of millions of years ago, a group of armored, segmented creatures called trilobites
covered Earths seafloors. Millions of years later, crinoids (sea lilies)
and brachiopods (clam-like animals) dominated the ocean bottoms. Jawless fish
and nautiloids (relatives of squids) once swam the seas in abundance, but millions
of years later sharks and ammonites (another squid relative) held sway.
Mass extinctions drove many of these changes, by eliminating seemingly well-adapted and very successful organisms, who were later replaced by equally successful but very different life forms. Indeed, many of the organisms that populate the globe today, including humans, probably would not be here had it not been for mass extinctions. These biotic catastrophes eliminated many successful and common organisms, paving the way for evolutionary innovations that produced a wide range of new and successful life forms. Thus, without mass extinctions, dinosaurs might still be stalking the landscape, and trilobites might be combing the seafloors.
The causes of these extinctions have long fascinated and challenged scientists. Seemingly simple causes, such as supernovas, have been proposed, analyzed and then rejected either for lack of concrete supporting evidence, or more often because the mass extinctions were shown to be complex phenomena that had complex underlying causes. Much recent thinking on what drives mass extinctions has focused on extraterrestrial impacts (meteorites, comets or asteroids) as a likely cause of one or all of the great extinctions in the history of life. However, 25 years of serious study of such events has shown that impacts were not the cause of most of the big extinctions.
Making an impact
In their 1997 book on mass extinctions, Anthony Hallam and Paul Wignall defined a mass extinction as an extinction of a significant proportion of the worlds biota in a geologically insignificant period of time. The current interest in such mass extinctions began in 1980: Nobel physics laureate Luis Alvarez, his geologist son Walter Alvarez, and two nuclear chemists, Frank Asaro and Helen Michel, made the bold proposal that a comet or meteorite 10 kilometers in diameter collided with Earth at the end of the Cretaceous 65 million years ago and caused the extinction of dinosaurs and many other organisms.
At about the same time, several paleontologists, notably Charles Pitrat and Keith Thomson, and later David Raup and John Sepkoski, were compiling the changing diversity of fossil organisms from the paleontological literature. They identified several diversity crashes, or mass extinctions, during the last 540 million years. Of these, five stood out as much larger than the rest: the end-Ordovician about 450 million years ago, the Late Devonian about 374 million years ago, the end-Permian about 251 million years ago, the end-Triassic about 200 million years ago, and the end-Cretaceous about 65 million years ago. Of these so-called big five Phanerozoic extinctions, the end-Permian extinction was the most severe mass extinction in the history of life on the planet. By some estimates, as much as 90 percent of marine species were decimated at the end of the Permian.
The coincidence of the impact hypothesis and the identification of several severe extinctions revived the old idea that the impact of extraterrestrial objects with Earth could provide a general theory of extinction. One of the earliest expressions of this concept came from French astronomer Pierre Laplace, who, in the 1790s, argued that large cometary impacts altered the spin and rotational axis of Earth, thereby producing major catastrophes. A significant drawback to Laplaces idea, and the similar proposals of others who followed, was that for nearly 200 years the means to test it did not exist.
The Alvarez team provided such a test an anomalously high concentration of the element iridium in sediments dated to the time of an impact, so high that it could only have come from an extraterrestrial source. Other signatures of an impact were soon identified in the rock record, such as shocked quartz grains. Thus, it became possible to look in the rock record for direct evidence of impacts associated with the five large mass extinctions. Critics of the impact hypothesis as the cause of the end-Cretaceous extinction remained, primarily for lack of a suitably aged impact crater, but these were largely silenced with the identification of the Chicxulub structure off the Yucatan Peninsula (see Geotimes, January 2004).
For the past quarter-century, the search for evidence linking the other mass extinctions to extraterrestrial impacts has continued. Despite the identification and dating of numerous impact structures, the evidence for extraterrestrial involvement in earthly mass extinction remains circumstantial and suspect at best. On the contrary, the underlying causes of each of the big five extinctions stands out as a unique set of circumstances. Furthermore, ongoing discussion of the timing and causes of the extinctions continues to demonstrate the need for better temporal resolution in the study of mass extinctions.
Near-end of the trilobites
The Late Ordovician extinction a two-pulse event is the oldest of the big five Phanerozoic extinctions. Current estimates suggest that 85 percent of marine species became extinct. Particularly significant, as pointed out by Peter Sheehan, is that extinctions in the Late Ordovician were spread across many taxa and not concentrated in particular food chains. Additionally, no evidence has been found of an extraterrestrial impact event at that time. This evidence argues against a cataclysmic cause and instead supports the idea of much less severe but widespread extinctions with global climate change as their underlying cause.
The most likely scenario involves a short but severe high-latitude ice age that changed global sea level. Norman Newell suggested in the 1960s that changes in sea level could cause extinction through reducing the available marine bottom habitats in shallow shelf and epicontinental environments.
Computer modeling adequately demonstrates that southward movement of Gondwana, and possible accompanying decreases in atmospheric carbon dioxide, led to glaciation. The onset of this glaciation closely correlates to a first pulse of extinction coincident with a drop in global sea level, a shift to a colder global climate and increased oceanic circulation. The end of glaciation closely corresponds to a second pulse of extinction with a rise in global sea level, warming of climate and the onset of sluggish oceanic circulation. The close correspondences are more than mere coincidence.
Such changes would have particularly affected bottom-dwelling marine organisms, such as the brachiopods and crinoids. But mobile marine animals also experienced extinctions. Trilobites were very common inhabitants of seafloors before the Late Ordovician, but after the extinctions they persisted at low diversity until their final demise in the Permian.
End of the placoderms
The second of the big five extinctions took place during the Late Devonian. It has long been clear that a major drop in diversity took place near the end of the Devonian, and there are two main extinctions usually identified. But it is impossible to decide how many individual extinctions took place and how long they took due to poor stratigraphic resolution. Estimates of the duration of the Late Devonian diversity drop vary from a few hundred thousand to more than 10 million years.
Estimates of the severity of the Late Devonian extinctions suggest a loss of about 27 percent of marine families and 70 to 80 percent of species of marine organisms. Particularly hard-hit groups include the brachiopods, ammonoids, trilobites, conodonts, jawless fish and the top predators of the Devonian seas, the armored placoderm fish. Devonian reef-building organisms (tabulate corals and stromatoporoids) were also devastated. Evidence for extraterrestrial impacts during the Middle and Late Devonian has been described, but none of the supposed impacts can be shown to coincide with these extinctions. Moreover, the Late Devonian looks more like a prolonged biotic crisis, and not simply a single (or even double) mass extinction.
Thus, explanations of the Late Devonian extinctions focus on changes in sea level, climate and oceanic anoxia (a shortage of oxygen) as causes. A particularly interesting explanation, proposed by Thomas Algeo and collaborators, links the extinctions to the spread of land plants during the Devonian. This spread intensified soil formation and chemical weathering, leading to the dumping of more organic matter and chemical nutrients into the oceans. This loading in turn accelerated weathering of silica on land, which created more calcium and magnesium carbonate, and thus removed carbon dioxide from the atmosphere. The increase in bioproductivity and the consequent accumulation of organic matter in shallow seas caused anoxia, the deposition of the characteristic Late Devonian black shales, and the extinctions of bottom-dwelling marine invertebrates all events now visible in the geologic record. Additionally, removing carbon dioxide from the atmosphere may have caused global cooling that led to a glacial age. However, conclusive evidence of the Late Devonian ice age has proved to be elusive, so more work on this topic is necessary.
Near-end of the brachiopods
The end-Permian mass extinction, which marks the Paleozoic-Mesozoic boundary at around 251 million years ago, has long been recognized as the most severe and sudden mass extinction in Earths history. Some estimates suggest as much as 90 percent of shelled marine invertebrate species were eliminated in less than 500,000 years.
Indeed, in the oceans, the end of the Permian signifies a remarkable change from Paleozoic seafloor communities dominated by brachiopods, crinoids, stromatoporoids, tabulate and rugose corals, and bryozoans to communities dominated by mollusks (especially ammonites, bivalves and gastropods). However, as in the earlier major extinctions, the end-Permian extinction was selective, and the story is not simple.
The end-Permian extinction followed a severe extinction during the Middle Permian (approximately 260 million years ago), and some groups devastated during that period simply persisted at lower diversity until their final demise at the end of the Permian. While there is no convincing evidence for an impact at the end of the Permian, the end-Permian extinction did coincide with the eruption of the Siberian traps, one of the greatest volcanic outpourings in Earths history. The climatic effects of the volcanism from acid rain to global warming are considered likely factors in the extinctions. Other scientists have proposed that the release of methane from seafloor sediments was also involved (see Geotimes, November 2004).
The timing of the end-Permian extinction, however, is also open to some question. Virtually all the detailed data on the marine extinction come from southern China, and problems of stratigraphic resolution hinder close comparison to other areas, in particular to terrestrial deposits. Recent claims of a sudden and massive terrestrial extinction of reptiles are also based on a single area (in South Africa) and are difficult to evaluate elsewhere. Thus, we really only know what might have happened on land at the end of the Permian in South Africa, but not elsewhere.
End of the conodonts
The end-Triassic extinction about 200 million years ago has long been portrayed as a land- and sea-based event of comparable magnitude to the end-Cretaceous extinction. At this time, ammonites nearly became extinct, and conodonts met their final demise. Also, dinosaurs rose to dominate the land at the expense of the Triassic archosaurs and amphibians.
There are several documented Late Triassic impact structures, the most famous of which is the 214-million-year-old Manacouagan Crater in Quebec, Canada (which predates the Triassic-Jurassic boundary by nearly 15 million years). But again, no widely accepted evidence for an impact has been demonstrated at the end of the Triassic.
At the end of the Triassic, the Pangean supercontinent was fragmenting across the Atlantic Ocean region. This produced a huge outpouring of lavas called the Central Atlantic Magmatic Province (or CAMP), with basalt flows and related intrusions now found in such far-flung (but then nearly contiguous) areas as New Jersey, Brazil and Morocco. This extensive volcanic field, comparable in size to the Siberian traps, may have played a role in the extinctions, although the precise mechanism has yet to be elucidated. Similar to explanations for the end-Permian, scientists have suggested that here again, the release of methane from the ocean floor was involved.
Accelerated biotic turnover during the Late Triassic has long been interpreted as a single, end-Triassic mass extinction event. In collaboration with Larry Tanner, my own analysis of the fossil record indicates that the groups usually claimed to have suffered a catastrophic extinction at the end of the Triassic, including ammonites, bivalves, conodonts and some archosaurs, experienced multiple or prolonged extinctions throughout the Late Triassic, and that other groups were relatively unaffected or subject to only regional effects.
Instead of a single mass extinction at the end of the Triassic, the Late Triassic was likely an interval of elevated extinction rates (a prolonged biotic crisis), encompassing several distinct extinction events during the last 15 million years of the Triassic. The focus on a single, cataclysm-caused extinction at the Triassic-Jurassic boundary may be diverting attention away from searching for the causes of the prolonged Late Triassic extinctions.
End of the dinosaurs
The end-Cretaceous extinction, about 65 million years ago, stands as the most
intensively studied extinction and the only one that coincides with a major
impact event. The so-called smoking gun is the huge Chicxulub crater (estimated
diameter about 300 kilometers) off the northern coast of Mexicos Yucatán
Peninsula. Chicxulub has been assigned a terminal Cretaceous age for about a
decade, but recent (and widely debated) work by Gerta Keller and colleagues
has called into question whether or not it predates the end-Cretaceous by some
300,000 years. Also still unresolved is the role that the Deccan traps
eruptions in India, comparable in volume to the Siberian traps and CAMP, played
in the end-Cretaceous extinctions.
The poster children of mass extinctions, the dinosaurs, disappeared at the end of the Cretaceous. Yet almost all we know of dinosaur extinction comes from a small area in eastern Montana, and those who study the extinction readily extrapolate this record to represent the global story of dinosaur extinction. Maybe this microcosm really does reflect the macrocosm. But it would be extremely useful to develop a database on dinosaur extinction elsewhere that is as detailed as the Montana record, especially in places like Argentina or southern China, where these data are likely to be present.
Current debate on the dinosaurs extinction focuses on whether diversity was collapsing long before the end-Cretaceous impact. The most recent article on this, by David Fastovsky and his collaborators, reignites the debate over whether dinosaur diversity was collapsing prior to the impact (they were on their way out already) or was increasing to a sudden and drastic collapse driven by the impact. Better stratigraphic resolution holds the promise of solving this debate.
End in sight?
In 1991, David Raup, one of the foremost scholars of mass extinctions, posed the question, Could all extinction be caused by meteorite impact? The answer to this question in 2005 is yes, it could, but no, it wasnt. After 25 years of feverish study of the biggest extinctions in the history of life, only one mass extinction coincides with a documented impact the end-Cretaceous extinction, as originally envisioned by the Alvarez team. Extraterrestrial impacts as a general cause of mass extinction (or, if you like, a general theory of mass extinction based on extraterrestrial impacts) can be laid to rest.
However, if we simply shovel dirt on Laplace and those who advocated impacts as causes of global catastrophes, we miss the tremendous benefits the earth sciences have received from the quest to link mass extinctions to impacts. The geological signatures of impacts are now well-understood and documented. Knowledge of the history of impacts on this planet has grown dramatically. Paleontologists, stratigraphers, geochemists and geophysicists have all been challenged to look at their data in new ways and to improve the stratigraphic resolution of the fossil record relevant to the extinctions.
Furthermore, we always knew that all of the extinctions were selective, but now we have learned that the failure of a mass extinction to eliminate some animals (for example, crocodiles did not go out with the dinosaurs) tells us something about the magnitude and nature of the environmental changes that took place during the mass extinction. The next 25 years will hopefully bring us the temporal resolution we need to take what the last 25 years have taught us and apply it, to reach a full understanding of the history of mass extinctions on this planet.