Smaller than meets the eye – the use of microfossils

Written by Heda Agić

Early days of fieldwork on the Digermul peninsula have yielded abundant fossils of macroscopic discs – attachment structures of large softbodied organisms such as Aspidella sp. that lived approximatelly 600-540 years ago. Such fossils are important since they represent some of the first visible records of complex life and the evolution of animals.

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Figure 1: Acritarch anatomy 101: round sphaeromorphs and process-bearing acantomorphs, sometimes showing an excystment opening.

 

Yet the quest for the early life does not end with Ediacaran discs and other impressions visible to the naked eye. Even older fossils, and a great diversity of early Paleozoic ones, resides in the world of small, observable only with the aid of a microscope. Some of those fossils are minute, tenth of a milimeter in diameter, organic-walled, single-celled spheres called acritarchs (Figure 1). They are usually found along with clumped pieces of particulate organic matter and kerogen, in fine-grained, organic-rich shales. Most suitable rocks for acritarch preservation are usually olive green, dark blue or black in colour (Figure 2). These microfossils may also occur in chert nodules or more rarely, in limestones.

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Figure 2: Olive green and blue green shales are excellent for preserving organic matter and organic-walled microfossils (Digermul 2014).

Acritarchs are prepared for microscopic observation by acid extraction from shales , clays, silts or limestones (using HF and HCl), whereby the rock matrix is dissolved and the individual carbonaceous fossils are picked carefully with a pipette and mounted onto glass preparates for light and scanning electron microscopy (SEM). Rock chips containing the specimens (usually in case of cherts) may also be cut and polished into thin sections (30 µm thickness) and the fossils are observed in cross section (Figure 3).

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Figure 3: Ediacaran microfossil Appendisphaera tenuis from Doushantuo, China, preserved in chert. (from: Xiao et al., 2014)

 

There is a lot of variation in size and shape among the acritarchs, but three main features of their body plan are shared: 1) resilient carbonaceous vesicle wall (Figures 4 & 5), able to withstand acid maceration; 2) processes (spiny protrusions) in different species (Figure 5); 3) round or split openings in the wall (Figure 4), called the excystment structure or a pylome. Processes were formed as the organism begun to develop a protective cyst, preparing for reproduction similar to modern algal zygotes, and these openings likely served for the release of gametes or daughter-cells as in many living protists.

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Figure 4: Cristallinium dubium from the Cambrian of Nova Scotia, Canada, preserved in shale (from: Palacios et al. 2012)

 

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Figure 5: Process-bearing Cambrian acritarch Skiagia ornata, extracted from shales (from: Moczydłowska, 2011). Scale bar: 15 µm.

Oldest acritarchs (leiosphaerids) are found in 1.8 billion year old rocks, while some other organic-walled fossils like bacterial sheaths and coccoids date as far back as 3.4 billion years.

Single-celled acritarchs are quite important for our understanding of the early life on Earth as they are the first record of complex and larger cells of Eukaryotes, a doman of organisms posessing a true nucleus (animals, plants, fungi, protists). By comparing ancient microfossils to modern microscopic marine organisms, it is possible to study the evolution and antiquity of different eukaryotic kingdoms.

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Figure 6: (Above & below images.) Size and shape of the spines surrounding the microfossil vesicles called processes (left, Ericiasphaera) and patterns on the acritarchs’ organic walls (right, Dictyosphaera macroreticulata) are helpful for distinguishing between different species. (from: Willman & Moczydłowska, 2006; Agić et al., 2014). Images obtained by SEM.micro 6b

Microfossils from the Proterozoic-Cambrian transition, such as those present on the Digermul peninsula, are particularly interesting for a better understanding of changing environments and biosphere revolution. As planktonic primary producers in the oceans, acritarchs would have depended on light and nutrient availability, so their diversity patterns through time may contribute additional information on the climate change in the late Neoproterozoic as well as the biotic turnover at the Ediacaran-Cambrian boundary. They would have provided a significant food source for the rapidly evolving animal groups.

 

Being abundant, planktonic, usually globally distributed and with rapidly evolving form-species, acritarchs are excellent index (guide) fossils that can be used in correlation of Proterozoic and early Paleozoic rocks. If an acritrach species occurs worldwide and over a short time period, it may be used to identify different a stratigraphic units or a biozone. This is helpful in estimating the age of the rocks, especially where other dating methods are not available.

 

Precambrian fossils, oddballs, Treptichnus pedum and moving camp – fieldwork summer of 2014

written by Anette Högström

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Fig 1. From days gone, fishing cabins on our way to sections south of camp at Manndrapselva.

 

We have moved camp from the Precambrian to the Cambrian, in the midst of wind and rain after packing a weeks worth of samples and specimens that will await pickup towards next week. Results have been manifold. Our quest for the oldest macroscopic organisms on the Digermul peninsula have given us large surfaces covered with mainly Aspidella sp., they look like holdfasts or attachment structures of soft bodied animals where only the holdfasts are preserved. They appear to have been quite rigid and were preserved sitting in the sediment on an ancient sea floor whereas the remainder of the animal may have looked like a frond standing up from the bottom. We now have large numbers of discs of varying size, but no clear frond or anything similar. Our last day on the southernmost exposure of the Ediacaran beds south of Manndrapselva revealed amazing discoveries. After tracing a bed we knew to contain a number of Aspidella discs we hit upon a spot where we fortunate to excavate a larger surface. It did require removing the overlying rock or overburden with the help of long crowbars, chisels, hammers and a large portion of stubbornness. The overburden finally gave way, and the hard work paid out! The surface showed an abundant and well preserved association of Aspidella and a few more odd structures. We were not sure of what all the features represent at the moment, but no obvious fronds were seen. A smaller float (a piece of rock found loose) contains a structure, albeit poorly preserved, that actually may prove to be a frond. This could be one of the more important finds as it may be the very first discovery of this type of structure connected to these discs.

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Fig 2. Surface covered with Aspidella.

Further up from the shore in a small valley still partially filled with snow but without name (quickly nicknamed “no-name valley”) the hunt for the Precambrian – Cambrian boundary is on. Treptichnus pedum is a trace fossil distinctly Cambrian and wherever it appears we know the boundary is close. This is one of the most exciting periods in the evolution of life, huge functional changes happen and animals more similar to the ones we know from today emerge in the oceans. Animals leaving behind a multitude of traces on these shallow marine bottoms, traces we are looking for roughly 550-540 million years later. Close to where the snow still lies Treptichnus appears on an under surface worthy of showing students. However, there is one trace still eluding us, it looks a bit like series of small stacked bananas, called Palaeopascichnus.

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Fig 3. Linn at work at the Treptichnus pedum level in “no-name valley”

 

This first week in the Precambrian at Manndrapselva ends with a tired group on the beach waiting for Trygve to come pick us up and drop us off at Breidvika where our search for early life will continue in the Cambrian.

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Fig 4. Awaiting pick up on the Precambrian pebble beach at Manndrapselva.

 

Runde disker og fotografering opp ned – felt sommeren 2014

Skrevet av Anette Högström

De første dagene i felt er bestandig fylt av spenning og forventninger; hva skal vi finne og kommer det vare noe helt nytt som ser dagens lys etter over 500 millioner år i sedimentene? At vare den første menneske som ser et fossil er spesielt. Vi vet at her ved Manndrapselva finns fossil av ei type som ikke finns noen annet sted i Skandinavia, de kalles for Ediacara fossil etter plassen Ediacara i Australia der man fant dem først. Disse fossil er rester av dyr uten skjelett eller skall, de eksisterte innen dyrene blev mer lik de vi finner i dag og intet som lever i dag er likt. Dette gjør Digermulhalvøya utrolig spennende.

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Fig 1: Lagene med disse allre første dyrene er akkurat nede ved stranden ved Manndrapselva.

 

Vi starter akkurat nede ved vannet der vi har leiret, her kommer vi vende og dele på så mye av steinen som vi kan for å se hva som gjømmer seg mellom lagene. De første funnen av runde disker med navnet Aspidella møts av mye glade tilrop, JAAA vi har funnet lagene der de finns, etter et tag er alle mer fokusert og vi tar fatt i store blokk, blokk som vi trenger vare 4 personer for å gjøre noe med. Den største vi sett, omtrent 14 cm, er å finne på et slikt blokk. For å ikke miste fingrer og tår sikrer vi blokket med tau til vi er ferdige i området.

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Fig 2; Den største Aspidella vi fant denne dagen

 

 

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Fig 3: Sikkring av et svært blokk som ikke vil stå opprett av seg sjøl.

 

Fotografering av disse runde disker kan vare utfordrende, å fota de som sitter på underside av ei klippe enda mer. Vi kryper inn og Wendy tar bilder av alle fossil og jeg prøver og holde tommestokken still.

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Fig 4: Fotografering opp ned

 

Reisen til Digermulen , nykokte krabber, and “the Ediacaran loo” – felt sommeren 2014

Skrevet av Linn Kristin Novis

Så var vi på vei til Norges ende for å lete etter spor fra det første flercellede liv på jorda. Etter en humpete flytur, møttes gjengen kalt, “Digermulen Early Life Research group” i Lakselv. For noen av oss deriblant undertegnede, var dette det første møte med denne samlingen bestående av en rekke forskere, med en særegen interesse for små rare skapninger som dominerte landjorda for over 500 millioner år siden.

Ferden fra Lakselv gikk videre til Sjursjokk, hvor våre lokale fiskere Trygve og Joakim møtte oss med båt. Full lastet til randen med utgravningsutstyr, telt og mat seilte vi nordover i Tanafjorden mot Digermulen. Joakim krabbefiskeren, dro opp en tegne og tok ut en gedigen kongekrabbe som han kokte og serverte oss underveis.

Fig 1-Kongekrabber
Kongekrabber

 

Trygve passet på at utstyret vårt kom trygt i land, ga oss en nyfisket laks til middags mat, og advarte oss mot sterk vind i natt, før han seilte skuta hjemmover.

Fig 2-Manndrapselva
Ankomst til leirplassen ved Manndrapselva.

 

Campen skulle nå etablerers, etter noe strev og mange kloke hoder som tenkte på kryss og tvers, var lavoen vår omsider reist.

Neste prosjekt var å lage den perfekte do. Utsikt er pri. nummer en, deretter stabilitet og komfort. De to sistnevnte kriteriene var værre å tilfredstille. Vi fant den perfekte plass, som ga oss det ene valget å bygge en stein-do. Iogforseig måtte dette jo være drømme doen til en geolog, og enda bedre da steinen var av typen nedre Ediacara, altså mere enn 550 millioner år gammel. Det som ikke var fullt så lett var å få dette byggverket til å stå stabilt. Jan Ove og undertegnede innså hvorfor vi hadde blitt geologer og ikke ingienjører. Etter flere runder med steinlegging sto mesterverket “The Ediacaran loo” ferdig.

Fig 3-View from the loo
Leiret sett fra “the Ediacaran loo”.

 

Leirplassen var etablert, åtte telt med stramme barduner lyste mot en utsikt over Tanafjorden og Varangerhalvøya.

Håper bare vi ikke blåser bort i natt da, to be continued…