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).
- 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 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
- 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
- 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
- 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
- Food for many marine reptiles
- Rudists: reef-forming clams
- Out-competed modern-style corals as reef builders of Late Cretaceous
- Inoceramids: gigantic scallop-relatives
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
- Plesiosaurs (both short- and long-necked)
Marine turtles survived the event, although some groups of marine turtles died out during
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.
- The Asiamerican clade of marsupials (although one species may have survived into
- 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)
- various clades of toothed birds (that is, all birds but Aves itself)
- Hypsilophodontians (FINALLY something killed them!!)
- basal neoceratopsians
Important to remember that there were LOTS of survivors (otherwise, there would be
no life today!!):
- Insects (show no sign of change in diversity, unlike at Permo-Triassic)
- Marsupials in Gondwana
- Multituberculates (although extinct today, survived and flourished in early part of
- Placentals (including hoofed mammals, primate relatives, etc.)
- Champsosaurs (a group of semi-aquatic archosauromorphs: died out about same
time as multituberculates)
- 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
- 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?
(Without those two aspects, the hypothesis is not scientific, but simply speculation)
- 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:
Poison Gas from Comets
- Idea that clades, like individuals have a “lifespan” with a fast growing “youth”,
long stable “adulthood”, and period of decline and decay at end: racial senescence or
- “Evidence” for this was “nonfunctional” structures in dinosaurs (pachycephalosaur
skulls, short tyrannosaurid arms, spikes on centrosaurine frills, etc.), bizarrely coiled
- However, these structures likely have functional significance
- Also, no evidence of predetermined “lifespans” of clades
Caterpillars ate all the food
- Shortly after WWI (when poison gas was used on battlefield), astronomers identify
cyanogens gasses in comet tails
- Thought that if Earth passed through tail of comet, the globe might be “gassed”
- Looked at dinosaur skeletons, showed necks bent backwards as in deaths on battlefield
- However, such positions are common in rock record: due to drying and shrinking of
Mammals ate the dinosaurs to death
- Perhaps caterpillars diversified and ate all the plants before the dinosaurs could
- No evidence for this
- ?How would it affect coccolithophorids, ammonoids, etc.?
Allergies to Angiosperms?
- Mammals indeed may have eaten eggs of dinosaurs, but...
- Why didn't they eat eggs of toothless birds, crocs, turtles, lepidosaurs, etc.?
- How could it affect marine community?
- Also, mammals and dinosaurs coexisted since Late Triassic!
- How would this affect all dinosaurs except for toothless birds?
- Why only at latest K, tens of millions of years after rise of angiosperms?
- How could it affect marine community?
- Why would it only affect certain taxa?
- Why at same time on land and in sea?
Will explore other hypotheses…
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Last modified: 14 July 2006