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:

• Classical biologist/Paleontologist: When last member of species or clade dies out.
• Geneticist: When that particular genome is no longer passed on.
• Ecologist: When species or clade range goes to zero.

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:

• Geologically rapid extinction of many distantly related taxa which are not immediately replaced in ecological space.

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:

• The boundaries between every period is a mass extinction of marine invertebrates (at least)
• The boundaries between the Paleozoic & Mesozoic and between the Mesozoic & Cenozoic Eras are very large mass extinctions The end of the Mesozoic is the boundary between the Mesozoic Era and Cenozoic Era, which is also the boundary between the Cretaceous Period (K) and the Tertiary Period (T):
  • So it is called the K-T boundary
  • NOT the largest of all mass extinctions!!
    • Paleozoic-Mesozoic (Permo-Triassic) extinction the largest in the last half-billion years
    • Two or three mass extinctions during Paleozoic are also larger than K-T
  • However, is most recent in time of the really big extinctions
  • Also, “liberated” the mammals: no large bodied dinosaurian competitors
  • Very well dated at 65.51 ± 0.3 Ma during magenetic chron C29R.

What died out?
Among the marine invertebrates:

• Coccolithophorids: chalk-forming nannoplankton algae
  • Had been astonishingly common in shallow Cretaceous seas: the reason the “Cretaceous” (Chalk) got its name!
  • Survived but at MUCH reduced numbers and reduced species diversity
  • Base of much of the food chain
• Ammonoids: shelled, coiled relatives of modern nautilus, squids, octopi, etc.
  • Very common and diverse during Mesozoic, lived in many marine habitats
  • Evolved very quickly: primary index fossils for Mesozoic
  • Once thought to be predators, but very likely planktonivores (and thus coccolithophorid-eaters)
  • Food for many marine reptiles
• Rudists: reef-forming clams
  • Out-competed modern-style corals as reef builders of Late Cretaceous
  • Planktonivores
• Inoceramids: gigantic scallop-relatives
  • Planktonivores

Among marine vertebrates, taxa that died out at K-T include:

• Mosasaurs
• Plesiosaurs (both short- and long-necked)

On land, victims include:

• The Asiamerican clade of marsupials (although one species may have survived into earliest Tertiary)
• Pterosaurs (only larger pterodactyloids known from Late K)
• Some kinds of dinosaurs:
  • Titanosaurs (last surviving sauropods)
  • Abelisaurs (last surviving ceratosaurs)
  • a few basal coelurosaurs (Dryptosaurus in eastern North America, for instance)
  • Tyrannosaurids
  • Ornithomimosaurs
  • Oviraptorosaurs
  • Therizinosauroids
  • Troodontids
  • Dromaeosaurids
  • Alvarezsaurids
  • various clades of toothed birds (that is, all birds but Aves itself)
  • Nodosaurids
  • Ankylosaurids
  • Hypsilophodontians (FINALLY something killed them!!)
  • Hadrosaurids
  • Pachycephalosaurs
  • basal neoceratopsians
  • Ceratopsids

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

• Insects (show no sign of change in diversity, unlike at Permo-Triassic)
• Amphibians
• Mammals:
  • Monotremes
  • Marsupials in Gondwana
  • Multituberculates (although extinct today, survived and flourished in early part of Tertiary)
  • Placentals (including hoofed mammals, primate relatives, etc.)
• Turtles
• Lepidosaurs
• Champsosaurs (a group of semi-aquatic archosauromorphs: died out about same time as multituberculates)
• Crocodilians
• Toothless birds (Aves)

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

• Does the proposed agent only affect dinosaurs, or does it affect the other known victims?
• Is it overkill? (i.e., is it so strong it should have killed EVERYTHING?)
• Is the cause terrestrial (from Earth) or cosmic (from space)?
• Is the cause biological or abiotic?
• Is the cause unique to the K-T or the same as other mass extinctions?
• Did the extinction occur instantaneously (over hours to weeks to months), gradually (over tens or hundreds of thousands or millions of years), or in between?
• Was it a single agent or multiple agents?
• Were the marine and terrestrial extinctions caused by the same or different causes?

• Is the hypothesis testable (i.e., falsifiable)?
• Would the agent leave a record independent of the extinction itself?

Some older proposed causes for the K-T event: