• 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

• 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

• Perhaps caterpillars diversified and ate all the plants before the dinosaurs could
• No evidence for this
• ?How would it affect coccolithophorids, ammonoids, etc.?

• 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...

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.

Predictions:

• 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.

Confirmed Potential K/Pg Causal Agents

While the above phenomena are largely insufficient to explain the event, there are three confirmed large-scale environmental changes during the end of the Maastrichtian that potentially are involved with the extinction. In order of appearance (and of increasing severity) they are:

• The Maastrichtian Regression: a global drop in sea level
• The Deccan Traps: a flood basalt invent in India
• The Chicxulub Impact: an asteroid slamming into Mexico

Maastrichtian Regression:
The Maastrichtian is last Age of Late Cretaceous Epoch; regression refers in this context to any period of sea-level drop. In this particular case, it was triggered by reduction in mid-ocean ridge activity. As the mid ocean ridges shrank, the water that they had displaced onto the continents throughout the Late Cretaceous drained away.

For the marine realm, this caused a very large drop in the area for the shallow marine communities. Additionally, it removed a major source for marine productivity and modified global circulation patterns. For the terrestrial realm it made climates more continental (hotter summers, colder winters) because now not every place was essentially close to the sea shore. This would also mean that habitats would shift.

The Maastrichtian regression unquestionably occurred (although as with all such changes, there were local and global smaller scale sea level rises mixed in with the pattern of the drop.) One would predict that the environmental effect of the Maastrichtian regression would play out over the period of a few million years.

Deccan Traps Volcanism:
The Maastrichtian has long been known as a period of intense volcanism in parts of the world (see "Global Diastrophism" above). For instance, 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.

The Deccan Traps were a GIGANTIC series of lava flows in western India. They were the most major flood basalt event since the Siberian Traps at the Permo-Triassic boundary and the Central Atlantic Magmatic Province volcanism that formed by the break up of Pangaea at the Triassic-Jurassic boundary. In some places the Deccan Traps are 2.4 km (1.44 MILES) thick. An area of 2 million km³ was covered. As with all flood basalts, it was not a single continuous event; instead, there would be eruptions; periods of cooling from sulfates; global warming from excess greenhouse gases; stabilization of the environment after the eruption had ended; then a new eruption.

The Deccan Traps began around half a million years before the K/Pg boundary. The lowermost beds erupted during a global magnetic normal time (subchron C30n), while the impact of the asteroid and the boundary itself would be later in a magnetic reversed interval (subchron C29r). Since the switch from C30n to C29r happened 350 kyr prior to the boundary, the asteroid impact could NOT have been the cause of the Deccan Traps. That said, there is some evidence that a few thousand years after the impact the intensity of the eruptions increased: this timing would be consistent with magma chambers being disrupted by impact-generated mega-earthquakes, and then percolating slowly up to the surface.

As with the Siberian Traps and CAMP, the Deccan Traps would provide both a cooling and a warming component. There is a 5-6°C increase in annual temperature in western North America during the pre-impact eruptive phase, that might reflect the volcanic greenhouse contributions. Furthermore, we would expect ocean acidification as a side effect.

So, Deccan Traps themselves were a MAJOR event, and might have contributed to the extinction event. Had just this eruption plus the Maastrichtian regression occurred, there probably would have been a mass extinction. But perhaps it would have been much less severe, and not an "era-ending" one. But Nature had one more, far more spectacular event to unleash.

The Chicxulub Impact:
The discovery of an asteroid impact at the end of the Cretaceous began in the latest 1970s and was published in 1980. Geologist Walter Alvarez was investigating a layer of clay in Gubbio, Italy at the K/Pg boundary. He wanted to determine length of time represented by the clay layer. He knew that the limestone below was latest Maatrichtian and those above were earliest Danian (early Paleogene). But how much time was there for the clay layer? A few years? Decades? Millennia? More? (The biostratigraphy above and below showed it couldn't be more than a million years.) Being clay he couldn't radiometrically date it, and there is no magnetic flip-flop right near the boundary. So he consulted his dad (Nobel winning physicist Luis Alvarez) for possible solution. After teaming up with a few chemists, the decided to look for meteoritic material as a possible clock. This was based on the following observations:

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 (the iridium spike) at base of clay. Using their method it would mean that there was a few million years missing, which was clearly not the case. So which assumption was not correct? They eventually realized that this wasn't a case of an average rate of infall. Instead, they hypothesized that an asteroid impacted Earth at the K/Pg boundary. Luis Alvarez had been part of the US nuclear weapons program, and later had worked with NASA on calculating the energy of impacts from asteroids and comets. Both of these sets of skills proved useful in determining the effect of the end-Cretaceous impactor

• Calculated probable size need to increase the rate of iridium infall suggested a 10-15 km diameter object (Manhattan-sized or Mount Everest-sized)
• Calculated that a blast of this size would have released about 1.8 x 10⁸ megatons!! (A "megaton" is the destructive force of 1,000,000 tons of TNT. The most powerful nuclear blast ever produced by humans was 50 Mt, and the largest standard H-bombs were in the 5-10 Mt range. The energy involved here as 1000-10,0000 times the total nuclear weapon yield available at the height of the Cold War.)

Although there had been a very good chance that the impact had hit oceanic crust that had since been subducted back into the mantle, the crater was actually recognized in 1990s. It is in the Yucatán Peninsula of Mexico. It is not visible from the surface except by radar scanning, because it is covered by 300-1000 m of Cenozoic sediments. But cores drilled during petroleum exploration had found a disrupted and melted layer right around the K/Pg boundary; with the publishing of Alvarez paper and subsequent search for evidence to test their hypothesis, these cores were re-examined and re-interpreted as being from a crater. Seismic and gravity scanning revealed the presence of a 180-km diameter crater, exactly the right size for hypothesized impactor. There are even sinkholes (cenotes) in the rock above that trace out the disrupted layer, because groundwater in the limestone preferentially drains down into the crustal fractures. The crater is named Chicxulub after a town near the first core where the disruption was recognized.

Between the initial Alvarez paper and the discovery of the crater, several proxies of the impact beyond the iridium spike were recognized. These include:

• Shocked quartz: grains of the common mineral quartz found with fracture planes within its crystalline structure. A volcanic blast might make a single direction of shock planes, but impacts and nuclear blasts produce multiple shock planes in the same crystal, and that is what is found in the K/Pg shocked quartz. More than 100 sites worldwide have shown these deposits.
• Melt Glass (Tektites): Debris that is blasted out of the atmosphere can melt as it re-enters, chilling into a glass when it reaches cooler temperature. These melt glass spherules and droplets are found in many locations around the world.
• Tsunami and Ejecta Deposits: The blast wave from the impact sent out tsunamis in the ocean. Similarly, ejecta (material thrown sideways rather than up and out of the atmosphere) splashed from the blast. Both of these have distinctive sedimentary structures, and are found at boundary sites particularly in the Western Hemisphere.

The map of the K/Pg sites and the thickness of the disrupted layers form a bullseye around Chicxulub, showing that this rather than some other spot was the source of the blast.

Effects of Chicxulub

An asteroid impact is a very different kind of causal agent, and its effects are more instantaneous than the other types of mass extinction causes we've explored. (Some of the effects, however, are extended over long periods.) Here is what we have reconstructed so far.

Phase I (the same day): Shockwave and Tsunami: A magnitude 10.1 earthquake would be expected from this blast. In the modern world this would likely topple buildings all over the planet; in a Cretaceous world it might be very disruptive to forests and cause landslides (and make dinosaurs fall over...), but wouldn't be a primary killing agent. A shockwave of hurricane-force winds would spread over southern North America and northern South America would also be locally dangerous, and the energy flash from the impact would incinerate everything in line-of-sight, but again these are not mass extinction-causers. A tsunami of 100-250 m would surge out, but again while regionally disruptive would not bring an era to an end.

Phase II (later the same day): "Easy Bake Oven" and the Canopy Collapse: However, other events of that first day would be globally catastrophic. Infalling material would mostly burn up in the atmosphere. Your average meteor doesn't put out much heat, but so much infalling material 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 deeper) or underwater (upper few microns of water might boil off, but that would be it). I have nicknamed this the "Easy Bake Oven" effect, and may be the reason that no land animal larger than 5 kg seems to have survived.

Related to this, some of the material WOULD make it down to surface, and seems to have sparked off global forest fires. This is called the canopy collapse, and is shown by an increase of soot and charcoal and by an increase of fern spores (the fern spike in sediments post-impact. (Ferns are excellent at recovering from periods of forest fires.)

Phase III (the first decade or so): Impact Winter: Material vaporized by the impact is kicked up into the stratosphere, blocking insolation. This was the primary killing agent suggested by the Alvarez team. Probes on Mars show big temperature drops when fine particles are spread to the high atmosphere. And in human history, the 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 analyzed.

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 (46.8°F) temperature drop in global surface temperature for 3-16 years and a greater than 30 year duration until recovery! Recovery on land goes much more quickly (basically once the skies are clear), but some deep-water sites show the cooling required 10 kyr to recover.

Phase IV (100 kyr or so): Greenhouse Warming: The impact site was covered by carbonate rocks; when you oxidize carbonate rocks, you release CO₂ (this is the same as when limestone is converted into cement). The Deccan Traps already released enough CO₂ to raise the atmosphere from ~500 to 1400 ppm; the new addition brought it up to 2300 ppm or so. Once the dust was cleared the full effect of the greenhouse gases could be in play. Recent studies show a warming of 5C° in the shallow ocean for 100 kyr; air temperature warming might be between 4.3 and 13.5C°C (average of 7.5°C). See the discussions of the P/Tr and the PETM a few lectures ago about the effects of this on the living world.

General Patterns of the K/Pg Impact

In the marine realm, plankton (including the larval ammonoids) and nekton suffer worse than benthos. Photosynthesizers, and the creatures that feed on them directly (and those that fed on THEM directly) suffer worse than bottom feeders (which eat food "stored" in sediment). Groups with symbiotic algae also suffer strongly. Among marine vertebrates, larger ones in the open seas suffer worse than those on the coast, and those with higher metabolic rates worse than those with lower ones.

(Note: the basic pattern is the exact opposite of the P/Tr, where the benthos got clobbered relative to nektonic and planktonic forms.)

In the terrestrial realm, freshwater animals and those that feed on the water ecosystem tended to do better than those which fed on land. Larger animals and medium-sized animals with high metabolic rates suffer worse than small animals and medium-sized animals with low metabolic rates.

For the terrestrial realm, it seems that "Easy Bake Oven" is the primary selective filter, with the Impact Winter taking out some survivors and the Greenhouse Summer even more. For the marine realm the "Easy Bake Oven" is likely not a factor, but the Impact Winter is the dominant selective force.

The pattern of extinction looks like it was global and essentially instantaneous: hours to days to months to a few years. This suggests that the Chicxulub impact is by far the major causal agent. But all three events (Chicxulub impact, Deccan Traps volcanism, Maastrichtian Regression) are known to occur, so the earlier scenarios may have destabilized the ecosystems to some degree.

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:

• Tyrannosaurids: top predators
• Dromaeosaurids, Troodontids: smaller predators
• Ornithomimosaurs, Oviraptorosaurs, Alvarezsaurids: omnivores
• Titanosaurs: rare in the Montana
• Nodosaurids, Ankylosaurids: last of the thyreophorans
• Parksosaurids
• Hadrosaurids: one of the two most common herbivore groups
• Pachycephalosaurs
• Leptoceratopsids
• 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: 18 November 2019