PLEASE NOTE:


*

JACK SEPKOSKI (1948-1999): SCHOLAR WHO TURNED THE STUDY OF MASS
EXTINCTION INTO A HARD SCIENCE

From Steven N. Koppes <s-koppes@uchicago.edu>

Benny,

I suspect that many members of the CCNet list knew Jack Sepkoski or
his extraordinary work. It was my sad duty today to issue this news
release regarding this death.

Steve


May 4, 1999
For immediate release

Contact: Steve Koppes
(773) 702-8366
s-koppes@uchicago.edu

Paleontologist J. John Sepkoski Jr., 1948-1999

University of Chicago paleontologist J. John Sepkoski Jr.,
whose work has had innovative and far-reaching impact on the
scientific understanding of the fossil record and the diversification
of animal life throughout much of Earth's history, died Saturday, May
1, of sudden heart failure related to high blood pressure. He was
50.
"Paleobiology is a small profession, so when we lose one of
our very greatest, it's really a tremedously painful experience,"
said Harvard University paleontologist Stephen Jay Gould, with whom
Sepkoski studied as a graduate student. "Jack was one of the leading
lights of the profession."
One of Sepkoski's major contributions was quantifying the
nature of life's diversity through time, said Douglas Erwin, a
research paleontologist at the Smithsonian Institution in Washington,
D.C., and editor of the journal Paleobiology.  "He exhaustively
documented the ups and downs of life through the last 600 million
years. By collecting the data and developing a series of statistical
methods to study it, he gave us a new way of understanding the
history of life in the oceans.."
During the 1980s and 1990s, there were many meetings of
paleontologists during which literally every speaker used a figure
from Sepkoski's work, Erwin said. "His contributions were fundamental
to everything that people were doing," he said.
Sepkoski's death was a huge loss for paleontology, said
University of Chicago paleontologist David Jablonski. "Jack was
extremely generous with his ideas and for that matter with the huge
database that he spent decades compiling. His combined approach of
mathematical modelling, paleoecology and massive data arrays really
changed the way we do paleontology," Jablonski said.
His work is discussed in the book Mystery of Mysteries: Is
Evolution a Social Construction? by Michael Ruse, published this year
by Harvard University Press. "One of the first chapters is on Charles
Darwin and one of the last chapters is on Jack," Jablonski said.
During the 1980s, using rigorous statistical analyses of the
fossil record, Sepkoski and his Chicago colleague David Raup put
forth the controversial theory that catastrophic extinctions of
marine animals may have occurred approximately every 26 million years
during the past 250 million years of Earth's history. These periodic
events also included the extinction of the dinosaurs 65 million years
ago. The extinctions previously were thought to have been random
events.
The theory helped open the possibility that mass extinctions
both on land and in the oceans were caused by some force external to
Earth, such as catastrophic comet and asteroid impacts of the type
that inspired the films "Deep Impact" and "Armageddon" and inspired
the popular Shriekback song "Nemesis." These findings prompted a
major interdisciplinry research effort on extinction events in the
fossil record.
The exact cause of periodic extinctions remains a mystery.
"The theory has been under attack, but I don't hink anyone's been
able to disprove it," said Sepkoski's wife, Christine M. Janis, a
Brown University paleontologist.
Sepkoski's taste in music was as unconventional as some of
his scientific theories. His favorite musical group was the punk rock
Sex Pistols. "He thought that the Velvet Underground album was the
best album of all time," Janis said.
Sepkoski was born July 26, 1948, in Presque Isle, Maine. "He
started collecting dinosaur bones and fossils in New Jersey when he
was 10 and had wanted to become a paleontologist since that time,"
said his former wife, Maureen Meter.
He earned his B.S. degree, magna cum laude, from the
University of Notre Dame in 1970 and his Ph.D. in geological sciences
from Harvard University in 1977. His Ph.D. research was on the field
geology and paleontology of South Dakota's Black Hills.
He taught at the University of Rochester from 1974 to 1978
first as an instructor, then as an assistant professor. He was
appointed Assistant Professor in the Department of Geophysical
Sciences at the University of Chicago in 1978. Sepkoski attained the
rank of Associate Professor in 1982 and Professor in 1986. He also
had been a research associate at the Field Museum of Natural History
in Chicago since 1980.
Sepkoski held visiting professorships at the California
Institute of Technology in 1986 and at Harvard Uniersity in 1990 and
1991. In 1988, Sepkoski visited the University of California at Los
Angeles as a senior fellow and lectured at the Polish Academy of
Sciences, where he was elected a foreign member.
The Paleontological Society bestowed its Charles Schuchert
Award upon Sepkoski in 1983. He served a term as the society's
president from 1995 to 1996 and founded the Paleontological Society
International Research Program, or PalSIRP, the society's program for
assisting paleontologists in the countires of the former Soviet Union
through small competitive grants.
He was co-editor of the journal Paleobiology, regarded as the
major journal of his field, from 1983 to 1986 and a member of its
editorial board from 1987 to 1989.
"Jack was admired by his family, friends and colleagues not
only for the brilliance of his research, but also for his untiring
devotion to scholarship and teaching," said Mike Foote, a University
of Chicago paleontologist. He taught popular, large-enrollment
courses in the University's Core curriculum, advanced undergradaute
courses in paleontology, and demanding, advanced graduate courses. He
took great satisfaction in mentoring and advising students and
colleagues, as well, Foote said.
When Sepkoski led paleontological field trips for
undergraduate and graduate students over the years, his son, David,
was a frequent companion. David Sepkoski went on to receive an M.A.
degree from the University of Chicago and is a doctoral student in
the history of science at the University of Minnesota.
He is survived by his wife, Christine M. Janis of Providence,
R.I.; his son, David Sepkoski of Minneapolis, Minn.; his father,
Joseph J. Sepkoski of Sparta, N.J.; two sisters, Carol Sepkoski of
Cambridge, Mass., and Diane Karl of Cedar Brook, N.J.; and his former
wife, Maureen Meter of Chicago. Sepkoski also had a dog, Ronnie.
Arrangements for a memorial service are pending. In lieu of
flowers, donations may be made to PalSIRP, c/o Dr. Thomas W. Kammer,
Treasurer, Paleontological Society, Dept. of Geology and Geography,
West Virginia University, P.O. Box 6300, Morgantown, WV 26506-6300.

####
Radio stations: The University of Chicago has an ISDN line. Please
call for information.
For more news from the University of Chicago, visit our Web site at
http://www-news.uchicago.edu.
sk/99-69
Steve Koppes
University of Chicago News Office
5801 South Ellis Ave. Room 200
Chicago, IL 60637-1473
773-702-8366
773-702-8324 (fax)

=======================
PAPERS BY JACK SEPKOSKI (1948-1999): SCHOLAR WHO TURNED THE STUDY OF
MASS EXTINCTION INTO A HARD SCIENCE


M. Foote & J.J. Sepkoski: Absolute measures of the completeness of the
fossil record. NATURE, 1999, Vol.398, No.6726, pp.415-417

Measuring the completeness of the fossil record is essential to
understanding evolution over long timescales, particularly when
comparing evolutionary patterns among biological groups with different
preservational properties. Completeness measures have been presented
for various groups based on gaps in the stratigraphic ranges of fossil
taxa and on hypothetical lineages implied by estimated evolutionary
trees. Here we present and compare quantitative, widely applicable
absolute measures of completeness at two taxonomic levels for a broader
sample of higher taxa of marine animals than has previously been
available. We provide an estimate of the probability of genus
preservation per stratigraphic interval, and determine the proportion
of living families with some fossil records. The two completeness
measures use very different data and calculations. The probability of
genus preservation depends almost entirely on the Palaeozoic and
Mesozoic records, whereas the proportion of living: families with a
fossil record is influenced largely by Cenozoic data. These
measurements are nonetheless highly correlated, with outliers quite
explicable, and we find that completeness is rather high for many
animal groups. Copyright 1999, Institute for Scientific Information
Inc.


M. Foote, J.P. Hunter, C.M. Janis, J.J. Sepkoski: Evolutionary and
preservational constraints on origins of biologic groups:
Divergence times of eutherian mammals. SCIENCE, 1999, Vol.283,
No.5406, pp.1310-1314

Some molecular clack estimates of divergence times of taxonomic
groups undergoing evolutionary radiation are much older than the
groups' first observed fossil record. Mathematical models of branching
evolution are used to estimate the maximal rate of fossil preservation
consistent with a postulated missing history, given the sum of species
durations implied by early origins under a range of species origination
and extinction rates. The plausibility of postulated divergence times
depends on origination, extinction, and preservation Fates estimated
from the fossil record. For eutherian mammals, this approach suggests
that it is unlikely that many modern orders arose much earlier than
their oldest fossil records. Copyright 1999, Institute for Scientific
Information Inc.


F.K. McKinney, S. Lidgard, J.J. Sepkoski, P.D. Taylor: Decoupled
temporal patterns of evolution and ecology in two
post-Paleozoic clades. SCIENCE, 1998, Vol.281, No.5378,
pp.807-809

Counts of taxonomic diversity are the prevailing standards for
documenting Large-scall patterns of evolution in the fossil record.
However, the secular pattern of relative ecological importance between
the bryozoan clades Cyclostomata and Cheilostomata is not reflected
fully in compilations of generic diversity or within-fauna species
richness, and the delayed ecological recovery of the Cheilostomata
after the mass extinction at the Cretaceous-Tertiary boundary is missed
entirely. These observations demonstrate that evolutionary success and
ecological dominance can be decoupled and profoundly different, even
over tons of millions of years. Copyright 1999, Institute for
Scientific Information Inc.


J.J. Sepkoski & A.I. Miller: Analysing diversification through time.
TRENDS IN ECOLOGY & EVOLUTION, 1998, Vol.13, No.4, pp.158-159


J.J. Sepkoski: Rates of speciation in the fossil record. PHILOSOPHICAL
TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES B-BIOLOGICAL
SCIENCES, 1998, Vol.353, No.1366, pp.315-326

Data from palaeontology and biodiversity suggest that the global biota
should produce an average of three new species per year. However, the
fossil record shows large variation around this mean. Rates of
origination have declined through the Phanerozoic. This appears to have
been largely a function of sorting among higher taxa (especially
classes), which exhibit characteristic rates of speciation (and
extinction) that differ among them by nearly an order of magnitude.
Secular decline of origination rates is hardly constant, however; many
positive deviations reflect accelerated speciation during rebounds from
mass extinctions. There has also been general decline in rates of
speciation within major taxa through their histories, although rates
have tended to remain higher among members in tropical regions.
Finally, pulses of speciation appear sometimes to be associated with
climate change, although moderate oscillations of climate do not
necessarily promote speciation despite forcing changes in species'
geographical ranges. Copyright 1999, Institute for Scientific
Information Inc.


J.J. Sepkoski: Biodiversity: Past, present, and future
JOURNAL OF PALEONTOLOGY, 1997, Vol.71, No.4, pp.533-539

D. Jablonski & J.J. Sepkoski: Paleobiology, community ecology, and
scales of ecological pattern. ECOLOGY, 1996, Vol.77, No.5,
pp.1367-1378

The fossil record provides a wealth of data on the role of
regional processes and historical events in shaping biological
communities over a variety of time scales. The Quaternary record with
its evidence of repeated climatic change shows that both terrestrial
and marine species shifted independently rather than as cohesive
assemblages over scales of thousands of years. Larger scale patterns
also show a strong individualistic component to taxon dynamics;
assemblage stability, when it occurs, is difficult to separate from
shared responses to low rates of environmental change. Nevertheless,
the fossil record does suggest that some biotic interactions influence
large-scale ecological and evolutionary patterns, albeit in more
diffuse and protracted fashions than those generally studied by
community ecologists. These include: (1) the resistence by incumbents
to the establishment of new or invading taxa, with episodes of
explosive diversification often appearing contingent on the removal of
incumbents at extinction events; (2) steady states of within-habitat
and global diversity at longer time scales (10(7)-10(8) yr), despite
enormous turnover of taxa; and (3) morphological and biogeographic
responses to increased intensities of predation and substratum
disturbance over similarly long time scales. The behavior of species
and communities over the array of temporal and spatial scales in the
fossil record takes on additional significance for framing conservation
strategies, and for understanding recovery of species, lineages, and
communities from environmental changes. Copyright 1999, Institute for
Scientific Information Inc.


J.J. Sepkoski: Paleontologists lend a helping hand. GEOTIMES, 1996,
Vol.41, No.10, pp.9-10

J.J. SEPKOSKI: EXTINCTION AND THE FOSSIL RECORD. GEOTIMES, 1994,
Vol.39, No.3, pp.15-17

J.J. SEPKOSKI & D.C. KENDRICK: NUMERICAL EXPERIMENTS WITH MODEL
MONOPHYLETIC AND PARAPHYLETIC TAXA. PALEOBIOLOGY, 1993, Vol.19,
No.2, pp.168-184

The problem of how accurately paraphyletic taxa versus monophyletic
(i.e., holophyletic) groups (clades) capture underlying species
patterns of diversity and extinction is explored with Monte Carlo
simulations. Phylogenies are modeled as stochastic trees. Paraphyletic
taxa are defined in an arbitrary manner by randomly choosing
progenitors and clustering all descendants not belonging to other taxa.
These taxa are then examined to determine which are clades, arid the
remaining paraphyletic groups are dissected to discover monophyletic
subgroups. Comparisons of diversity patterns and extinction rates
between modeled taxa and lineages indicate that paraphyletic groups can
adequately capture lineage information under a variety of conditions of
diversification and mass extinction. This suggests that these groups
constitute more than mere 'taxonomic noise' in this context. But,
strictly monophyletic groups perform somewhat better, especially with
regard to mass extinctions. However, when low levels of paleontologic
sampling are simulated, the veracity of clades deteriorates, especially
with respect to diversity, and modeled paraphyletic taxa often capture
more information about underlying lineages. Thus, for studies of
diversity and taxic evolution in the fossil record, traditional
paleontologic genera and families need not be rejected in favor of
cladistically-defined taxa. Copyright 1999, Institute for Scientific
Information Inc.


C.C. LABANDEIRA & J.J. SEPKOSKI: INSECT DIVERSITY IN THE FOSSIL RECORD
SCIENCE, 1993, Vol.261, No.5119, pp.310-315

Insects possess a surprisingly extensive fossil record. Compilation of
the geochronologic ranges of insect families demonstrates that their
diversity exceeds that of preserved vertebrate tetrapods through 91
percent of their evolutionary history. The great diversity of insects
was achieved not by high origination rates but rather by low extinction
rates comparable to the low rates of slowly evolving marine
invertebrate groups. The great radiation of modern insects began 245
million years ago and was not accelerated by the expansion of
angiosperms during the Cretaceous period. The basic trophic machinery
of insects was in place nearly 100 million years before angiosperms
appeared in the fossil record. Copyright 1999, Institute for Scientific
Information Inc.


J.J. SEPKOSKI: 10 YEARS IN THE LIBRARY - NEW DATA CONFIRM
PALEONTOLOGICAL PATTERNS. PALEOBIOLOGY, 1993, Vol.19, No.1,
pp.43-51

A comparison is made between compilations of times of origination and
extinction of fossil marine animal families published in 1982 and 1992.
As a result of ten years of library research, half of the information
in the compendia has changed: families have been added and deleted,
low-resolution stratigraphic data have been improved, and intervals of
origination and extinction have been altered. Despite these changes,
apparent macroevolutionary patterns for the entire marine fauna have
remained constant. Diversity curves compiled from the two data bases
are very similar, with a goodness-of-fit of 99%; the principal
difference is that the 1992 curve averages 13% higher than the older
curve. Both numbers and percentages of origination and extinction also
match well, with fits ranging from 83% to 95%. All major events of
radiation and extinction are identical. Therefore, errors in large
paleontological data bases and arbitrariness of included taxa are not
necessarily impediments to the analysis of pattern in the fossil
record, so long as the data are sufficiently numerous. Copyright 1999,
Institute for Scientific Information Inc.


J.J. Sepkoski, 1992. A compendium of fossil marine animal families, 2nd
edition. Milwaukee Public Museum Contributions in Biology and
Geology, No. 83. 156 pp.

J.J. Sepkoski, 1992. Phylogenetic and ecologic patterns in the
Phanerozoic history of marine biodiversity. Pp. 77-100. In: N.
Eldredge (ed.) Systematics, Ecology, and the Biodiversity
Crisis. Columbia University Press; New York.


J.J. SEPKOSKI: A MODEL OF ONSHORE-OFFSHORE CHANGE IN FAUNAL DIVERSITY
PALEOBIOLOGY, 1991, Vol.17, No.1, pp.58-77

Onshore-offshore patterns of faunal change occurred at many taxonomic
scales during the Paleozoic Era, ranging from replacement of the
Cambrian evolutionary fauna by the Paleozoic fauna to the environmental
expansion of many orders and classes. A simple mathematical model is
constructed to investigate such change. The environmental gradient
across the marine shelf-slope is treated as a linear array of discrete
habitats, each of which holds a set number of species, as observed in
the fossil record. During any interval of time, some portion of the
species in each habitat becomes extinct by background processes, with
rates of extinction varying among both clades and habitats, as also
observed in the record. After extinction, species are replaced from
within the habitat and from immediately adjacent habitats, with
proportions dependent on surviving species. This model leads to the
prediction that extinction-resistant clades will always diversify at
the expense of extinction-prone clades. But if extinction intensity is
highest in nearshore habitats, extinction-resistant clades will expand
preferentially in the onshore direction, build up diversity there, and
then diversify outward toward the offshore. Thus, onshore-offshore
patterns of diversification may be the expectation for faunal change
quite independently of whether or not clades originate onshore. When
the model is parameterized for Paleozoic trilobites and brachiopods,
numerical solutions exhibit both a pattern of faunal change and a time
span for diversification similar to that seen in the fossil record.
They also generate structure similar to that seen in global
diversification, including logistic patterns of growth, declining
origination but constant extinction within clades through time, and
declining overall extinction across clades through time. Copyright
1999, Institute for Scientific Information Inc.
J.J. Sepkoski, 1991. Diversity in the Phanerozoic oceans: a partisan
review. Pp. 210-236. In: E. Dudley, (ed.) The Unity of Evolutionary
Biology. Proceedings of the Fourth International Congress of Systematic
and Evolutionary Biology. Dioscorides Press; Portland, Oregon.


J.J. Sepkoski, 1990. The taxonomic structure of periodic extinction.
Pp. 33-44. In: V. Sharpton and P. Warda (eds.), Global
Catastrophes in Earth History. Geological Society of America
Special Paper 247.


J.J. SEPKOSKI: PERIODICITY IN EXTINCTION AND THE PROBLEM OF
CATASTROPHISM IN THE HISTORY OF LIFE. JOURNAL OF THE GEOLOGICAL
SOCIETY, 1989, Vol.146, No.Pt1, pp.7-19

A.I. MILLER & J.J. SEPKOSKI: MODELING BIVALVE DIVERSIFICATION - THE
EFFECT OF INTERACTION ON A MACROEVOLUTIONARY SYSTEM.
PALEOBIOLOGY, 1988, Vol.14, No.4, pp.364-369

J.J. SEPKOSKI: ALPHA, BETA, OR GAMMA - WHERE DOES ALL THE DIVERSITY GO.
PALEOBIOLOGY, 1988, Vol.14, No.3, pp.221-234

D.M. RAUP & J.J. SEPKOSKI: TESTING FOR PERIODICITY OF EXTINCTION
SCIENCE, 1988, Vol.241, No.4861, pp.94-96

J.J. SEPKOSKI: IS THE PERIODICITY OF EXTINCTIONS A TAXONOMIC ARTIFACT -
REPLY. NATURE, 1987, Vol.330, No.6145, pp.251-252

D.H. ERWIN, J.W. VALENTINE, J.J. SEPKOSKI: A COMPARATIVE-STUDY OF
DIVERSIFICATION EVENTS - THE EARLY PALEOZOIC VERSUS THE
MESOZOIC. EVOLUTION, 1987, Vol.41, No.6, pp.1177-1186

J.J. SEPKOSKI: ENVIRONMENTAL TRENDS IN EXTINCTION DURING THE PALEOZOIC.
SCIENCE, 1987, Vol.235, No.4784, pp.64-66

J.J. SEPKOSKI & D.M. RAUP: WAS THERE 26-MYR PERIODICITY OF EXTINCTIONS?
NATURE, 1986, Vol.321, No.6069, p.533

D.M. RAUP & J.J. SEPKOSKI: PERIODIC EXTINCTION OF FAMILIES AND GENERA.
SCIENCE, 1986, Vol.231, No.4740, pp.833-836

J.J. SEPKOSKI: A KINETIC-MODEL OF PHANEROZOIC TAXONOMIC DIVERSITY .3.
POST-PALEOZOIC FAMILIES AND MASS EXTINCTIONS
PALEOBIOLOGY, 1984, Vol.10, No.2, pp.246-267

B.D. WOODBURY & J.J. SEPKOSKI: DIVERSITY AND FAUNAL PATTERNS OF
ENDOBIONT AND EPIBIONT COMMUNITIES THROUGH THE PHANEROZOIC
AMERICAN ZOOLOGIST, 1984, Vol.24, No.3, p.A145

D..M. RAUP & J.J. SEPKOSKI: PUBLISHING CHRONOLOGY. NATURE, 1984,
Vol.309, No.5966, p.300

D. JABLONSKI, J.J. SEPKOSKI, D.J. BOTTJER, P.M. SHEEHAN: BIOLOGICAL
DIVERSITY. SCIENCE, 1984, Vol.224, No.4655, p.1294

J.J. SEPKOSKI, A.H. KNOLL: PRECAMBRIAN-CAMBRIAN BOUNDARY - THE SPIKE IS
DRIVEN AND THE MONOLITH CRUMBLES. PALEOBIOLOGY, 1983, Vol.9,
No.3, pp.199-206

D.M. RAUP, J.J. SEPKOSKI: PERIODICITY OF EXTINCTIONS IN THE GEOLOGIC
PAST. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE
UNITED STATES OF AMERICA-BIOLOGICAL SCIENCES, 1984, Vol.81,
No.3, pp.801-805

D. JABLONSKI, J.J. SEPKOSKI, D.J. BOTTJER, P.M. SHEEHAN:
ONSHORE-OFFSHORE PATTERNS IN THE EVOLUTION OF PHANEROZOIC SHELF
COMMUNITIES. SCIENCE, 1983, Vol.222, No.4628, pp.1123-1125

D.M. RAUP, J.J. SEPKOSKI, S.M. STIGLER: MASS EXTINCTIONS IN THE FOSSIL
RECORD. SCIENCE, 1983, Vol.219, No.4589, pp.1240-1241

J.J. SEPKOSKI: MAMMALIAN EVOLUTION AND THE GREAT AMERICAN INTERCHANGE
SCIENCE, 1982, Vol.215, No.4538, pp.1351-1357

D.M. RAUP & J.J. SEPKOSKI: MASS EXTINCTIONS IN THE MARINE FOSSIL RECORD
SCIENCE, 1982, Vol.215, No.4539, pp.1501-1503

R.E. CHAPMAN, P.M. GALTON, J.J. SEPKOSKI, W.P. WALL: A MORPHOMETRIC
STUDY OF THE CRANIUM OF THE PACHYCEPHALOSAURID DINOSAUR
STEGOCERAS. JOURNAL OF PALEONTOLOGY, 1981, Vol.55, No.3,
pp.608-618

J.J. SEPKOSKI, R.K. BAMBACH, D.M. RAUP, J.W. VALENTINE: PHANEROZOIC
MARINE DIVERSITY AND THE FOSSIL RECORD. NATURE, 1981, Vol.293,
No.5832, pp.435-437

J.J. SEPKOSKI: A FACTOR ANALYTIC DESCRIPTION OF THE PHANEROZOIC MARINE
FOSSIL RECORD. PALEOBIOLOGY, 1981, Vol.7, No.1, pp.36-53

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*

LETTERS TO THE MODERATOR, 5 May 1999
------------------------------------

(1) WHERE HAVE ALL THE CRATERS GONE?
    Andrew Glikson <andrew.glikson@anu.edu.au>

(2) ETA AQUARIDS & THE LYRIDS
    Mark Kidger <mrk@ll.iac.es>

(3) XF11 & AN10: LEGITIMATE SCARES
    Jens Kieffer-Olsen <JKO@dst.dk>


===========
(1) WHERE HAVE ALL THE CRATERS GONE?

From Andrew Glikson <andrew.glikson@anu.edu.au>

Dear Benny,

A couple of comments in response to the queries raised by Timo Niroma
(CCNet 5.5.99):

(1) For craters larger than 20 km-diameter - lunar mare crater counts,
terrestrial crater counts in long-lived stable cratons, and
astronomical observations are consistent with an impact flux of 4-6 *
10^-15 km^-2yr^-1 since the end of the Late Heavy Bombardment. Coupled
with the observed crater size vs cumulative crater size frequency
relationship of N µ Dc^-1.8 (N = cumulative number of craters of
diameter > Dc), these rates imply formation on Earth of more than 450
craters larger than 100 km-diameter and more than 50 craters larger
than Dc³ 300 km-diameter since ~3.8 b.y. ago.

(2)  Dominant crater removal mechanisms (erosion, burial, metamorphism,
subduction) fully explain the fact that less than 180 craters have been
identified to date.  In Australia about 1-2 new craters are discocvered
each year - mostly through geophysical surveys and drilling in sedimentary
basins.  As larger craters have a better chance of preservation, the
fact that only 4 craters larger than 150 km in diameter (Vredefort,
Sudbury, Morokweng, Chicxulub) were found to date - out of the
predicted number of about 100 or so - provides a measure of the degree
of crater preservation.

(3) The late Eugene Shoemaker suggested an increase in the rate of
cratering during the Phanerzoic.  From the 3He/4He flux we live in a
period of peak bombardment, although considerations regarding the
mobility of Helium in sediments may modify such a conclusion.  As
suggested by many, the tendency for medium to large impacts to occur in
clusters, typically at late stages of geological eras (late Ordovician,
late Devonian, late Triassic, Jurassic-Cretaceous boundary, late
Eocene), suggests significant correlations with major biological
extinctions, as in the classic case of the K-T boundary.

Andrew Glikson
andrew.glikson@anu.edu.au

==============
(2) ETA AQUARIDS & THE LYRIDS

From Mark Kidger <mrk@ll.iac.es>

Dear Benny:

Re: Meteors down under/NASA News

I should point out that, although the Eta Aquarids are a very active
shower (here in the Canary Islands we get to observe them very well),
NASA is being a bit misleading with its headline story. In fact, the
bright Moon which is approaching the Eta Aquarid radiant in the dawn
sky will make observing this shower practically impossible this year.

NASA also was a bit fast off the mark writing off the Lyrid shower as
"visually disappointing" given that the first IMO shower report,
supported by observers in the Canary Islands and, most recently, by new
Cuban data, show that the shower was a factor of 2-3 ABOVE its normal
level for several hours on the morning of maximum. Far from being
disappointing, it was probably the finest show from the Lyrids for a
number of years.

Mark

=============
(3) XF11 & AN10: LEGITIMATE SCARES

From Jens Kieffer-Olsen <JKO@dst.dk>

Re: RELATIVE RISKS & THE PROVISIONAL NATURE OF TRUTH IN SCIENCE
    From Michael Martin-Smith <martin@miff.demon.co.uk>

Indeed the long-term expectation among ordinary folks will be that
anything other than than a trivial impactor, say up to 10m, is
predicted or subject to an advance warning.

We know that this level of forewarning takes a long time to establish.
In the meantime I find it perfectly legitimate that scares such as XF11
and AN10 stimulate the universal desire to come to grips with the
threat from outer space.

Neither Brian Marsden nor Andrea Milani deserve any criticism for their
choice of method for communicating their observations to the public.
The only ones to blame are those scientists, who might keep mum about
similar discoveries still unknown to the rest us.

--
Jens Kieffer-Olsen

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