Prehistoric plankton prognosticate mass extinction

A recent study shows drastic shifts in a community of macroplanktonic fauna in the years preceding the mass extinction event at the end of the Ordovician (443.7 Million years ago).  These findings suggest that careful analysis of community structure using species abundances may prove to be a fine-tuned metric in understanding the effects of climate change on biological communities.

Climate change is one the most pressing issues facing scientists today and has reoccurred throughout history, with devastating consequences.  Understanding the consequences of climate change to biological communities in the past may help us to predict future changes to life on earth as the world warms and communities shift in response.

One group of scientists from Buffalo, NY, the Czech Republic, and Canada; are studying these such changes.  They chose to delve into the forgotten past, to the ancient late Ordovician (~447-444 million years ago).  This was a warm period in history where life in the sea reigned supreme.  Where cephalopods and trilobites were some of the most complex creatures, and life was just beginning to colonize the land.  But this was also a period of great climatic change.  During these few million years the world was heading toward the mass extinction event at the boundary between the Ordovician and Silurian periods.  It was a time of cooling, as the great land mass of Gondwana moved over the South Pole, ice caps began to form, sea levels dropped significantly, and deep ocean currents were altered.  The extinction event culminated around 447.3 million years ago as life was unable to adapt to the rapidly changing conditions and around 85% of all marine species went extinct, one of the five main extinction events in earth’s history.

ordovician sea

William B.S. Berry’s depiction of a marine community in the Ordovician period, dominated by a diversity of marine invertebrates (graptolites, trilobites, brachiopods), corals, primitive fish, and cephalopods – (University of California’s Museum of Paleontology)

One of these biological groups to be affected by this great change were the graptolites.  A curious-looking consumer, these macroplanktonic creatures were considerably diverse during this date.  They have been misrepresented in literature for years with fictitious attachments to seaweed, and only recently has the accurate reconstruction of the morphology of these organisms from fossil evidence been examined, see paper by Jörg Maletz below.

The graptolites were divided into two main groups by their associated habitat type.  There were the epipelagic forms, those which lived in the upper surface waters of the ocean, and the mesopelagic forms which lived in the deeper sections of the ocean.  The mesopelagic graptolites lived in the deep oxygen-poor level of the ocean and were dependent upon the coastal upwelling of nutrients and organic matter falling from above.  The epipelagic graptolites lived in the mixed, oxygenated, sunny, surface waters.  To study the changes in the community composition of these strange beasts, the scientists chose to analyze marine fossils from two localities during the same 3 million year time frame.  One dig site in the Yukon was most located on a mid-continental shelf of the ocean and had an ancient community dominated by epipelagic trilobites. The other location in Nevada represented a continental shelf edge, with presumably much deeper ocean depths, and a subsequent fossil assemblage dominated by the mesopelagic type graptolites.

sheets et al 2016

Sheets et. al 2016

In the shallow site, they found that as the sea levels fell, species diversity and evenness of the epipelagic graptolites fell, and the community became dominant by only a few members able to withstand the decline in habitat and the changing conditions.  They also found that in the deep location, along the shelf edge, that the mesopelagic graptolites were disproportionately affected more than the epipelagic species.  This was hypothesized as due to a change in ocean currents affecting the rate of coastal upwelling and the disappearance of low-oxygen conditions that supported the phytoplankton community the mesopelagic graptolites loved to eat.  Eventually, as ocean levels continued to drop and communities continued to shift, they reached a tipping point that resulted in many of these graptolite species becoming extinct.

ocean fossils

Examples of graptolite fossils used to determine community composition – Charles E. Mitchell

These changes would not have been noticed with coarse level analysis as the general character of the two sites remained the same throughout the study period.  However the attention to detail and calculations of community evenness, abundance, and estimation to account for rare species not detected (rarefaction) proved to be significant factors in detecting ecological change.

This study will prove to be invaluable in giving the scientists the tools to accurately determine community alterations in the face of climate change, and it also brings to mind the importance of those who study zooplankton and the need to fund such intellectual pursuits that may save the earth.

For more information on: statistical methods used in diversity analysis, the use of neodymium isotopic signatures to determine sea level, modeling rate and composition of deposition in the ancient oceans, and more!; read –

Sheets, H. D., Mitchell, C. E., Melchin, M. J., Loxton, J., Štorch, P., Carlucci, K. L., & Hawkins, A. D. (2016). Graptolite community responses to global climate change and the Late Ordovician mass extinction. Proceedings of the National Academy of Sciences, 201602102.

Maletz, J. (2015). Graptolite reconstructions and interpretations. Paläontologische Zeitschrift, 89(3), 271-286.

http://www.ucmp.berkeley.edu/ordovician/ordovician.php

https://www.sciencedaily.com/releases/2016/07/160719111114.htm

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