To see the world in a grain of sand: Planktonic foraminifera and Evolution.

Planktonic foraminifera. (Credit: Paul Pearson, Cardiff University)

Planktonic foraminifera. (Credit: Paul Pearson, Cardiff University)

“To see the world in a grain of sand…”, this is the first line of William Blake´s poem “Auguries of Innocence” which describe a series of paradoxes about innocence, evil and corruption. But in a biological sense, this line can also describe how “a grain of sand” could gives a glimpse of how evolution works using the remains of planktonic foraminifera which resemble grains of sand to the naked eye and date back hundreds of millions of years.

Foraminifera are an important group of single celled protozoa with shells of different composition and granuloreticulose pseudopodia.  The first record of the group is from the Early Cambrian and extend to the present day. Their size range is from about 100 micrometers to almost 20 centimeters long.

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

Planktonic foraminifera are ideal subjects for testing how species evolve over time. They are a diverse extant clade that have an exceptional fossil record, due to extremely large population sizes and widespread species distributions. They also can record the climate and environmental conditions on their calcium carbonate shells.

It seems that gradual morphological trends do not strictly reflect the rate of speciation or its mode within the clade. In a paper published in 1998, Kucera & Malmgren,  showed that gradual change in the Cretaceous planktonic foraminifera Contusotruncana fornicata probably resulted in a shift in the relative proportion of high conical to low conical forms through time, yet isotopic data indicated a rapid separation of the population.

 Globigerina bulloides and Legionella inflata, two examples of planktonic foraminifera.

Globigerina bulloides and Globoconella
inflata, two examples of planktonic foramininfiera.

Using stratigraphic, phylogenetic and ecological data from the exceptional fossil record of Cenozoic macroperforate planktonic foraminifera, Dr Thomas Ezard from the  University of Southampton, explains how the fossil record contains signals of biological processes that drive genetic evolution. He used a complete phylogeny of those Cenozoic foraminifera to provide palaeontologically calibrated ages for every divergence within the clade that are independent of molecular data. Their  hypothesis is that speciation provokes a burst of rapid genetic change, giving molecular evolution a punctuational component. This rapid burst helps isolate the new species from its ancestor.

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The study shows how the fossil record contains signals of biological processes that drive genetic evolution and promotes the importance of using fossil records in conjunction with the molecular models.

References:

Ezard, T. H. G., Thomas, G. H., Purvis, A. (2013), Inclusion of a near-complete fossil record reveals speciation-related molecular evolution. Methods in Ecology and Evolution, 4: 745–753. doi: 10.1111/2041-210X.12089

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MICROFOSSILS AND THE OCEAN HISTORY.

Forams from deep-sea. Credit: Miriam Katz, Rensselaer Polytechnic Institute. (Originally published by Micropress.)

Forams from deep-sea. Credit: Miriam Katz, Rensselaer Polytechnic Institute. (Originally published by Micropress.)

Microfossils from deep-sea are crucial elements for our understanding of past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. Over millions of years, these skeletons accumulate in the deep ocean to become a major component of biogenic deep-sea sediments.

The importance of microfossils as tool for paleoclimate reconstruction was recognized early in the history of oceanography. John Murray, naturalist of the CHALLENGER Expedition (1872-1876) found that differences in species composition of planktonic foraminifera from ocean sediments contains clues about the temperatures in which they lived.

Following this pioneering work, Schott working on sediments of the METEOR Expedition (1925-1927) introduced quantitative counting of species within the fossil assemblages on the sea floor and realized that surface water temperature changed as the climate fluctuated between glacial and interglacial conditions.

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

In 1955, Emiliani, who was then a student of Harold Urey at the University of Chicago,  published a paper entitled “Pleistocene temperatures” where introduced isotope stratigraphy to paleoceanography. He used the density of a heavy oxygen isotope in planktonic foraminifera from deep sea cores to outline oxygen isotope stages for the Quaternary, believing these would reflect surface temperature changes and the ice volume changes.  He concluded that the last glacial cycled had ended about 16,000 years ago, and found that temperature increased steadily between that time and about 6000 years ago. Many of Emiliani’s findings are still valid today, however in 1970 several improvements to Emiliani’s work were made, such as a revision of the temperature scale.

Oxygen isotope records have also been obtained from well-preserved microfossil materials in the Late Cretaceous  when bottom waters appear to have been much warmer than at present.

This concepts of paleotemperature reconstruction, as first developed for planktic foraminifera, apply to other groups of microfossils. Diatoms and radiolarians are susceptible to different set of dissolution parameters than calcareous fossils, resulting in a different distribution pattern at the sea floor and have been used for temperature estimates in the Pacific and in the Antartic Oceans, especially where calcareous fossils are less abundant. Diatom assemblage are also used in reconstructions of paleoproductivity.

Climatic modes and sea-level fluctuations indicated by calcareous nannofossils of the Oligocene deposits from the Romanian Carpathians. (Melinte, 2004)

Climatic modes and sea-level fluctuations indicated by calcareous nannofossils of the Oligocene deposits from the Romanian Carpathians. (Melinte, 2004)

The calcareous nannoplankton represents good proxy for the sea-level fluctuations. The group exhibit  a clear latitudinal distribution pattern, for instance, the presence of mixed nannofloral assemblages (taxa of low-middle latitudes together with high ones) are indicative of the sea-level rise,  while endemic assemblages characterize periods of low sea-level.

By studying cores from those ocean sediments, its possible determine the ages of the rocks, the ocean environment and some atmospheric conditions using the information  provided by the microfossils present in that core, as well as stable isotope analysis and magnetic stratigraphy.

Each layer of the core recorded the geological history of the ocean basins, changing climates, evolving biota and the events that could altered the course of Earth history.

References:

Armstrong, Howard A. and Martin D. Brasier.  Microfossils.  Blackwell Publishing, 2005.

Berger, W. H., Sea level in the late Quaternary: patterns of variation and implications, Int J Earth Sci (Geol Rundsch) (2008) 97:1143–1150

The impact winter model

The Chicxulub asteroid impact (Image credit: NASA)

The Chicxulub asteroid impact (Image credit: NASA)

The Chicxulub asteroid impact was the final straw that pushed Earth past the tipping point.  The K-Pg extinction that followed the impact was one of the five great Phanerozoic  mass extinctions. The impact released an estimated energy equivalent of 100 teratons of TNT and produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere. The dramatic decrease of sunlight that reached the Earth surface caused a drastic short-term global reduction in temperature. This phenomenon is called “impact winter”.

Cold and darkness lasted for a period of months to years.  Photosynthesis stopped and the food chain collapsed. Three-quarters of the plant and animal species on Earth disappeared. Marine ecosystems lost about half of their species, and freshwater environments shows low extinction rates, about 10% to 22% of genera.

This difference in extinction rates between marine and freshwater ecosystems could be explained by differential effects of an impact winter on marine and freshwater biota and the difference in the use for detrital foods.

Arkhangelskiella cymbiformis and Globotruncana linneiana (d’Orbigny, 1839).

Arkhangelskiella cymbiformis and Globotruncana linneiana (d’Orbigny, 1839).

Three factors can be associated with the impact winter in marine and fresh water enviroments. First, starvation caused by the stop of photosynthesis. Second, the loss of dissolved oxygen. Third, the low temperatures. The flux of organic detritus to the sea floor also declined abruptly and remained low for about 3 Myr after the impact.

Marine extinction rates were greater among pelagic than benthic organisms. Calcareous nanoplankton (primarily the coccolithophores) and planktonic foraminifera had the highest extinction rates among the marine plankton, possibly because they commonly lack cysts or resting stages.  About 70% of planktonic foraminifera became extinct at the K/Pg boundary. The food webs supported by plankton were severely affected. For instance, ammonites were plankton feeder and they, like mosasaurs, plesiosaurs and pliosaurs that fed on them, became extinct at the  K-Pg boundary.

Mosasaurus hoffmani, Late cretaceous of Europe by Nobu Tamura. From Wikimedia Commons.

Mosasaurus hoffmani, Late cretaceous of Europe by Nobu Tamura. From Wikimedia Commons.

In case of freshwater communities, they were adapted to rapidly changing environments. For instance, starvation can be offset by dormancy, which is much more common among freshwater than marine organisms. Dormancy could have lowered extinction rates in inland waters compared to marine waters.

Because the late Cretaceous climate was warm, a major challenge for aquatic organisms, especially in inland waters, may have been the persistence of low temperatures. Inland waters had an advantage for preventing extinction caused by prolonged cold:  the presence of thermal refugia in the drainage networks of inland waters derived not only from  geothermal waters, but also from groundwater.

The abundance of refugia combined with greater adaptation to stressful conditions may explain lower extinction frequencies in freshwaters than in marine waters. Even when mortality within all taxa may have been equal to or even greater in freshwaters than in marine waters, the factors that protect some survivors against extinction  are more evident in the freshwater environment.

Jeletzkytes spedeni, a fossil ammonite from USA. From Wikimedia Commons.

Jeletzkytes spedeni, a fossil ammonite from USA. From Wikimedia Commons.

It seems clear that the terrestrial and marine extinction were separated in time by a matter of months to years.  These extinctions had two different mechanisms: an impact winter in the marine environment and a heat pulse and subsequent fires in the terrestrial environment, although an impact winter would also affect the terrestrial environment. A more comprehensive analysis also shows three separate spatial domains (terrestrial, marine, and freshwater), which provide us a more understandable picture of the K-Pg extinction.

References:

Douglas S. Robertson, William M. Lewis, Peter M. Sheehan and Owen B. Toon, K-Pg extinction patterns in marine and freshwater environments: The impact winter model, Journal of Geophysical Research: Biogeosciences, JUL 2013, DOI: 10.1002/jgrg.20086.

Schulte, P., et al. (2010), The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary, Science, 327, 1214–1218, doi:10.1126/science.1177265