxies shows that Lake Sihetun was (3) Subsequently, ecophenotypic variation in Eo- governed by an oxygen-controlled Phase 2 and a sestheria middendorfii has been analysed, using an ap- temperature-controlled Phase 3. The dioecious Eo- proach that minimizes ontogenetic variation. Three sestheria middendorfii was a generalist adapted to life distinct phenotypic morphogroups corresponding in permanent waters. Its high tolerance towards en- to different lake phases and excavation areas have vironmental gradients is expressed in a pronounced been recognized, implying that palaeoenvironmen- ecophenotypic variability. This indicates that careful tal changes had a great effect on carapace size and analyses of carapace size and shape in combination shape. Ecophenotypic differences are most pro- with sedimentological proxies and a palaeocom- nounced between phases 2 and 3 of Lake Sihetun. munity analysis can establish clam shrimps as im- In addition to morphological disparity, the min- portant marker organisms for ecosystem changes in eralogical and microstructural cuticle preservation Mesozoic lakes. of clam shrimps is introduced as a proxy for pal- aeoenvironmental conditions. Biominerals have been confirmed within two of the three extant spinicaudatan families: Cyzicidae build in calcium phosphate biominerals and possibly calcite. Leptes- theriidae biomineralize with calcite, and carapaces of the Limnadiidae are either hardly mineralized or they lack biominerals. Calcium phosphate biomin- eralization is a prerequisite for fluorapatite preser- vation, which is widespread in fossil clam shrimps. Important additional fossil minerals are carbon residues, dolomite, quartz, and silicates, which carry environmental instead of genetic signals. Calcite has not been detected in fossil material. Excellent mi- crostructure preservation implies oxygen deficient, alkaline lake waters (Phase 2). In turn, oxygenated conditions are identified by an amalgamation of growth increments and the precipitation of silica and silicates. In the final study, a partial response of lake com- munities to the proposed abiotic changes in the pal- aeoenvironment between phases 2 and 3 has been demonstrated. The studied section of Lake Sihetun yields three arthropod-dominated associations as well as two assemblages. Response curves to envi- ronmental gradients indicate that components of associations 1 and 2 (Eosestheria middendorfii and mayfly larvae) were generalists, explaining their presence in both phases 2 and 3. In turn, the so- called Transitional Fauna (Association 3 and the 2 Zusammenfassung Die Seesedimente der Yixian Formation (Bar- Zysten goldbrauner Algen (Herbstlagen) angezeigt
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