"Nature was quick to pass the sponge of her deluges over these awkward sketches [i.e., dinosaurs and other Mesozoic megafauna], these first nightmares of Life. And yet, what curious prints might have been made of all these creatures! Alas, the vision is lost forever." -- Auguste comte Villiers de L'Isle-Adam, L'Ève future (1886)
"Night comes to the Cretaceous." -- J.L. Powell, 1998
BIG QUESTION:What caused the Cretaceous-Paleogene extinction and the end of the Age of Dinosaurs?
The Cretaceous-Paleogene Extinction (66.0 Ma)
We'll start our look at particulars of mass extinctions by looking at the most famous of all: the end of the Age of Dinosaurs. This is the
boundary between the Mesozoic Era and the Cenozoic Era (and thus between the Cretaceous and Paleogene Periods, the Late Cretaceous and Paleocene Epochs, and the Maastrichtian and Danian Ages). Currently dated (as of 2013) as being 66.0 Ma, this is the extinction event that eliminated the dominance of dinosaurs and allowed the adaptive radiation of mammals.
(By the way, this event is by no means the largest of mass extinctions: we'll see that one next lecture! It was also not the final extinction of the dinosaurs, for as we'll see later in this course Dinosauria is still alive and flying!)
This event goes by many names:
The K/Pg extinction (or boundary event): "K" is the formal symbol for the Cretaceous, and "Pg" for the Paleogene
The Cretaceous/Teritary extinction (or boundary event), or its abbreviation "K/T": the Tertiary is the former name for the first Period
of the Cenozoic Era; in modern stratigraphy the Tertiary is no longer used and instead we break it into the Paleogene and Neogene Periods.
The Maastrichtian/Danian extinction (or boundary event)
The Terminal Cretaceous, or Terminal Maastrichtian extinction
And other terms along these lines
Some of the major victims and survivors of the event:
In the marine realm, among the main victims were:
Coccolithophorids: chalk-forming nannoplankton phytoplankton, a
major part of the base of the food chain. These are not entirely wiped out, but are greatly reduced in diversity
Foraminferans, both planktonic and benthic "armored" zooplankton with calcarous shells. These form the food source for many of the planktonivores of the seas. They too underwent incredible decline,
but actually rallied and did better in the Cenozoic than in the Cretaceous
Ammonoids: shelled, coiled relatives of modern nautilus, squids, octopi, etc. They had a diverse diet: primitive ones were predators on macroscopic food, but derived ones seem to have been primarily planktonivores. Unlike their shelled cousins the nautiloids (which survive today as Nautilus and Allonautilus (whose larvae are macroscopic and hang out at the sea floor), larval ammonoids were microscopic (about 0.5 mm in diameter) and planktonic. Phenomenally abundant in the Cretaceous (they were major index fossils for the Mesozoic), they died utter at the K/Pg.
Belemnoids: another group of squid relatives, these active predators. Also die out at the K/Pg.
Inoceramids: gigantic benthic scallop relatives. Once thought to be suspension feeders and planktonivores, they are now interpreted to be have lived in dysoxic (low oxygen) environments where methane was seeping out of the sea floor; symbiotic chemosymbiotic bacteria in their tissues would absorb the methane and convert it into food for the clams.
Rudists: reef-forming clams. Planktonivorous suspension feeders and almost certainly with symbiotic algae (as in today's giant clams). These were the main reef-formers of the Late Cretaceous, having outcompeted the corals in the carbon dioxide-rich warm tropical seas of that Epoch.
In contrast, there does not seem to be too much in terms of extinction among smaller bottom-dwelling organisms.
In the terrestrial/continental realm, major victims include:
One major group of plants (which actually might have died out earlier)
Temporary decrease in the amount of insect feeding damage on leaves, but no major insect extinction
Many groups of Mesozoic mammals. (However, the ancestors of marsupials and of placentals survived, as did the monotremes (already present), and
some Mesozoic groups that ultimately died out later in the Cenozoic, such as the multituberculates.)
Many types of crocodyliform, including terrestrial armored herbivores
All the larger types of dinosaurs, and many clades of birds
Indeed, pretty much all fully-terrestrial animals greater than 5 kg mass goes extinct
In the terrestrial realm there is a transition from a gymnosperm-dominated flora to an angiosperm (flowering plant)-dominated one.
The toothless crown-group birds survived, as did the living groups of amphibians, turtles, lepidosaurs, and crocodylians. The long-snouted
champsosaurs (distant kin to the archosaurs) survived and thrived in the early Cenozoic, but have subsequently died out.
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?
Is the hypothesis testable (i.e., falsifiable)?
Would the agent leave a record independent of the extinction itself?
(Without those two aspects, the hypothesis is not scientific, but simply speculation)
Here are but some older proposed causes for the K/Pg event:
Discussed last week, the idea that most mass extinctions were caused by intense climate change brought about by geologic upheavals.
This was one particular extinction to inspire that idea. The latest Cretaceous and earliest Cenozoic of western North America--for a long time the best studied region for the end of the Mesozoic--does indeed have a major pulse of mountain building. However, this is by no means a global phenomenon.
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 senility
"Evidence" for this was "nonfunctional" structures in dinosaurs (pachycephalosaur skulls, short tyrannosaurid arms, spikes on centrosaurine frills, etc.), bizarrely coiled ammonoids, etc.
However, these structures likely have functional significance
Also, no evidence of predetermined "lifespans" of clades
Poison Gas from Comets
Shortly after WWI (when poison gas was used on battlefield), astronomers identify cyanogen 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 neck ligaments
Caterpillars ate all the food
Perhaps caterpillars diversified and ate all the plants before the dinosaurs could
No evidence for this
How would it affect coccolithophorids, ammonoids, etc.?
Mammals ate the dinosaurs to death
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!
Allergies to Angiosperms?
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?
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 (dinosaur 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.
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-falsifiable) hypothesis:
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 Cretaceous)
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 (Manhattan-sized).
Calculated probable effects of impact of an asteroid this size:
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 devastate 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).
Additional short term possible effects include:
Impact Winter: Material vaporized by impact kicked high up in atmosphere, reducing amount of incoming sunlight
Observations on Mars showed big temperature drops whenever fine particles spread to high atmosphere
In human history, eruption of Tambora in Indonesia in 1815 produced chilling effects worldwide for more than a year later; later eruptions, such as the 1991 eruption of Mount Pinatubo, while not as severe, were better studied and analysed
Dust and ash (and most especially, sulfate aerosols) 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 food chain might be lost
Estimates of duration of the Impact Winter have varied from a year or so to a few months to just a few weeks, but a model published in January 2017 puts a duration for a 26°C temperature drop in global surface temperature for 3-16 years and a greater than 30 year duration until recovery!
Longest term effect: Greenhouse Summer:
Proxy evidence shows jump in carbon dioxide levels from around 500 parts per million to around 2300 ppm
Deccan Traps outgassing (see below) may have been responsible for taking it to 1400; the rest is from the vaporized carbonate rock from the impact site.
Net effect was an estimated global temperature jump of 7.5°C temperature worldwide (although possibly as much as 13.5°). (For contrast, people are concerned about a 2°C temperature rise by the end of the century)
Warming would last for many tens of thousands of years, until the excess carbon dioxide is resorbed by the Earth's systems
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 dependent 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
Thick units probably formed by tsunami found at K/Pg in Caribbean, 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 Caribbean, 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 Deccan Traps
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 from sulfates, warms from excess greenhouse gases, conditions stabilize, 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 extinctions!
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 themselves 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
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
Suggestions that all these systems were in effect:
Some suggestion of million-year scale decline in some groups, but not as strong as once thought
Change in flora of western North American Interior, consistent with climate/ecosystem
changes due to Regression
Many extinctions, however, seemingly instantaneous
In marine realm, planktonic forms and creatures that eat them (and those that ate them, and so on) suffered greater than benthic detritivores, consistent with shut down of photosynthesis. Impact Winter seems to have been the primary killing agent. (Ammonoids were doubly-hit: their larval forms were plankton, and they ate plankton!)
In terrestrial realm, basic pattern is that animals dependent on large food supply and/or metabolism: larger and/or fully terrestrial creatures
survive better than smaller and/or subaquatic forms. A combination of the "Easy Bake Oven" and "Impact Winter" seem to have been the main killing agents, with the longer "Greenhouse Summer" picking up many of the survivors.
But, there are complications:
There is very different sampling in different parts of the world. For example, only western North America has a continuous record of terrestrial deposits for the 10 million years or so up to and through the event
It is possible that different effects were in play in different parts of the world
Given the problem of stratigraphic resolution, it would be hard to separate out the effects of Deccan Traps volcanism and
the Chicxulub Impact except where the record was exceedingly complete