| 1. |
- Agić, Heda, 1989-
(författare)
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A new species of small acritarch with porous wall structure from the early Cambrian of Estonia, and implications for the fossil record of eukaryotic picoplankton
- 2016
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Ingår i: Palynology. - : Informa UK Limited. - 0191-6122 .- 1558-9188. ; 40:3, s. 343-356
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Tidskriftsartikel (refereegranskat)abstract
- The Proterozoic-Phanerozoic transition records a general trend of decrease in phytoplankton cell size, in contrast tothe earlier and much larger Ediacaran acritarchs. Particularly minute, unornamented but sculptured organic-walledmicrofossils have been recovered from the lower Cambrian Lükati Formation in northern Estonia. The lack of anysignificant thermal alteration in the formation allowed for excellent preservation of fine microstructures on thesemicrofossils. Among the rich palynomorph assemblage in Lükati, a new species of tiny, spheroidal eukaryoticmicrofossil is recorded: Reticella corrugata gen. et sp. nov. It is characterised by a corrugated and flexible vesicle wallthat is densely perforated by nano-scale pores. Despite its unique morphology, the new species shares diagnosticcharacters with fossil and extant prasinophyte algae. R. corrugata is among the smallest microfossils with typicaleukaryotic morphology (conspicuous wall sculpture) and contributes to the diversity of the size class of smallacritarchs. Size, abundance, inferred prasinophyte affinity and eukaryotic wall sculpture make this new taxon alikely member of the early eukaryotic picoplankton.
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| 2. |
- Agić, Heda, 1989-
(författare)
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Fossil Focus : Acritarchs
- 2016
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Ingår i: Palaeontology Online. ; 6:11, s. 1-13
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 3. |
- Agić, Heda, 1989-, et al.
(författare)
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Reproductive cyst and operculum formation in the Cambrian-Ordovician galeate-plexus microfossils
- 2016
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 138:2, s. 278-294
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Tidskriftsartikel (refereegranskat)abstract
- Unicellular organic-walled microfossils from the Cambrian-Ordovician transition in Estonia (ca. 490-480 million years ago) exhibit rare characters reflecting their function as reproductive algal cysts. The studied assemblages record the evolutionary history of phytoplankton in the early Paleozoic Era: novel morphologies appearing through the Cambrian and subsequently diversifying in the Ordovician. Well preserved specimens were extracted following a standard palynological method and studied by light transmitted microscopy. The galeate plexus acritarchs Caldariola, Priscogalea and Stelliferidium have revealed exceptionally preserved morphological elements and a rare structure among both fossil and extant protists – an opening with operculum (lid) in reproductive cysts, in addition to lavish vesicle ornamentation and sculpture. Analogous morphology is observed in the living dasycladalean alga Acetabularia (Chlorophyta), which possesses an intrinsic lid-forming apparatus used during organism’s reproductive stage. Based on the observations on the fossil material and studies on the Acetabularia lid-formation, we propose a model of operculum formation in the galeate plexus microorganisms. Due to strong morphological and ecological similarities between galeate fossils and dasycladalean cysts, and the antiquity of this algal order, galeates may be positioned within green algae, more specifically Dasycladales. Unique morphology of the operculum-bearing microbiota would have required a high degree of intracellular complexity for its development, suggesting that advanced intracellular machinery was present already in the early Paleozoic phytoplankton. Additionally, minute prasinophyte microfossils Reticella corrugata are reported for the first time in the Upper Cambrian strata.
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| 4. |
- Petryshyn, Victoria, et al.
(författare)
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Stromatolites in Walker Lake (Nevada, Great Basin, USA) record climate and lake level changes ~ 35,000 years ago
- 2016
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier BV. - 0031-0182 .- 1872-616X. ; 451, s. 140-151
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Tidskriftsartikel (refereegranskat)abstract
- Walker Lake is a closed-basin remnant of the large Pleistocene glacial Lake Lahontan system that has experienced multiple high amplitude (100–200 m) changes in water level over the past ~ 40,000 years in response to changes in climate. A laminated carbonate stromatolite composed of varying proportions of calcite fans and micrite was collected from a paleoshoreline located at approximately 58 m above present lake level. Radiocarbon dating revealed that the stromatolite spans approximately 2000 years of growth, from 35,227 to 33,727 calibrated years before present (YBP), a time period during which paleolake-level is not well-constrained. Distinct laminae were drilled along the growth axis, and the resulting powders were collected for clumped isotope analyses to generate formation temperatures (lake water temperatures) during stromatolite formation from which δ18Owater was calculated.Results indicate that the stromatolite experienced an initial increase in temperature and water δ18O values followed by a decrease in both during the course of accretion. The resulting temperature and isotopic data were input into a Rayleigh distillation model for lakewater evaporation in order to estimate the magnitude of lake level and volume fluctuations over the course of accretion. Modeling results reveal a lake level decrease of between 8.1 and 15.6 m, followed by an increase of between 4.3 and 8.8 m during the course of stromatolite growth.The results of this study indicate that Walker Lake experienced significant lake volume change over the course of 2000 years, perhaps as a response to precipitation changes driven by fluctuations in the polar jet stream and accompanying changes in regional climate, and/or evaporation-induced changes in lake level. These results add to a growing body of research indicating that stromatolites and other lacustrine tufas represent a detailed and extensive terrestrial archive that can potentially be used to reconstruct the timing and magnitude of climate change.
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