Assessing the record and causes of Late Triassic extinctions
Introduction
As early as 1963, Newell identified a major extinction (more than one third of all animal families) at the end of the Triassic. Newell (1963) stated specifically that 24 of 25 ammonoid families became extinct, and he drew specific attention to the replacement of many groups of amphibians and reptiles by the dinosaurs. The loss of species at the Triassic–Jurassic boundary (TJB) is now identified routinely as one of the “big five” mass extinctions of the Phanerozoic, implying a level of suddenness and severity that distinguishes it in the stratigraphic record (e.g., Hallam, 1981, Hallam, 1990a, Raup and Sepkoski, 1982, Raup and Sepkoski, 1984, Olsen et al., 1987, Olsen et al., 2002a, Olsen et al., 2002b, Benton, 1995, Sepkoski, 1996, Sepkoski, 1997, Kemp, 1999, Lucas, 1999, Pálfy et al., 2002). Indeed, Raup (1992) estimated that about 76% of species became extinct at the TJB. Sepkoski (1982) identified the end-Triassic extinction as one of four extinctions of intermediate magnitude (end-Cretaceous, end-Triassic, Late Devonian, Late Ordovician), based on a global compilation of families of marine invertebrates. Overall, this assumption of intense and sudden biotic decline at the system boundary remains largely unquestioned, with a few notable exceptions Teichert, 1990, Hallam, 2002.
In addition to inspecting the palaeontological data to evaluate the timing and severity of extinction, with particular attention to the record of biotic turnover at the TJB, in this paper we examine critically the potential effects, and therefore the feasibility, of the various mechanisms that have been suggested as responsible for Late Triassic extinction. These proposed mechanisms include both gradualistic and catastrophic processes. The former may encompass sea-level change Newell, 1967, Hallam, 1990a, which may result in habitat reduction (from regression) or anoxia (from transgression), and climate change, specifically widespread aridification (Tucker and Benton, 1982). The catastrophic processes proposed to explain the biotic events include: bolide impact Olsen et al., 1987, Olsen et al., 2002a, Olsen et al., 2002b, the effects of which may encompass a sudden increase in atmospheric opacity; outgassing during voluminous volcanism McElwain et al., 1999, Marzoli et al., 1999, Wignall, 2001, McHone, 2003, with climatic effects of both CO2 and SO2 emissions proposed as forcing mechanisms; and sudden release of methane hydrates from the sea floor Pálfy et al., 2001, Retallack, 2001, Hesselbo et al., 2002, the consequences of which may include significant greenhouse warming.
Section snippets
Defining the boundary
There is no internationally agreed global stratotype section and point (GSSP) for the TJB, although recent proposals of TJB GSSPs in Nevada, Canada, Peru and Great Britain are currently under consideration. It has long been agreed to use the lowest occurrence (LO) of the ammonite Psiloceras planorbis (J. de C. Sowerby) to define the base of the Hettangian Stage at the base of the Jurassic (e.g., Maubeuge, 1964, George, 1969, Morton, 1971, Cope et al., 1980). Unfortunately, this definition is
Extinctions and the compiled correlation effect
Most of Newell's (1963) original estimates of extinction at the TJB lacked explicit quantitative documentation. Pitrat (1970), in contrast, calculated that 103 families of marine invertebrates became extinct during or at the end of the Triassic, but that another 175 continued from the Triassic through into the Jurassic. He claimed elimination of approximately 20% of roughly 300 extant families, most severely affecting the cephalopods (loss of 31 families), but also marine reptiles (loss of 7
Age of the TJB and timing of extinctions
Until recently, the age of the TJB has been known only imprecisely, resulting in numerical estimates of 208.0±7.5 Ma (Harland et al., 1990) and 205.7±4.0 Ma (Gradstein et al., 1994) cited commonly in the literature. In recent years, however, improved age determinations of volcanics proximal to the boundary in both terrestrial and marine sections have led to greatly enhanced understanding of the timing of the extinctions. The clearest association has been in the Mesozoic rift basins of eastern
Mechanisms of extinction
As discussed above, the palaeontological record does not support the interpretation of a single catastrophic end-Triassic extinction. Rather, many major biotic groups, but not all, suffered significant declines in diversity through the Late Triassic, possibly with episodes of extinction scattered among the Carnian–Norian boundary, during the Early Norian, at the Norian–Rhaetian boundary, spread throughout the Rhaetian, as well as at the system boundary. At this time, stratigraphic resolution is
Conclusions
(1) Although the Late Triassic witnessed significant biotic decline, the appearance of a sudden mass extinction event at the TJB seems to be a consequence largely of stage-level correlation (the CCE). The most prominent faunal groups of the marine realm cited in identifying this “event,” such as ammonoids, bivalves, and conodonts, instead experienced gradual to step-wise extinction throughout the Norian, particularly during the middle to upper Norian, and Rhaetian. The terrestrial record of
Acknowledgements
The authors gratefully acknowledge the helpful comments provided by Carolyn Shoemaker, Kevin Mullins and John McHone during preparation of this manuscript. Mike Benton, Tony Hallam and Paul Wignall provided insightful reviews that improved this work significantly.
Lawrence H. Tanner is a sedimentary geologist with interests in Early Mesozoic paleoenvironments and paleoclimates, mass extinctions, volcaniclastic sediments and volcanic processes. He is a Professor of Geosciences at Bloomsburg University of Pennsylvania. He trained at Williams College (BA), Tulsa University (MS) and the University of Massachusetts-Amherst (PhD). His field experience includes extensive work in the Canadian Maritimes, the Colorado Plateau region of the Southwestern U.S. and
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Lawrence H. Tanner is a sedimentary geologist with interests in Early Mesozoic paleoenvironments and paleoclimates, mass extinctions, volcaniclastic sediments and volcanic processes. He is a Professor of Geosciences at Bloomsburg University of Pennsylvania. He trained at Williams College (BA), Tulsa University (MS) and the University of Massachusetts-Amherst (PhD). His field experience includes extensive work in the Canadian Maritimes, the Colorado Plateau region of the Southwestern U.S. and southern Italy.
Spencer G. Lucas is a paleontologist and stratigrapher who specializes in the study of Late Paleozoic, Mesozoic and Early Cenozoic vertebrate fossils and continental deposits, particularly in the American Southwest. He is Curator of Paleontology and Geology at the New Mexico Museum of Natural History and Science. Trained at the University of New Mexico (BA) and Yale University (MS and PhD), he has extensive field experience in the western United States as well as in northern Mexico, Costa Rica, Jamaica, Kazakhstan, Soviet Georgia and the People's Republic of China.
Mary Genevieve Chapman has been a Research Geologist with the Astrogeology Team of the U.S. Geological Survey in Flagstaff, Arizona for 20 years. Her expertise includes planetary geology, particularly the study of outflow channels and volcanic deposits on Mars, Icelandic–Martian analog environments, and the sedimentology of volcaniclastic deposits on the Colorado Plateau. She received her education at the University of Utah (BS) and Northern Arizona University (MS), and is currently engaged in doctoral studies at Keele University, UK.