GEOL 104 Dinosaurs: A Natural History
Fall Semester 2015
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).
- 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.
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:
- 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 Paleogene Period (Pg):
- 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/Pg boundary
- Formerly, the first Period of the Cenozoic was the "Tertiary" Period, so that this extinction was called the Cretaceous-Tertiary (or K/T)
- 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/Pg
- However, is most recent in time of the really big extinctions
- Also, "liberated" the mammals: no large bodied dinosaurian competitors
- Dated at 65.51 ± 0.3 Ma (although recalibration may bring this down to 66.0 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 foram-eaters)
- Their larvae were microscopic (0.5 mm diameter) and lived as plankton; this is different from their living cousins the nautiloids (which have larvae 15 mm or diameter and which hang out on the sea floor)
- Food for many marine reptiles
- Belemnoids: another group of squid relatives
- Hard part on inside of body (unlike ammonoids)
- A common food for marine reptiles
- Rudists: reef-forming clams
- Out-competed modern-style corals as reef builders of Late Cretaceous
- May have had symbiotic algae to help feed their tissues (like modern giant clams do)-
- Inoceramids: gigantic scallop-relatives
- May have had symbiotic algae to help feed their tissues (like modern giant clams do)-
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
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.
- Bennettitalians: a group of Mesozoic plants related to the ancestors of the flowering plants
- Various types of mammals
- Various types of land-dwelling crocs (including the herbivorous crocs)
- Pterosaurs (only larger pterodactyloids known from Late K)
- Most kinds of dinosaurs present at the time:
- Saltasaurid, nemegtosaurid, and antarctosaurid titanosaurs (last surviving sauropods)
- Abelisaurids and noasaurids (last of the ceratosaurs)
- Caenagnathoid oviraptorosaurs
- Hesperornithine, and a few other basal euornithine bird groups
- Hadrosaurids (both lambeosaurines and hadrosaurines)
- Ceratopsids (although only chasmosaurines are currently known from the very end of the Cretaceous)
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)
- Prototheres (in the form of monotremes)
- Allotheres (in the form of multituberculates (although extinct today, survived and flourished in early part of
- Metatheres (in the form of marsupials and a few close relatives)
- Eutheres (in the form of placentals and a few close relatives)
- Champsosaurs (last survivors of a group of
semi-aquatic freshwater archosauromorphs important in the Mesozoic and early Cenozoic: die out in early Cenozoic)
- Modern-style crocodilians
- Aves (only dinosaurs to survive the event)
Looking for Causal Agents
Many hypotheses proposed for the K/Pg 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 telluric (from Earth) or cosmic (from space)?
- Is the cause biological or abiotic?
- Is the cause unique to the K/Pg 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?
Here are but some older proposed causes for the K/Pg 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 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
- Large animals would suffer worst; small animals and burrowers might survive
- Organisms on the surface of the water (or creatures that fed off of these) would be
hit worse then bottom-dwellers
- Would affect whole planet simultaneously (essentially every part but the poles would
get clobbered once per day until the supernova faded)
Fits prediction. However, problem because it is an untestable (and thus non-
- Cannot observe remnants of the star, because supernova would have dispersed (and
besides, the Solar System and all other star systems have moved greatly since the
- Would leave NO geologic signature other than the extinction itself.
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:
1. Meteors impact the Earth's atmosphere all the time
2. Some chemical elements more common in meteors and such than on Earth's surface:
these should be traceable in minute quantities in sediment
3. Find the average infalling rate of these elements today; use this rate and observed
amount at the Gubbio clay layer to find out how much time
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
- Calculated probable size need to add this much iridium: suggested a 10-15 km diameter object (Manhatten-sized).
- Calculated probable effects of impact of an asteroid this size:
- Short term:
- Release lots of energy near impact, form huge crater: 1.8 x 108 megatons!!
- Burst of light would vaporize material for kilometers around, just like thermonuclear weapons
- Blast wave would devaste nearby region; it would be felt around the world, but decrease with distance
- Shockwaves from impact would generate huge tsunamis ("tidal" waves)
- Newly recognized minutes-to-hours event: the "Easy Bake Oven Effect":
Material that was thrown up above atmosphere and reentered generates substantial
infrared radiation. This heat raises air temperature by only about 10C° (18F°), but would be fully absorbed by rock, leaf, flesh, and
any other opaque material. It is predicted that the increase in infrared radiation would be 8-10x that of high noon at the hottest spot of the Earth,
and persist for many minutes to hours. Living tissue would bake, unless underground 10 or more cm (heat wouldn't have time to make it that deep) or
underwater (upper few microns of water might boil off, but that would be it).
- Longer term:
- Material vaporized by impact kicked high up in atmosphere: reduced amount of incoming sunlight
- Observations on Mars showed big temperature drops due to high-level particles
- In human history, eruption of Tambora in Indonesia in 1815 produced chilling effects
worldwide for more than a year later
- Dust and ash would block out sunlight, reducing photosynthesis and killing off
plants on land and surface algae in water; herbivores feeding on these would die;
carnivores feeding on these would starve (after a brief feast)
- Collapse of foodwebs would require long term to recover, as many parts of each foodchain might be lost
- Additional possible effects include:
- Superacid rain
- Global firestorms
- Global tsunami
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
- Animals with larger total food requirements die more those with less
- In marine communities, foodwebs tied into photosynthesis (that is, direct from the phytoplankton) would be hit harder than bottom feeders
(which feed on the accumulated decayed remains of organisms)
- Additionally, taxa dependant on symbiotic algae would be devastated
- Some geologic record other than just iridium might remain
- Effects would be global and essentially instantaneous: hours to days to months to a few years
Biotic prediction fits most of the predictions; search for geological signature was on.
- Quartz is one of the most common of all minerals
- When subjected to intense heat & pressure, forms shock planes
- Shocked quartz has been found in over 100 K/Pg boundary sites worldwide
Melt Glass (Tektites):
- Material thrown up by impact would melt during reentry, form glassy spheres
- These have been found at some K/Pg sites
Tsunami ("tidal wave") and ejecta deposits:
- Thick units probably formed by tsunami found at K/Pg in Carribbean, Gulf Coast of Texas, Mexico, Central America, and South America
- Thinner but widespread deposits of ejecta (material flung through the air) at K/Pg in Carribbean, Gulf Coast of Texas, Mexico, Central America, and
- Chances were that the impact was in ocean basins, but most Cretaceous ocean basins have been recycled by plate tectonics
- Some early leads were in Siberia (too early); Manson, Iowa (too small and too early (within Late K))
- Nearly all geological lines of evidence (tektites, tsunami deposits, ejecta deposits, shocked quartz, etc.) were more abundant in Western Hemisphere,
and especially in the Gulf of Mexico, than the rest of the world: pointed to impact in that region!
- In Yucatan, Mexico: disrupted layers at K/Pg boundary in buried rock
- Seismic and gravity scan suggested a
crater 180 km across: the right size!
- Although not visible as a crater because buried under 300-1000 m of Cenozoic rock, it
can be seen using sensitive satellite and other data
- Crater was named Chicxulub, after nearby town
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
- GIGANTIC series of lava flows in western India
- A major flood basalt event, like the Siberan Traps at the Permo-Triassic boundary or the Central Atlantic Magmatic Province volcanism
that formed by the break up of Pangaea at the Triassic-Jurassic boundary
- Some places the Deccan Traps are 2.4 km (1.44 MILES) thick
- Second or third largest volcanic event in last half-billion years: only Siberian Traps (and probably CAMP) are bigger
- Was not a single continuous event: erupts, cools, life returned, reerupted, etc.
- If single eruptions like Tambora can effect global climate for a few weeks, what of this much larger, longer scale event?
- Once thought to have begun millions of years prior to impact (67 Ma) and continued through boundary into early part of Tertiary, but now
known to have all formed in 1 million years or less, centered on K/Pg boundary
- Begins during a magnetic normal time (C30N) while the impact occurs in the middle of a magnetic reversed time (C29r),
so the beginning of volcanism can be no closer than 350 kyr from the impact
- Would have similar climatic effects to impact (lots of ash and dust, blotting out skies and reducing incoming sunlight)
- May have also caused longer term global climate changes due to greenhouse gasses
- Only larger volcanic events--the Siberian Traps and CAMP--coincide directly with Permo-Triassic and Triassic-Jurassic
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: last Age of Late Cretaceous Epoch; Regression: any period of sea-level drop
- Change in mid-ocean ridge activity meant ocean levels drop
- Shallow seaways typical of Late Cretaceous drain, exposing lots of land
- Climates worldwide becomes more continental (more extreme: hotter summers, colder winters)
- Change in climate produces change in ocean circulation (and amount of nutrients)
- Change in climate produces changes in growing seasons and ranges of plants, and thus the animals that ate them
Maastrichtian Regression clearly happens (latest Maastrichtian terrestrial rocks on top of earlier Maastrichtian shoreline rocks on top of
earlier marine rocks).
- Gradual change over period of millions of years
All three events (Chicxulub impact, Deccan Traps volcanism, Maastrichtian Regression) are known to occur. Can we separate their effects in the
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:
- China and Siberia: only moderately known, and in no one region is there a long continuous record
- Europe: good for Spain, southern France, Transylvania, but not a complete record for any one spot. (And because Europe was an archipelago, the
differences between each site may have to do with the differences between islands rather than change over time.)
- South America: someday soon, hopefully. Great fossils, but correlations are poor at present (but better than 10 years ago.)
- Australasia, Antarctica: only very few fossils
- Africa: Sadly, no dinosaur-bearing units from later Late Cretaceous
- India & Madagascar: fairly good Maastrichtian fossils, but only from a single time slice in each
- Eastern North America: very few good dinosaur fossils
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:
- Tyrannosaurids: top predators
- Dromaeosaurids, Troodontids: smaller predators
- Ornithomimosaurs, Oviraptorosaurs, Alvarezsaurids: omnivores
- Titanosaurs: rare in the Montana
- Nodosaurids, Ankylosaurids: last of the thyreophorans
- Thescelosaurids, zephyrosaurs
- Hadrosaurids: one of the two most common herbivore groups
- Ceratopsids: the other most common herbivore group
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 hadrosaurines, lambeosaurines) die out earlier, while long-snouted forms
(chasmosaurines, long-snouted hadrosaurines) 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:
- Dinosaurs are NOT extinct: toothless birds (Aves) are dinosaurs, and survive today
- Some latest Cretaceous dinosaurs were being killed off by long-term climatic changes due to regression (and volcanism?)
- Latest Maastrichtian non-avian dinosaurs (including toothed birds) were probably finished off by asteroid impact
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Last modified: 24 April 2016