RICERCA ITALIANA     

The end and the beginning: loss and recoveries from the end-Ordovician mass extinction in the North Gondwana

Università degli Studi di Modena e Reggio Emilia

Abstract

The second major extinction event in life history occurred in the Late Ordovician. It was followed by an important recovery in the Early Silurian: the end and the beginning, precisely.
After having cleared up all "inorganic" elements of this crisis (geodynamic evolution, climate change, ocean circulation patterns, regional anoxia, O2/CO2 ratio), how did marine faunas respond to this dramatic episode? How could they react and trigger recovery processes? How important were palaeogeographic, palaeoclimatic and palaeoecological frameworks?
We will try to give an answer to the previous questions with the aid of sole pelagic faunas (conodonts, graptolites and cephalopods), main victims of the extinction, in a specific palaeogeographic setting, the North Gondwana. Biodiversity change evaluation, as well as palaeoclimatic information obtained by means of geochemical analyses, will allow the precise and detailed monitoring of marine biotic response to the Late Ordovician extinction event.

Principal Investigator

Annalisa FERRETTI Università degli Studi di MODENA e REGGIO EMILIA

Research Objectives

Only very recently scientists are suggesting that the sixth great mass extinction, involving present world, has already started.
Major crises of the life history are being riexamined with renewing interest in order to obtain new data on the dynamics and causes of these episodes and to have a better comprehension of what is happening nowadays.
The Big Five mass extinctions classically identified in the Phanerozoic have greatly attracted scientists since the pioneer works of Newell (1967) and Raup & Sepkoski (1982). The Late Ordovician crisis represents the second severest extinction episode. The majority of research carried out on this topic has been mostly devoted to unravelling the biotic response of benthic faunas, which are more strictly dependent on ecological features. Pelagic faunas, on the contrary, have been often ignored. In a time slice for which palaeogeographic elements are continuosly moving and palaeoclimatologic data are contributing to define a more and more precise Ordovician "scenario", is the chapter on the Late Ordovician extinction already finished?
On this basis, we intend to propose the project: THE END AND THE BEGINNING: LOSS AND RECOVERIES FROM THE END-ORDOVICIAN MASS EXTINCTION IN THE NORTH GONDWANA.
The aim of the project is a study of the distinctive extinction/radiation pattern of pelagic faunas during and after the end-Ordovician crisis in order to unravel the variety of mechanisms, intensities and magnitudes of the >>>

First Results

The main expected partial result consists in the collection of data in the field, for all the time intervals included in the present proposal. The systematic and biostratigraphical study of the selected fossil groups will be carried out as well. In detail:

- mapping, measurements and sampling of already mentioned sections;
- lithostratigraphic and sedimentological analysis of the sections under study;
- lab processing (acid treatment for conodonts, thin-sections and preparation of peels for microfacies analysis);
- systematic identification of the investigated fossil groups;
- biostratigraphical analysis;
- palaeoecological analysis.

The general meeting scheduled for October 2005 with foreign collaborators will verify the results so far obtained in comparison with the scheduled objectives.The expected results are the final results of the project. The details are explained in the second phase. The basic purpose of the project is to assess biotic response to global change in the Northern Gondwana area during the second severest extinction of life history, the Late Ordovician event. It is strongly desirable that our study, focused on some pelagic taxa often ignored, might produce new data for improving the already existing information about this topic so as to understand why these extinction events did occur, in which way they developed and how and why marine faunas responded to them.

Timescale

24 months

National and international background

Compilations of global marine biodiversity through the Phanerozoic (Sepkoski 1979, 1981, 1990) have identified intervals of progressive increase and long-term plateaux punctuated by five sharp events of mass extinction, the so called Big Five (Raup & Sepkoski 1982). Of these, the end-Ordovician and end-Permian are the two most severe crises. Since the hypothesis of Raup & Sepkoski (1984) about the cyclicity and similarity of mechanisms of major extinction events, only a few studies have been carried out on the comparison of the five crisis showing a correlation in the pattern of biotic response (Hallam & Wignall 1997; Harries & Little 1999; Rong & Shen 2002). In fact, most palaeontologists chiefly analyse patterns of extinction and survival that characterise each mass extinction. As regards the Late Ordovician episode, indeed, many elements still need a clear definition, first of all a precise palaeoclimatic background as well as a definite palaeogeographic "scenario" if a final answer on the Late Ordovician extinction is really expected.
The major features and general state of the art about the Late Ordovician great event are outlined by the answers to the following questions.

-HOW SEVERE WAS THE ORDOVICIAN EXTINCTION? Taxonomic loss during the Late Ordovician extinction was particularly heavy at lower levels. An estimated 85% of species disappeared, 61% of genera (Jablonski 1991) and 12-24% of families (Sepkoski 1997; Benton 1995) but few o no >>>