Click here to pick up where the print version left off.

Mass extinctions

Late Devonian. Two new studies suggest that the end-Devonian extinction event, said to have occurred around 365 million years ago, may not have been so big after all. At the annual meeting of the Geological Society of America in Boston in November 2001, Richard Bambach and Andrew Knoll of Harvard University argued that the observed decline in diversity of marine life in the late Devonian was due more to a decline in origination of new forms than to an increase in the extinction of old ones. At the same meeting, Johnny Waters of West Georgia State University and colleagues presented new data on fossil echinoderms outside of North America and Europe. These new data suggest the echinoderms did not in fact decline in diversity at the boundary as much as had previously been thought.

Permian-Triassic. A study of rocks in eastern Greenland (Geology, v. 29, n. 4, p. 351) suggests that the P-T mass extinction occurred roughly simultaneously on land and in the sea. Richard Twitchett and colleagues examined rocks that contain not only well-preserved marine fossils but also microfossils from terrestrial plants, for the first time making it possible to look at events in both environments simultaneously. They conclude that collapse of marine and terrestrial ecosystems began at the same time and took between 10,000 and 60,000 years.

Triassic-Jurassic. Two recent studies indicate that the T-J boundary was more abrupt than previously thought, and support an at least partially extraterrestrial cause. Peter Ward of the University of Washington and colleagues examined rocks in British Columbia and concluded that the event lasted less than 10,000 years. The report suggests that a sudden decline in marine productivity, perhaps associated with volcanism, may have been a culprit. (Science, v. 292, p. 1148). More recently, Paul Olsen and colleagues (Science v. 296, p. 1305) report discovery of a significant iridium anomaly at the T-J boundary in Pennsylvania, providing strong support for an extraterrestrial impact, perhaps combined with volcanism, as a cause for the extinction.

Cretaceous-Tertiary. An analysis of helium from clay at the K-T boundary points to the impact of a single asteroid or comet, rather than a more protracted event of volcanism or multiple impacts. The study (Science, v. 291, p. 1952) found a near constant level of helium-3, usually associated with extraterrestrial impacts, in the boundary clay at three sites, suggesting a single impact and a length of time for the formation of the layer of no more than about 10,000 years.

Late Pliocene. Although it is not one of the "big 5" mass extinctions, the turnover of mollusks, corals and other taxa in the Plio-Pleistocene was a major regional event in the western Atlantic. The event has been ascribed variously to changes in temperature and to the nutrient content of the sea water. At the meeting of the American Astronomical Society in January 2002 and in a published report (Physical Review Letters, v. 88: 081101) a new hypothesis suggests that an extraterrestrial event may have been at least partly responsible for the change. Narciso Benitez, an astronomer at Johns Hopkins University, and colleagues suggest that a star may have exploded relatively nearby and showered Earth with cosmic rays, destroying the protective ozone layer and exposing the Earth to harmful ultraviolet radiation. The resulting increase in ultraviolet radiation destroyed much of the oceanic plankton, contributing to the extinction of many marine animals dependent on plankton for food.

Cambrian-Precambrian

Several studies highlighted the role of extraterrestrial events in the origin and early evolution of life.

The Earth and Moon underwent intense bombardment from asteroids and other bits of interplanetary debris between 4.0 and 3.9 billion years ago, according to a new analysis of lunar meteorites. The new study, by B. Cohen of the University of Tennessee and colleagues at the University of Arizona, and reported (Science, v. 290: p. 5497, 2000) suggests that during this interval, the Earth and Moon underwent bombardment up to 100 times as intense as anything before or after. Because this bombardment is very close to the time that most authorities believe life originated on Earth, this "lunar cataclysm hypothesis" raises the possibility that the impacts may have nudged life along, perhaps by contributing chemical precursors.

Life on Earth probably did not originate in space, but scientists working on the origin of life take seriously the idea that some of the basic chemical building blocks of life - organic macromolecules - may have come from extraterrestrial sources such as comets or meteorites. Once present on the early Earth, these molecules may then have come together in the earliest cells close to 4 billion years ago. A new study bolsters this idea still further with the report of the first sugar-related compounds in meteorites. George Cooper of NASA and colleagues report in the Dec. 27 issue of Nature (v. 414, p. 879) that the much-studied Murchison and Murray meteorites may host several kinds of sugar molecules. Their study is different than earlier studies because they used a technique that gave them greater confidence that the molecules are not terrestrial contaminants.

The oldest known fossils are more than 3.5 billion years old, and they are undoubtedly from organisms that lived in the sea. When life first made its way onto land has been less clear, but it may have been not too much later according to a new study. Yumiko Watanabe of Pennsylvania State University and colleagues report in the Nov. 30, 2000, issue of Nature (v. 408, p. 574) on their examination of 2.6 to 2.7 billion-year-old rocks from South Africa, in which they found evidence of mats of bacteria that developed on the soil surface. In addition to extending the record of land life by more than a billion years, the discovery, the authors suggest, may imply that Earth had a protective ozone shield earlier than previously thought. Such a shield would probably have been necessary for life to live outside of water, which partially guards from the destructive effects of ultraviolet radiation.

It has been known for several decades that eukaryotic cells were present in rocks as old as 1.4 billion years. It is now increasingly clear, based on molecular and geochemical evidence, that eukaryotes originated much earlier in Earth history, almost certainly by the late Archean (2.5 to 2.7 billion years ago). An analysis of 1.5 billion-year-old fossils from northern Australia pushes back the record of structurally complex fossil eukaryotic cells, and further supports the idea that they were already diversifying or ready to diversify at this earlier time. Emmanuelle Javaux of Harvard University and colleagues studied microscopic fossils from the Mesoproterozoic Roper Group and found fossils called acritarchs, which are believed to be cysts of single-celled eukaryotic algae. The fossils show a variety of spiky extensions that, according to the researchers, suggest sophisticated cellular processes of growth and remodeling, that do not go on in prokaryotic cells. The study appeared in Nature (v. 412, p. 66).

In 1998, Indian and German paleontologists described marks on rocks found in India as the oldest known burrows made by multicellular animals and stated that the specimens were 1.1 billion years old. The date was criticized by another Indian paleontologist, who said he had found 540-million-year-old fossils in rocks immediately overlying the supposed burrows. The Geological Society of India asked a team to investigate the controversy. Their report concluded that they could not confirm the younger date. A new study suggests that the "burrows" are even older than the original report claimed. Writing in Geology (v. 30, n. 2, p. 131, 2002), a team of researchers from the University of Ottawa and Massachusetts Institute of Technology report new radiometric dating on rocks associated with the controversial marks. They conclude that the supposed burrows are around 1.6 billion years old - more than twice as old as the oldest known animal trace fossils. Could marks this old really have been made by animals? One of the authors of the original study now has his doubts: Adolf Seilacher is quoted as saying that the old age "makes it unlikely that these are animal trace fossils" (Science, v. 295, p. 1209).

The famous fossils first known from Ediacara, in Australia, are widely acknowledged to be the earliest complex multicellular fossils, but not much more is agreed about them. They date from the latest part of the Precambrian, between 600 and 545 million years ago, and are found on several continents, but exactly what they are has remained controversial. Do they represent the ancestors of later animals, are they an independent "experiment" in multicellularity that left no descendants, or are they something else? A new study of very well-preserved Ediacara-type fossils from China and Russia by German researchers Michael Steiner and Joachim Reitner argues for the "something else" interpretation, specifically that at least some Ediacara-type forms were bacterial colonies. The researchers specifically suggest that the frond-shaped Ediacara forms known as Charniidae were not related to modern sea pens (Pennatulaceans), as has previously been suggested. Rather, they say, the Ediacara forms may have been some kind of primitive colonial form that became extinct with the coming of large animals in the Cambrian explosion around 543 million years ago. The new study appears in the December 2001 issue of Geology (v. 29, n. 12, p. 1119).

Phanerozoic diversity

It is one of the most reproduced images in modern paleontology. But is it true? The "Sepkoski curve": since its initial publication 20 years ago it has proven useful and reliable, but a new study calls this icon into question. The Paleobiology Database at the NCEAS (National Center for Ecological Analysis and Synthesis) at the University of California, Santa Barbara, is an assemblage of studies that take sampling into account. A preliminary analysis by NCEAS in the May 22 Proceedings of the National Academy of Sciences (v. 98, n. 11, p. 5955) suggests that one of the central features of the Sepkoski curve may not hold when sampling is taken into account: total diversity may not have increased since the Paleozoic. Two other independent studies give similar suggestions. Peters and Foote (Paleobiology, v. 27, n. 4, p. 583) and Powell and Kowalewski (Geology, v. 30, n. 4, p. 331, 2002).

One of the other marked features of the Sepkoski curve is the dramatic increase in diversity it shows for the Ordovician. This event, which paleontologists call the "Ordovician Radiation," took place between around 470 and 430 million years ago. It saw the decline of trilobites and the rise of articulated brachiopods, clams and snails. But the causes of this event have remained obscure. A new analysis by Arnold Miller and Sean Connolly of the University of Cincinnati may shed some light on this important episode. Earlier studies had argued that brachiopods took over from trilobites because trilobites suffered from higher rates of extinction. Miller and Connolly find, however, that brachiopods also show higher rates of origination of new forms. The authors do not find strong support in their analysis for previous suggestions that changing oceanic productivity might have affected evolution. Trilobite origination, they argue, declined in the more "crowded" seas of the Ordovician, while changes of rates in extinction remain unexplained. Miller and Connolly's studies are published in the fall 2001 and winter 2002 issues of the journal Paleobiology (v. 27, n. 4, p. 779-795, 2001; v. 28, n. 1, p. 26, 2002).

Other notable developments

The oldest known crustacean had been of late Cambrian age, but older specimens have now been found in early Cambrian rocks. Now a group of researchers led by David Siveter of the University of Leicester in England has found extraordinarily well-preserved fossils that are clearly members of the Crustacea from early Cambrian rocks, approximately 511 million years old, of Shropshire. The new find suggests that arthropods had already split into their major groups by early Cambrian time, perhaps in the Precambrian, prior to the Cambrian Explosion, when animals with hard skeletons diversified rapidly. The researchers report the find in the July 20 issue of Science (v. 293, p. 5529, 2001).

The oldest known crinoids have been found in rocks of early Ordovician age (about 500 million years old) in western Utah and southeastern Idaho. The new fossils point to some surprising conclusions about the early history of the group. All previously known crinoids are very different from any possible ancestor, so the origin and early evolution of the group had remained unclear. Now Thomas Guensburg and James Sprinkle report on four new types of crinoids. These types have primitive echinoderm characteristics but lack characteristics that would link them to other stalked crinoids with which they are usually grouped. Guensburg and Sprinkle conclude that crinoids originated separately from other stalked forms, sometime in the Cambrian, and that the initial diversification of crinoids slightly preceded the expansion of other groups in the Ordovician. The study appears in the February Geology (v.29, n. 2, p. 131, 2001).

Back to index

Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers © 2024 American Geological Institute. All rights reserved. Any copying, redistribution or retransmission of any of the contents of this service without the express written consent of the American Geological Institute is expressly prohibited. For all electronic copyright requests, visit: http://www.copyright.com/ccc/do/showConfigurator?WT.mc_id=PubLink