Dinosaurs ruled the Earth for 160 million years, beginning about 225 MY ago and ending 65 MY ago. In the geological strata, the layer of clay known as the Cretaceous-Tertiary (K/T) boundary layer marks the transition from the era of the dinosaurs (the Cretaceous) to the following post-dinosaur era (the Tertiary). This boundary layer is well marked and recognized world-wide and has been long known to mark one of the largest mass extinctions in the fossil record.
What has always clearly marked this boundary layer is the
fossils above and below. Below, in the older Cretaceous sediments,
the tiny foraminifera - single-celled organisms that have inhabited
the oceans for more than 500 million years - come in a variety of shapes
In the younger, Tertiary sediments, there are only tiny, less ornate foraminifera.
Other creatures, prominently the ammonites, the fish of the oceans (except they are cephalopods like the octopus and the chambered nautilus) in the Paleozoic and Mesozoic Eras, some 400 to 65 million years ago, abruptly disappeared.
And of course, the terrible reptiles, the dinosaurs, disappeared from the face of the Earth. Clearly, something happened 65 million years ago to cause a mass extinction.
The Gubbio Clay
In the late 1970s, a geologist at UC Berkeley, Walter Alvarez was conducting research on the geological strata surrounding the so-called K/T boundary layer, in Gubbio, Italy. A core sample of rock, taken across the boundary layer (but not from Gubbio) is shown here:
core sample across K/T boundary layerThe slice of rock shown covers about 40 cm and about 200,000 years of depositional history. The actual "boundary layer" is about 17 cm thick and is surrounded by "pre-extinction" sedimentary deposits below and "post-extinction" sedimentary deposits above. Note that throughout most of the world, the boundary layer is only 2-3 cm thick. These thick deposits are found only in the Gulf of Mexico region and in Haiti.
Alvarez brought samples of the boundary layer back to his lab at Berkeley and ran some standard tests. One of those tests revealed that the boundary clay was heavily enriched in the element iridium, i.e., the amount of iridium (per gram of material) was greater than in normal, terrestrial rocks. Iridium is an element that has very few uses: one of these is as a hardening agent for gold. So the initial tests were thought contaminated by material sloughing off the gold wedding bands worn by the scientists working with the clay. But more tests revealed no such contamination. So, what does the enhanced abundance of iridium mean?
Iridium is known as a siderophile - or iron-loving - element. Iridium attaches itself to iron and goes where the iron goes. If you want to find or mine iridium, the best place is an iron deposit. And this means that most of the iridium on Earth went to the core when the Earth differentiated. Consequently, the abundance of iridium in almost all terrestrial rocks is very small. Where could the iridium have come from?
Walter Alvarez obtained the answer after consulting with his father, Luis Alvarez, a nobel prize recipient for his work in physics: the iridium came from space, from an asteroid. Meteorites have lots of iridium, especially the undifferentiated and the iron meteorites. So, if an asteroid hit the Earth, and if the debris from the collision was distributed into a cloud surrounding the planet, and then if that debris cloud gradually rained down to Earth, we would find a small sedimentary layer with an enhanced level of iridium.
The Alvarezes estimated that the likely culprit would have been an asteroid perhaps 10 km in diameter, in order to produce the necessary amount of iridium.
Later tests confirmed that the iridium level is enhanced in the K/T boundary clay throughout the world, not just in Italy, by a factor of 10-50 compared to normal rocks.
So what happened? Presumably an asteroid hit the earth. The collision through an enormous amount of ejecta, including the vaporized asteroid, into the atmosphere. Somehow (darkness? shock waves? global fires?), this event precipitated global mass extinctions.
Many scientists, including the Alvarezes and Eugene Shoemaker - famous for identifying meteor crater in Arizona as an impact crater, for training astronauts how to collect rocks on the moon, for Comet Shoemaker/Levy IX - went so far as to claim that all mass extinctions on Earth were likely caused by asteroid impacts and that extraterrestrial events such as this have been the single most important contributor to biological change in the history of Earth. Darwin's fittest were the few creatures that could survive the impact of an asteroid, they claimed. But this kind of evolution is profoundly non-Darwinian in that it posits extraterrestrial catastrophism as the primary mechanism for evolution.
Other evidence for the asteroid impact hypothesis
Soon, new evidence was being found in support of the impact hypothesis. These include:
The K/T Extinction: the devil is in the details
Extinctions are identified on the basis of changes in the fossil record. So, in order to associate the K/T extinctions with the impact, one must demonstrate that:
How do fossils form?Fossils form in sedimentary rocks, not in igneous rocks (formed from hot, molten rock that, once extruded from a volcano or a mid-ocean ridge, solidifies) or in metamorphic rock (which forms from either igneous or sedimentary rocks that are subjected to enormous pressures and temperatures several km beneath the surface of the Earth).
Sedimentary rocks from from the (usually gradual) deposition of eroded material on stream beds or lake bottoms. So only certain environments will permit the formation of sedimentary rocks, at any given point in time.
Typically, a sedimentary rock from the Cretaceous-Tertiary eras can
be dated to within the closest million year interval, sometimes to within
+/- 250,000 years. Even if we can obtain an accuracy of 50,000 years,
is that sufficient to demonstrate than an extinction event was sudden,
almost instantaneous (a year, a few years, a few decades)?
What is known from the fossil record?15% of all marine families went extinct. This was concentrated on plankton, marine predators, and shallow water communities. 25% of all terrestrial families went extinct. Only small species (below ~25 kg) survived. Higher planets, birds, mammals and dinoflagellates were generally unaffected.
Dinosaurs: The dinosaurs arose 225 million years ago and flourished until the end of the Cretaceous, 65 million years ago. But the dinosaurs that lived 180 MY ago were not the same species as those that died out 65 MY ago. And, whereas there were 60 species of dinosaurs on Earth 75 MY ago, only 18 of those species are found in strata from 65 MY ago. So, clearly dinosaurs were suffering, probably from some long term climatic changes. No dinosaur fossils are found above the K/T boundary; however, no dinosaur fossils are found within a meter below the K/T boundary. The last appearance of dinosaur eggshells and footprints occur 2 million years prior to the K/T boundary. Did the dinosaurs die out 100s of thousands of years before the impact? Or, are conclusions in this regard too hard to draw because there are so few large animals and even fewer fossils preserved?
Icthyosaurs: These fish-lizards died out 30 million years before the K/T extinction.
Plesiosaurs: These creatures had decreased from 6 families to only 2, at the approximate time of the K/T extinction.
Vertebrate species: More than 50% of vertebrate species survived across the K/T boundary, in the fossil record of what is now the western United States. Not including very rare fossil species, for which often only a single fossil is known, 70% survived. The extinctions were concentrated in only five of 12 taxa of vertebrates. Hit hard were sharks & relatives (less than 20% survival), bird-hipped dinosaurs (0% survival), reptile-hipped dinosaurs (0% survival, except birds), lizards (less than 30% survival) and marsupials (less than 10% survival). Completely unaffected were frogs & salamanders, champsosaurs, and placentals; barely affected were turtles (85% survival), crocodilians (80% survival), bony fishes (70% survival) and multituberculates (50% survival). One must ask what kind of event would cause this kind of survival and extinction pattern. Clearly, the K/T extinction did not affect all types of vertebrates equally. What kind of changes would cause the noted extinctions? A quick, sustained temperature drop is consistent with the survival and extinction patterns for only 4 groups; acid rain is consistent with only 3 groups; global wildfire is consistent with 5 groups; local wildfires are consistent with 9 groups; global marine regression and habitat fragmentation (the shallow epicontintental seas that formed in the late Cretaceous receded quickly, producing the largest single increase in non-marine area during the past 250 million years) is consistent with 11 of 12. Thus, the vertebrate fossil record is not consistent with an impact caused extinction.
Mosasaurs: These giant marine reptiles (up to 17-m in length) lived from about 90-65 MY ago. They appear to have been diversifying and undergoing a mass radiation shortly before the K/T event, with most of this occurring from 75-65 MY ago. They apparently went extinct abruptly at the K/T boundary.
Rudistid bivalves: These are marine creatures that live on reefs. Their fossil remains are considered a proxy for tropical, shallow-marine ecosystems. Rudistids survived the end of Jurassic, late Aptian and Cenomanian-Turonian extinctions. At the end of the Cretaceous, they became extinct suddenly, but the demise of the reef systems occurred 1.5 - 3.0 million years before the K/T boundary.
Ammonites: The abundance of ammonites (from K/T strata in Chile) declined gradually and disappear 5 meters below the K/T boundary. This suggests their disappearance was due to environmental changes in the late Cretaceous (the Maastrichtian).
Radiolarians: These are protozoans with silica shells; they are closely related to foraminifera, but are exclusively marine and planktonic. Radiolarians are absent from the vast majority of K/T boundary sections. In modern seas, siliceous plankton are only abundant in areas of high surface productivity, where wind and current systems cause continuous upwelling of nutrient-rich deep waters. In New Zealand strata, where radiolarians are abundant in the fossil record of the late Cretaceous and early Tertiary, 100% of radiolarian species survived and crossed the K/T boundary. They show no indications of a mass extinction.
Benthic (deep water) Foraminifera: There is general agreement that foraminifera underwent a major mass extinction, at least at low to middle latitudes, at or near the K/T boundary. 12% of these species went extinct 300,000 years prior to the K/T boundary, 57% appear to go extinct at the boundary, 31% of the species (i74% of all fossil individuals) survived. Thus, these data strongly support the existence of a sudden and major biotic crisis, but one that appears to have acted on an already highly stressed ecosystem.
Caribbean foraminifera: These creatures appear to have lived right up to the K/T boundary and then disappeared.
Sea Urchins (echinoids): 36% of these species went extinct in the late Maastrichtian, but it is unclear how sudden or how close to the K/T boundary this occurred. Another 15% went extinct during the early Paleocene, i.e., after the K/T boundary. Post-Cretaceous survivors were dramatically smaller than their predecessors. The extinctions seemed to have hit adults at depth harder than planktonic larvae, feeding at the surface. The extinction pattern could be due to a decrease in phytoplankton abundance at the end of the Cretaceous; this change would have had to be small enough to not seriously affect surface larvae but large enough to affect deep water feeders. The conclusion drawn from this group is that an asteroid may have been the final blow, but that the climate was already stressed for this type of creature. Furthermore, the continued extinctions after the K/T boundary suggest a very long term stress exacerbated but not caused by an impact.
The last 50,000 years on EarthAt the end of the last ice age, ~40,000 years ago, the Mastodons and saber tooth tigers and many other creatures perished due to natural climatic change.
In the last 500 years, the dodo bird as well as many other island avian species perished, when they could not withstand the predation and diseases brought by new species to previously isolated islands.
In the last 150 years, many species (buffalo, alligator, grizzly bear, rhinos, elephants,whales, carrier pigeon) have been driven (nearly) to extinction by hunting.
In the last 50 years, many species, such as wild cats (lions, tigers, leopards), the spotted owl, snail darter, many birds and butterflies, etc., are being driven to extinction through loss of habitat in competition with humans.
In the last 3000 years, we have seen the explosive abundance of cows, horses, dogs, cats, rats, mice, sheep, people, and cockroaches, all due to the increasing human control over the environment.
Now, we imagine that an asteroid hits the Earth in the year 2080 and
wipes out humanity (and many other species). Or we imagine that a
nuclear war does the same thing. What if the most successful surviving
species (rats) reach modern human intelligence in 100 million years and
investigate the geologic record of this epoch? With the accuracy
we currently have, they would conclude that ALL the above creatures died
out suddenly and catastrophically, that they all lived right up to the
asteroid impact (or nuclear war) and then were dramatically wiped out.
But, in fact, we know that there are numerous reasons for these extinctions,
most of which have nothing to do with the impact (or war), and that in
fact some species were proliferating at the expense of others, yet the
fossil record would merely show a catastrophic wipe out!
Was the Asteroid the Cause of the Extinctions, or the Straw that broke the Camel's back?What else was going on at the time of the K/T extinctions? We've seen that there were major environmental changes associated with the recession of intercontinental seas.
In addition, there were enormous volcanic eruptions at about the same
time, forming the Deccan traps in India. Large scale emissions of
CO2 are associated with volcanic eruptions; thus, it is possible
that the Deccan flats eruptions could have influenced climate change.
What about other Extinctions?
In 2001, buckyballs enriched relative to terrestrial abundances in Helium
and Argon isotopes were found in the layer of strata that marks the Permian-Triassic
extinction, 250 million years ago. This extinction was much more
profound, in terms of the level of extinctions that occurred, than the
K/T event. However, despite searches, no iridium enrichments have
been found associated with this extinction. Did a comet (rich in helium
and argon, not rich in iridium) wipe out the trilobites?