GEOL 104 Dinosaurs: A Natural History

Fall Semester 2006
The Cretaceous-Tertiary Extinction I: Definitions and Dramatis Personae

One of the most interesting aspects of dinosaur history is the extinction of the (nonavian) dinosaurs and the shift from the 4th to the 5th Amniote Radiation.

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).

Under phylogenetic taxonomy, 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 Tetanurae, and Avetheropoda, and Coelurosauria, and Maniraptoriformes, and Maniraptora, and Eumaniraptora, and Avialae…) are not extinct!

Extinctions happen throughout the fossil record. Extinctions are in part what makes the dinosaur faunas of the Lakota different from the Morrison (other factor is speciation).

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-T include:

(NOTE: Some popular and scientific books and articles show ichthyosaurs as victims of the K-T extinction. However, this clade was extinct TENS OF MILLIONS OF YEARS prior to the K-T boundary)
Marine turtles survived the event, although some groups of marine turtles died out during Cenozoic.

On land, victims include:

NOTE: Other groups of dinosaurs (e.g., other sauropod clades, coelophysoids, spinosaurids, carnosaurs, etc.) were already LONG EXTINCT and were thus NOT victims of the K-T boundary.

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

Many hypotheses proposed for the K-T Extinction. In evaluating the hypotheses, must consider:

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

Some older proposed causes for the K-T 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 hypotheses…

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    Last modified: 14 July 2006