•The disappearance of non-avian dinosaurs was just one part of a larger event: the Cretaceous-Paleogene (K/Pg) mass extinction (formerly called the Cretaceous-Tertiary or K/T extinction).
•Diverse groups of land and sea organisms died out at this time, 66 million years ago.
•Three major environmental changes (the Maastrichtian Regression; the Deccan Traps volcanism; and the Chicxulub asteroid impact) occurred in this same general time. While the first two seem to have generated some level of extinction, it is the impact that seems to be the primary driver of this extinction.
•Survivorship on land seems to be controlled mostly by favoring small body size and ability to feed from stored food resources; in the sea it is the latter which is the most important factor.
•The western North American (Laramidian) fossil record of the last 15 million years of the Cretaceous is the most complete in the run up to the extinction event; there are some losses of groups before the impact that seem to have been driven by other environmental factor. That said, these do NOT seem to have been the cause of the loss of the non-avian dinosaurs.
Definitions and Dramatis Personae
One of the most interesting aspects of dinosaur history is the extinction of the non-avian 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:
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.
(All of these essentially mean the same thing: there are no more of that kind of organism).
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 [or Ornithoscelida], and Theropoda, and Neotheropoda, and Averostra, and Tetanurae, and Avetheropoda,
and Coelurosauria, and Tyrannoraptora, and Maniraptoriformes, and Maniraptora, and Metornithes, and Pennaraptora, and Eumaniraptora, and Avialae, and Pygostylia, and Ornithothoraces, and Euornithes, and Carinatae, and Ornithurae) 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 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 Paleogene Period (Pg):
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 magnetic chron C29R.
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)
(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:
Bennettitalians: a group of Mesozoic plants related to the ancestors of the flowering plants
Allotheres (in the form of multituberculates and gondwanatheres (although extinct today, both groups survived, and multituberculates flourished, in early part of Cenozoic))
A few other mammal groups
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)
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 victims?
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:
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 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 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?
Will explore other hypotheses...
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 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).
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:
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 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, 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-Jurassicextinctions!
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 geological record?
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.
Survival of Aves vs. other birds and small dinosaurs may have been from their ability to access a "pantry" that others could not: seed.
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
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 (83.6-72.1 Ma) and complete Maastrichtian (72.1-66.0 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 Paleogene 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:
Nodosaurids, Ankylosaurids: last of the thyreophorans
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