GEOL 104 Dinosaurs: A Natural History

Fall Semester 2014
The Cretaceous-Paleogene Extinction: All Good Things...

Definitions and Dramatis Personae
One of the most interesting aspects of dinosaur history is the extinction of the nonavian dinosaurs and the rise of mammals to dominance.

How did this occur?

First, some definitions:
Extinction: not recognized as a natural phenomenon until Cuvier. Different definitions (or at least different emphases) according to different types of scientists:

(All of these essentially mean the same thing: there are no more of that kind of organism).

Only two types of taxa can go extinct: species and clades. Old-fashioned gradistic paraphyletic groups could "go extinct" even though their descendents (and thus their genome) persisted on.

So, in this sense, Dinosauria (and Saurischia, and Theropoda, and Neotheropoda, and Averostra, and Tetanurae, and Avetheropoda, and Coelurosauria, and Tyrannoraptora, and Maniraptoriformes, and Maniraptora, and Metornithes, and Eumaniraptora, and Avialae, and Pygostylia, and Ornithothoraces, and Euornithes, and Ornithurae, and Carinatae) are not extinct!

Extinctions happen throughout the fossil record.

What interests us is Mass Extinction:

The effect of mass extinctions observed by William "Strata" Smith and others: the reason for dividing the Geologic Column into Eras and Periods is because of mass extinctions:

What died out?
Among the marine invertebrates:

Among marine vertebrates, taxa that died out at K/Pg include:

(NOTE: Some popular and scientific books and articles show ichthyosaurs as victims of the K/Pg extinction. However, this clade was extinct TENS OF MILLIONS OF YEARS prior to the K/Pg boundary)

Marine turtles survived the event, although some groups of marine turtles died out during Cenozoic. Also, one group of marine crocs (the dyrosaurids) survived the extinction event, only to die out early in the Cenozoic.

On land, victims include:

NOTE: Other groups of dinosaurs (e.g., other sauropod clades, stegosaurs, coelophysids, megalosauroids, carnosaurs, etc.) were already LONG EXTINCT and were thus NOT victims of the K/Pg event.

Important to remember that there were LOTS of survivors (otherwise, there would be no life today!!):
On land:

Looking for Causal Agents
Many hypotheses proposed for the K/Pg Extinction. In evaluating the hypotheses, must consider:


Most significantly: (Without those two aspects, the hypothesis is not scientific, but simply speculation)

Here are but some older proposed causes for the K/Pg event:

  • Racial Senescence
  • Poison Gas from Comets
  • Caterpillars ate all the food
  • Mammals ate the dinosaurs to death
  • Allergies to Angiosperms?
  • Diseases

    Will explore other hypothese...

    Modern Approaches to the Cretaceous-Paleogene Extinction
    The global nature of the K/Pg extinction would seem to favor some causal agent which could affect the whole planet. Cosmic (extraterrestrial) phenomena might be a good possibility.

    1971: Suggestion by Dale Russell (dino paleontologist) and Wallace Tucker (astrophysicist): a supernova killed the dinosaurs.

    Supernovae are exploding stars: put out TREMENDOUS amount of energy. If a star in a nearby solar system exploded, it would bombard surface of planet with radiation, bringing radiation sickness, cancer, etc.

    Modern analogue: during 1950s through 1970s, greatest fear about nuclear war was radioactive fallout.

    Predictions:

    Fits prediction. However, problem because it is an untestable (and thus non- falsifiable) hypothesis:

    So, remains as a potential but no reason should be supported. Was the leading candidate during the 1970s.

    The Chicxulub Impact
    1980: Walter Alvarez was investigating a layer of clay in Gubbio, Italy at the K/Pg boundary. Wanted to determine length of time represented by the clay layer. Consulted dad (Nobel winning physicist Luis Alvarez) for possible solution. Suggestion:

    The element used: iridium (a platinum-like metal, common in metallic asteroids but very rare in Earth's crust).

    When examined Gubbio clay, found a huge increase in iridium ( iridium spike) at base of clay: clearly not an "average" of infall.

    Hypothesized: an asteroid impacted Earth at the K/Pg boundary

    Modern analogue: fear of nuclear war during 1980s concerned with nuclear winter, the likely consequence to a large-scale nuclear war first proposed shortly after (and suggested by) the Alvarez scenario

    Predictions:

    Biotic prediction fits most of the predictions; search for geological signature was on.

    Shocked Quartz:

    Melt Glass (Tektites):

    Tsunami ("tidal wave") and ejecta deposits:

    Crater:

    So, great evidence for an impact at K/Pg independent of extinction. Also, pattern consistent with proposed effects (although some versions of the superacid rain, global fires, and global super tsunamis do not have good evidence and are probably "overkill" scenarios).

    Question, though: was the extinction just from impact?

    Media (and some professional scientists) act as if Chicxulub impact was only global change occurring at K/Pg boundary.

    However, equally good geological evidence for some other big changes:

    Deccan Traps Volcanism:
    Long known that a period of intense volcanism begins in later part of Cretaceous. In North America, associated with change in mountain building in Rockies (the beginnings of the Laramide Orogeny). But the biggest aspect of this volcanism is the Deccan Traps

    Some try to dismiss Deccan Traps as a side effect of Chicxulub crater, but begins a little too early (see paleomagnetic data above).

    So, Deccan Traps themsevles were a MAJOR event, and might have contributed greatly to the extinction event.

    But there were even earlier, longer term geologic changes:

    Maastrichtian Regression:

    Maastrichtian Regression clearly happens (latest Maastrichtian terrestrial rocks on top of earlier Maastrichtian shoreline rocks on top of earlier marine rocks).

    Predictions:

    All three events (Chicxulub impact, Deccan Traps volcanism, Maastrichtian Regression) are known to occur. Can we separate their effects in the geological record?

    And which, if any, seems to have an effect on dinosaur diversity?

    What does the dinosaur record show?
    Only a few spots on Earth have late Maastrichtian dinosaur record, and fewer still showing both the earlier Campanian (83.5-71.3 Ma) and complete Maastrichtian (71.3-65.5 Ma) record to see the change over the last several millions of years:

    Only good, continuous record from mid Campanian through earliest Paleogene is western North America.

    The Montana Group (late Late Cretaceous dinosaur-bearing rocks of western North America) spans the Campanian and Maastrichtian, and has earliest Tertiary rocks right above it. Similar groups of rock are found in other parts of western North America (in the Southwest and in Utah, for instance). What does the dinosaur record of the Montana Group show us?

    Throughout the Montana Group are the same basic groups of dinosaurs:

    Currently, changes in these are best seen in hadrosaurids and ceratopsids; definite changes in tyrannosaurids, ankylosaurids, and pachycephalosaurs; other taxa too poorly sorted out at species level to be certain.

    Pattern among big ornithischians: short-snouted forms (centrosaurines, short-snouted saurolophines, lambeosaurines) die out earlier, while long-snouted forms (chasmosaurines, long-snouted saurolophines) remain common until the K/Pg boundary. This might reflect changing abundance of some form of vegetation, but that is not definite.

    This pattern consistent with long-term (millions of year scale) change associated with Maastrichtian Regression (and possible vegetation change). However, no evidence that latest Maastrichtian dinosaurs were declining WITHIN latest Maastrichtian: might well have continued on to live in post-Maastrichtian if not for Deccan Traps &/or Chicxulub impact.

    So, what caused the dinosaurs to die out?

    Three equally valid answers:

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    Last modified: 8 July 2014