The Cretaceous-Paleogene (K-Pg) boundary, dated about 65 million years ago, marks the base of the Paleogene period and consequently of the Cenozoic era. (“K” is traditionally used as an abbreviation for the Cretaceous period (from the German term Kreide) in order to avoid confusion with the abbreviation “C” of the Cenomanian epoch and the abbreviation “C-T” of the Cenomanian-Turonian boundary.) It is more popularly known as Cretaceous-Tertiary (K-T) boundary, but the term Tertiary has become informal and is not used in the geologic timescale. From the chronostrati- graphic point of view, the K-T (or K-Pg) boundary was formally defined at the base of a dark clay bed at the El Kef stratotypic section (Tunisia). This bed is commonly called the K-T boundary clay, and its basal part is characterized by a millimeter-thick reddish layer, called K-T airfall layer, containing meteoritic impact evidence such as anomalous-concentrated iridium, siderophile trace elements in chondritic proportions, microdiamonds, nickel-rich spinels, shocked quartz, and altered microtektites.
The K-Pg or K-T boundary is widely known, because it represents one of the five major mass extinction events recorded in the earth’s history, which affected the most famous paleontological group, the dinosaurs. However, the dinosaurs were not the only victims of this biotic catastrophe. It is considered that 75% to 80% of then extant species were extinguished; this included the total extinction of dinosaurs (Tyrannosaurus, Triceratops, and Ankylosaurus among others), plesiosaurs, mosasaurs, ichthyosaurs, primitive birds (Enantiornithes, Hesperornithiformes), ammonites, belemnites, rudists, and orbitoid fora- minifers. Many other groups were severely affected, such as mammals (~80% extinguished), osteichthian fishes (~96%), nautiloids (~50%), gastropods (~80%), bivalves (~60%), brachiopods (~70%), scleractinian corals (~80%), thermospheric ostracods (~75%), planktic foraminifers (~96%), radiolarians (~85%) and calcareous nannofossils (~75%). Marsupial mammals became extinct except for those in Australia and South America. Remarkably, insects, amphibians, lepidosaurs (lizards, snakes, etc.), crocodilians, turtles, and insec- tivore mammals and birds were not very affected by the K-T extinction. Moreover, although their populations were initially decimated, many species of terrestrial plants and of marine phytoplankton (diatoms or dinoflagellates) also tended to survive, due surely to their capacity to form resistant cysts, spores, or seeds.
The K-T extinction was therefore selective, with a species’ survival depending on its position in the food chains. In the open ocean, the food chain is and was based on the microscopic phytoplankton (unicellular algae), such as coccoli- tophorids (calcareous nannoplankton), dinoflagellates, or diatoms. The marine protozoons and animals at successively higher levels in this food chain (phytoplanktivorous and carnivorous) were very strongly affected (planktic foraminifers, ammonites, carnivorous fishes, and marine reptiles such as plesiosaurs, mosasaurs, and ichthyosaurs). However, those animals whose diet was suspensivorous or detritivorous—or those who lived on these—tended to survive, at least partially (e.g., benthic foraminifers and many bivalves, bryozoans, brachiopods, and fishes). In the terrestrial environment, the food chain is and was based on plants, so the herbivorous and carnivorous animals directly or indirectly dependent on this vegetation—all dinosaurs and many birds and mammals—became extinct. On the contrary, those terrestrial animals whose diet was detritivorous (e.g., insects), who were potentially scavengers (e.g., cocodilians, turtles) or insectivorous (ancestral mammals and many birds, amphibians and lepidosaurs) tended to survive.
The most widely accepted theory for explaining the K-T mass extinction event is the meteoritic impact theory proposed by Luis Alvarez (1968 Nobel Prize winner in physics), Walter Alvarez, Frank Asaro, and Helen Michels in 1980. They discovered a reddish-greenish sedimentary layer at Gubbio (Italy) that marked the boundary between Cretaceous and Tertiary sediments, and it contained an anomalous concentration of iridium— hundreds of times grater than normal. Iridium is extremely rare in the earth’s crust but abundant in chondritic and iron meteorites. For this reason, Alvarez’s team proposed that a collision with a large asteroid (about 10 kilometers in diameter) caused the K-T extinction event 65 million years ago and the deposition of the reddish-greenish layer or airfall layer at Gubbio. Almost at the same time that Alvarez’s team proposed this collision, Jan Smit suggested the same theory after studying the Caravaca section (Spain), which was a more expanded and continuous section than that at Gubbio. He proved that the airfall layer coincides with the largest, most sudden planktic foraminiferal mass extinction in evolutionary history, suggesting a clear relation of cause and effect between the impact event and the K-T catastrophic mass extinction. Since then similar airfall layers have been found at sections worldwide, such as the El Kef section, suggesting the totality of the K-T event.
The discovery of the ~180-kilometer-diameter Chicxulub crater on the northern Yucatan Peninsula (Mexico) supported strongly the Alvarez group’s theory, because such a crater was coincident in age with the K-T boundary and has just the size to have been formed by the predicted 10-kilometer-diameter meteorite. In addition, strange and thick K-T clastic sediments were discovered around the entire Gulf of Mexico, suggesting the destabilization of continental margins in the region as result of giant tsunamis and earthquakes generated from the impact point at Yucatan.
Most geologists and paleontologists agree with Alvarez’s impact theory, although alternative scenarios have been suggested for explaining the K-T extinction. They include sea-level regressions, increases of the volcanic activity linked to the Deccan Traps (an extensive basalt province in India), or multiple causes, that is, all the aforementioned causes operating at the same time. However, these hypotheses have not been accepted by the geologic and paleontologic communities, because they would be more compatible with uncorroborated gradual extinction patterns occurring hundreds of thousands of years before and after the K-T boundary.
According to Alvarez’s theory, the impact raised a vast dust cloud of fine ejecta in the atmosphere and triggered global firestorms due to the fall of incandescent melted fragments worldwide (tektites and microtektites). The dust and soot cloud generated a global atmospheric darkening, sudden climatic cooling and acid rain, a phenomenon known as “impact winter” (similar to a nuclear winter). The blockout of sunlight caused the cessation of all photosynthesis and the severe disruption of food chains, initiating the catastrophic mass extinction event at the K-T boundary. The dust and fine ejecta that covered the atmosphere after the Chicxulub impact were deposited slowly, probably over months or a few years, forming the K-T airfall layer worldwide.
Atmospheric carbon dioxide content increased rapidly as result of the emissions from the impact site and the widespread fires as well as the temporary cessation of photosynthesis and primary productivity. After the dust cloud settled, the high concentration of carbon dioxide initiated a greenhouse effect period that lasted about 10,000 or 15,000 years. The K-T boundary clay was deposited during this period of global decrease in primary productivity and of increased greenhouse effect after the impact winter. All postimpact paleo- environmental aftermaths of longer term (decrease in primary productivity, low biodiversity, greenhouse effect, disruption in the water column stratification, etc.) have been recorded in the K-T boundary clay worldwide. Only the most cosmopolitan, generalist, and opportunistic species were able to survive the environmental damage of both the impact winter and greenhouse effect periods.
Terrestrial plants and marine phytoplankton, and consequently primary productivity, recovered progressively over thousands of years, reestablishing the food chains and decreasing the concentration of carbon dioxide in the atmosphere. Many niches remained vacant after the K-T mass extinction, so the opportunistic surviving species began to occupy them. Small and new marine and land species appeared in the earliest Tertiary, following a model of “explosive” adaptive radiation. For instance, mammals diversified, evolving from the small insectivorous mammals that had survived the K-T event and becoming dominant on land. Similar adaptive radiations occurred in other marine and terrestrial groups. Thanks to the K-T extinction of the dinosaurs, the mammal groups could diversify during the Tertiary, including those that formed the evolutionary lineage of the primates that culminated with the appearance of the species Homo sapiens.
See also Chicxulub Crater; Cretaceous; Dinosaurs; Evolution, Organic; Extinction, Mass; Foraminifers; Fossil Record; Geologic Timescale; Geology; Nuclear Winter; Paleogene; Paleontology; Permian Extinction
Alvarez, L. W., Alvarez, W., Asaro, F., & Michel, H.V. (1980). Extraterrestrial cause for the Cretaceous- Tertiary extinction. Science, 208, 1095-1108.
Alvarez, W. (1997). T. rex and the crater of doom. Princeton, NJ: Princeton University Press.
Ryder, G., Fastowsky, D., & Gartner, S. (Eds.). (1996). The Cretaceous-Tertiary event and other catastrophes in Earth history. Geological Society of America Special Paper 307. Boulder, CO: Geological Society of America.
Smit, J., & Hertogen, J. (1980). An extraterrestrial event at the Cretaceous-Tertiary boundary. Nature, 285, 198-200.