DINOSAUR EXTINCTION


Two radically different concepts have been proposed for the extinction of dinosaurs and many other organisms that did not survive into the Tertiary Period; 1) the impact of a bolide (asteroid, meteorite, or comet) with the earth and 2) a volcansim induced ecological collapse.
 


Mass Extinction:  Affected other organisms also.  Terminal Cretaceous extinctions at the Cretaceous-Tertiary (K/T) boundary.  Fifteen percent of families of marine shelled invertebrates: ammonoid cephalopods suffered total extinction, clams and snails suffered greatly, shell protozoans (Foraminifera) had heavy losses, mosasaurs and pleisosaurs (marine reptiles) suffered total extinction,  on land dinosaurs and pterosaurs suffered total extinction, many types of marsupial mammals were affected, and some types of plants were affected (ex.: cycad diversity decreased).

Evidence:  Patterns of extinctions versus Process of extinction.  Pattern: gradual or sudden (catastrophic).  Have to look at the latest part of Cretaceous, Maastrictian Stage, 71 to 65 million years ago.  (Paleocene dinosaur fossils probably transported.)

Hell Creek Formation, Montana:  Only place we have with continuous deposition of continental rocks across K/T boundary that contains dinosaur bones.  One set of researchers say dinosaur bones indicate a gradual decline in diversity during the Maastrictian rocks of the Hell Creek.  Others, basically using the same data, say relatively sudden disappearance of dinosaurs.  Of course much of the evidence depends on the naming of genera and species (controls how diverse the fauna is assumed to be), the bias of the rock record (are we seeing all the dinosaurs that really lived here at the time, the limited number of localities with continuous continental sedimentation across the boundary (maybe dinosaurs were more diverse in other parts of the world, where we don't have a record across the K/T boundary), etc.  However, general consensus here seems to favor a gradual extinction.

Marine Evidence:  The data of marine extinctions (particularly foraminifera) seem to indicate a more rapid extinction.  But even here there is some disagreement (see discussion on page 413 and 414 in Fastovsky and Weishampel, 1996).

Bolide Impact:  caused dust cloud in atmosphere and rapid global cooling.
The Smoking Gun: The Chicxulub Crater on the Yucatan peninsula.
1. 1977 Paper by Luis and Walter Alvarez - Gubio clay layer with high iridium content at K/T boundary.  Many other iridium rich clay layers found around the world at K/T.
2.  Shocked quartz grains also found at boundary.
3.  C-13/C-12 ratios from planktonic foraminifera in ocean sediments suggest that primary oceanic productivity (amount of organic matter synthesized by organisms from inorganic materials and sunlight) decreased by over 90% at K/T boundary.
4.  Studies indicate that the marine organisms that were most affected across the K/T boundary (i.e. suffered the most extinctions) are the ones that depend directly upon primary productivity  (phytoplankton) for their food source.
4.  Studies by  paleobotanists of the U.S.G.S  and the Canadian Geological Survey of pollen distribution across the K/T boundary in western North America appears to show extensive extinction across the K/T boundary coincident with the iridium anomaly.

SO WHY DID THE DINOSAURS GO EXTINCT AT THE K/T BOUNDARY? (AND DID THEY SUDDENLY GO EXTINCT OR WAS THE EXTINCTION GRADUAL?):  Verdict still out.  But many feel that the Bolide impact is the most compatible with all the data and the most parsimonious explanation.  However, many also favor a more gradual extinction due to climate change resulting from a volcanism; and of course, they say that this is more compatible with all the data.

The following quote from Fastovsky and Weishampel (1996) in their recent textbook, THE EVOLUTION AND EXTINCTION OF THE DINOSAURS (Cambridge University Press) illustrates the bolide impact view:  "Our  view is that the patterns of extinction are concordant with the asteroid impact hypothesis.  For us, therefore, the extinction of the dinosaurs remains best explained by the impact of an asteroid with the Earth. ......We opt for the theory that we think explains all of the data most satisfactorily." (p. 427).
 

SUMMARY STATEMENT

     The dinosaurs and many other plants and animals experienced extinction during the latest portion of the Cretaceous Period.  There are several hypotheses to explain this mass extinction event at the end of Cretaceous time.  However, only two theories are supported by accumulated factual data, the catastrophic bolide (meteorite, asteroid, or comet) impact theory and the gradualistic volcanism theory.  Both theories propose that dinosaurs (and many other organisms) became extinct because of a change in climate.  The impact theory proponents say that a large meteorite, asteroid, or comet (or perhaps several) hit the Earth, throwing up a large cloud of atmospheric dust which blocked the radiation coming from the Sun.  Of course, this would cause it to get colder (somewhat like a supposed nuclear winter scenario), plants on land and primary producers in the ocean would die, thus the food chain would be broken.  Proponents of the volcanism theory also believe the climate got colder during the later part of the Cretaceous (last few million years), but because of increased volcanism during Late Cretaceous time.  They propose that volcanoes blew huge amounts of volcanic ash into the atmosphere, which blocked solar radiation and thus caused the climate change and the gradual mass extinctions.  The gradualistic volcanism proponents do not think the volcanism occurred suddenly, but occurred over several million years during latest Cretaceous time and the climate gradually got colder.  However, some proponents of volcanism also propose a tremendous greenhouse warming right at the end of Cretaceous time due to the increased output of carbon dioxide from the Deccan Traps volcanism (in particular, Dewey McClean).
     A major line of evidence for both theories is several iridium rich (iridium is a rare element that is relatively abundant in meteorites, comets, and asteroids, but also in volcanic ash) clay layers that occur near the Cretaceous/Tertiary boundary.  Shocked quartz grains also have been found in the clay layers and both sides claim that the stress causing them supports their view (shocked quartz grains show stress cleavage and have been associated with meteorite impact sites, volcanic ash and debris at nuclear explosion sites).
     Although the evidence is still hotly debated, most paleontologists and geologists favor the gradualistic volcanism theory, whereas most physicists and astronomers favor the catastrophic bolide impact theory.  A recent book by Officer and Page (1996) discusses the controversy in great detail.  They favor the gradualistic volcanism theory.  The complete reference for this book is the following:  Officer, Charles and Jake Page; 1996; The Great Dinosaur Extinction Controversy;Addison-Wesley Publishing Company, Inc.; 209 p.