Cretaceous–Paleogene boundary

The Cretaceous–Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K-T) boundary,[a] is a geological signature, usually a thin band of rock. K, the first letter of the German word Kreide (chalk), is the traditional abbreviation for the Cretaceous Period and Pg is the abbreviation for the Paleogene Period.

The K–Pg boundary marks the end of the Cretaceous Period, the last period of the Mesozoic Era, and marks the beginning of the Paleogene Period, the first period of the Cenozoic Era. Its age is usually estimated at around 66 Ma (million years ago),[2] with radiometric dating yielding a more precise age of 66.043 ± 0.011 Ma.[3]

The K–Pg boundary is associated with the Cretaceous–Paleogene extinction event, a mass extinction which destroyed a majority of the world's Mesozoic species, including all dinosaurs except for birds.[4]

Strong evidence exists that the extinction coincided with a large meteorite impact at the Chicxulub crater and the generally accepted scientific theory is that this impact triggered the extinction event.

KT boundary 054
Badlands near Drumheller, Alberta, Canada, where glacial and post-glacial erosion have exposed the K–Pg boundary
Cretaceous Paleogene clay at Geulhemmergroeve
Complex Cretaceous-Paleogene clay layer (gray) in the Geulhemmergroeve tunnels near Geulhem, the Netherlands. Finger is on the actual K–Pg boundary.

Possible causes

Alvarez impact hypothesis

Stevns klint
Cliffs at Stevns, Denmark; highest iridium occurrence in Alvarez analysis.
The K–Pg boundary exposure in Trinidad Lake State Park, in the Raton Basin of Colorado, USA, shows an abrupt change from dark- to light-colored rock.
White line added to mark the transition.

In 1980, a team of researchers consisting of Nobel Prize-winning physicist Luis Alvarez, his son, geologist Walter Alvarez, and chemists Frank Asaro and Helen Michel discovered that sedimentary layers found all over the world at the K–Pg boundary contain a concentration of iridium many times greater than normal (30 times the average crustal content in Italy and 160 times at Stevns on the Danish island of Zealand).[5] Iridium is extremely rare in the earth's crust because it is a siderophile element, and therefore most of it sank with iron into the earth's core during planetary differentiation. As iridium remains abundant in most asteroids and comets, the Alvarez team suggested that an asteroid struck the earth at the time of the K–Pg boundary.[5] There were other earlier speculations on the possibility of an impact event, but no evidence had been uncovered at that time.[6]

The evidence for the Alvarez impact theory is supported by chondritic meteorites and asteroids which have an iridium concentration of ~455 parts per billion,[7] much higher than ~0.3 parts per billion typical of the Earth's crust.[5] Chromium isotopic anomalies found in Cretaceous–Paleogene boundary sediments are similar to those of an asteroid or a comet composed of carbonaceous chondrites. Shocked quartz granules and tektite glass spherules, indicative of an impact event, are also common in the K–Pg boundary, especially in deposits from around the Caribbean. All of these constituents are embedded in a layer of clay, which the Alvarez team interpreted as the debris spread all over the world by the impact.[5]

Using estimates of the total amount of iridium in the K–Pg layer, and assuming that the asteroid contained the normal percentage of iridium found in chondrites, the Alvarez team went on to calculate the size of the asteroid. The answer was about 10 km (6.2 mi) in diameter, about the size of Manhattan.[5] Such a large impact would have had approximately the energy of 100 trillion tons of TNT, or about 2 million times greater than the most powerful thermonuclear bomb ever tested.

One of the consequences of such an impact is a dust cloud which would block sunlight and inhibit photosynthesis for a few years. This would account for the extinction of plants and phytoplankton and of organisms dependent on them (including predatory animals as well as herbivores). However, small creatures whose food chains were based on detritus might have still had a reasonable chance of survival. Vast amounts of sulfuric acid aerosols were ejected into the stratosphere as a result of the impact, leading to a 10–20% reduction in sunlight reaching the Earth's surface. It would have taken at least ten years for those aerosols to dissipate.[8][9]

Global firestorms may have resulted as incendiary fragments from the blast fell back to Earth. Analyses of fluid inclusions in ancient amber suggest that the oxygen content of the atmosphere was very high (30–35%) during the late Cretaceous. This high O
level would have supported intense combustion. The level of atmospheric O
plummeted in the early Paleogene Period. If widespread fires occurred, they would have increased the CO
content of the atmosphere and caused a temporary greenhouse effect once the dust cloud settled, and this would have exterminated the most vulnerable survivors of the "long winter".[8]

The impact may also have produced acid rain, depending on what type of rock the asteroid struck. However, recent research suggests this effect was relatively minor. Chemical buffers would have limited the changes, and the survival of animals vulnerable to acid rain effects (such as frogs) indicates that this was not a major contributor to extinction. Impact theories can only explain very rapid extinctions, since the dust clouds and possible sulphuric aerosols would wash out of the atmosphere in a fairly short time—possibly under ten years.[10]

Chicxulub Crater

Chicxulub crater
Chicxulub impact structure
Yucatan chix crater
Imaging from NASA's Shuttle Radar Topography Mission STS-99 reveals part of the 180 km (110 mi) diameter ring of the crater. The numerous sinkholes clustered around the trough of the crater suggest a prehistoric oceanic basin in the depression left by the impact.[11]
Impact crater/structure
Diameter150 km (93 mi)
Depth20 km (12 mi)
Impactor diameter10–15 kilometres (6.2–9.3 mi)
Age66.043 ± 0.011 Ma
Cretaceous–Paleogene boundary[12]
Bolide typeCarbonaceous chondrite
Coordinates21°24′0″N 89°31′0″W / 21.40000°N 89.51667°WCoordinates: 21°24′0″N 89°31′0″W / 21.40000°N 89.51667°W
Country Mexico
Chicxulub crater is located in North America
Chicxulub crater
Chicxulub crater
Location of Chicxulub crater

When it was originally proposed, one issue with the "Alvarez hypothesis" (as it came to be known) had been that no documented crater matched the event. This was not a lethal blow to the theory; while the crater resulting from the impact would have been larger than 250 km (160 mi) in diameter, Earth's geological processes hide or destroy craters over time.[13]

The Chicxulub crater ( /ˈtʃiːkʃʊluːb/; Mayan: [tʃʼikʃuluɓ]) is an impact crater buried underneath the Yucatán Peninsula in Mexico.[14] Its center is located near the town of Chicxulub, after which the crater is named.[15] It was formed by a large asteroid or comet about 10 to 15 kilometres (6.2 to 9.3 miles) in diameter,[16][17] the Chicxulub impactor, striking the Earth. The date of the impact coincides precisely with the Cretaceous–Paleogene boundary (K–Pg boundary), slightly less than 66 million years ago,[12] and a widely accepted theory is that worldwide climate disruption from the event was the cause of the Cretaceous–Paleogene extinction event, a mass extinction in which 75% of plant and animal species on Earth suddenly became extinct, including all non-avian dinosaurs.

The crater is estimated to be over 150 kilometres (93 miles) in diameter[14] and 20 km (12 mi) in depth, well into the continental crust of the region of about 10–30 km (6.2–18.6 mi) depth. It makes the feature the second of the largest confirmed impact structures on Earth, and the only one whose peak ring is intact and directly accessible for scientific research.[18]

The crater was discovered by Antonio Camargo and Glen Penfield, geophysicists who had been looking for petroleum in the Yucatán during the late 1970s. Penfield was initially unable to obtain evidence that the geological feature was a crater and gave up his search. Later, through contact with Alan Hildebrand in 1990, Penfield obtained samples that suggested it was an impact feature. Evidence for the impact origin of the crater includes shocked quartz,[19] a gravity anomaly, and tektites in surrounding areas.

In 2016, a scientific drilling project drilled deep into the peak ring of the impact crater, hundreds of meters below the current sea floor, to obtain rock core samples from the impact itself. The discoveries were widely seen as confirming current theories related to both the crater impact and its effects.

The shape and location of the crater indicate further causes of devastation in addition to the dust cloud. The asteroid landed right on the coast and would have caused gigantic tsunamis, for which evidence has been found all around the coast of the Caribbean and eastern United States—marine sand in locations which were then inland, and vegetation debris and terrestrial rocks in marine sediments dated to the time of the impact.

The asteroid landed in a bed of anhydrite (CaSO
) or gypsum (CaSO4·2(H2O)), which would have ejected large quantities of sulfur trioxide SO
that combined with water to produce a sulfuric acid aerosol. This would have further reduced the sunlight reaching the Earth's surface and then over several days, precipitated planet-wide as acid rain, killing vegetation, plankton and organisms which build shells from calcium carbonate (coccolithophorids and molluscs).[20]

Deccan Traps

Before 2000, arguments that the Deccan Traps flood basalts caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68 Ma and lasted for over 2 million years. However, there is evidence that two thirds of the Deccan Traps were created within 1 million years about 65.5 Ma, so these eruptions would have caused a fairly rapid extinction, possibly a period of thousands of years, but still a longer period than what would be expected from a single impact event.[21][22]

The Deccan Traps could have caused extinction through several mechanisms, including the release of dust and sulphuric aerosols into the air which might have blocked sunlight and thereby reduced photosynthesis in plants. In addition, Deccan Trap volcanism might have resulted in carbon dioxide emissions which would have increased the greenhouse effect when the dust and aerosols cleared from the atmosphere.[22]

In the years when the Deccan Traps theory was linked to a slower extinction, Luis Alvarez (who died in 1988) replied that paleontologists were being misled by sparse data. While his assertion was not initially well-received, later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions that produced the Indian Deccan Traps, and these may have contributed to the extinctions.[23]

Multiple impact event

Several other craters also appear to have been formed about the time of the K–Pg boundary. This suggests the possibility of nearly simultaneous multiple impacts, perhaps from a fragmented asteroidal object, similar to the Shoemaker–Levy 9 cometary impact with Jupiter. Among these are the Boltysh crater, a 24 km (15 mi) diameter impact crater in Ukraine (65.17 ± 0.64 Ma); and the Silverpit crater, a 20 km (12 mi) diameter impact crater in the North Sea (60–65 Ma). Any other craters that might have formed in the Tethys Ocean would have been obscured by erosion and tectonic events such as the relentless northward drift of Africa and India.[24][25][26]

A very large structure in the sea floor off the west coast of India was interpreted in 2006 as a crater by three researchers.[27] The potential Shiva crater, 450–600 km (280–370 mi) in diameter, would substantially exceed Chicxulub in size and has been estimated to be about 66 mya, an age consistent with the K–Pg boundary. An impact at this site could have been the triggering event for the nearby Deccan Traps.[28] However, this feature has not yet been accepted by the geologic community as an impact crater and may just be a sinkhole depression caused by salt withdrawal.[26]

Maastrichtian marine regression

Clear evidence exists that sea levels fell in the final stage of the Cretaceous by more than at any other time in the Mesozoic era. In some Maastrichtian stage rock layers from various parts of the world, the later ones are terrestrial; earlier ones represent shorelines and the earliest represent seabeds. These layers do not show the tilting and distortion associated with mountain building; therefore, the likeliest explanation is a regression, that is, a buildout of sediment, but not necessarily a drop in sea level. No direct evidence exists for the cause of the regression, but the explanation which is currently accepted as the most likely is that the mid-ocean ridges became less active and therefore sank under their own weight as sediment from uplifted orogenic belts filled in structural basins.[29][30]

A severe regression would have greatly reduced the continental shelf area, which is the most species-rich part of the sea, and therefore could have been enough to cause a marine mass extinction. However, research concludes that this change would have been insufficient to cause the observed level of ammonite extinction. The regression would also have caused climate changes, partly by disrupting winds and ocean currents and partly by reducing the Earth's albedo and therefore increasing global temperatures.[31]

Marine regression also resulted in the reduction in area of epeiric seas, such as the Western Interior Seaway of North America. The reduction of these seas greatly altered habitats, removing coastal plains that ten million years before had been host to diverse communities such as are found in rocks of the Dinosaur Park Formation. Another consequence was an expansion of freshwater environments, since continental runoff now had longer distances to travel before reaching oceans. While this change was favorable to freshwater vertebrates, those that prefer marine environments, such as sharks, suffered.[32]

Supernova hypothesis

Another discredited cause for the K–Pg extinction event is cosmic radiation from a nearby supernova explosion. An iridium anomaly at the boundary is consistent with this hypothesis. However, analysis of the boundary layer sediments failed to find 244
,[33] a supernova byproduct which is the longest-lived plutonium isotope, with a half-life of 81 million years.

Multiple causes

It is possible that more than one of these hypotheses may be a partial solution to the mystery, and that more than one of these events may have occurred. Both the Deccan Traps and the Chicxulub impact may have been important contributors. For example, the most recent dating of the Deccan Traps supports the idea that rapid eruption rates in the Deccan Traps may have been triggered by large seismic waves radiated by the impact.

See also

References and notes

  1. ^ This former designation has as a part of it a term, 'Tertiary' (abbreviated as T), that is now discouraged as a formal geochronological unit by the International Commission on Stratigraphy.[1]
  1. ^ Gradstein, Felix M.; Ogg, James G.; Smith, Alan G., eds. (2004). A geologic time scale 2004. Cambridge, UK: Cambridge University Press. ISBN 978-0-521-78142-8.
  2. ^ "International Chronostratigraphic Chart" (PDF). International Commission on Stratigraphy. 2012. Archived from the original (PDF) on 2013-07-17. Retrieved 2013-12-18.
  3. ^ Renne; et al. (2013). "Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary". Science. 339 (6120): 684–7. Bibcode:2013Sci...339..684R. doi:10.1126/science.1230492. PMID 23393261.
  4. ^ Fortey, R (1999). Life: A Natural History of the First Four Billion Years of Life on Earth. Vintage. pp. 238–260. ISBN 978-0-375-70261-7.
  5. ^ a b c d e Alvarez, LW; Alvarez, W; Asaro, F & Michel, HV (1980). "Extraterrestrial cause for the Cretaceous–Tertiary extinction". Science. 208 (4448): 1095–1108. Bibcode:1980Sci...208.1095A. CiteSeerX doi:10.1126/science.208.4448.1095. PMID 17783054.
  6. ^ De Laubenfels, MW (1956). "Dinosaur Extinctions: One More Hypothesis". Journal of Paleontology. 30 (1): 207–218. Archived from the original on 2007-09-28. Retrieved 2007-05-22.
  7. ^ W. F. McDonough; S.-s. Sun (1995). "The composition of the Earth". Chemical Geology. 120 (3–4): 223–253. Bibcode:1995ChGeo.120..223M. doi:10.1016/0009-2541(94)00140-4.
  8. ^ a b Pope, K.O.; Baines, K.H.; Ocampo, A.C. & Ivanov, B.A. (1997). "Energy, volatile production, and climatic effects of the Chicxulub Cretaceous/Tertiary impact". Journal of Geophysical Research. 102 (E9): 21645–64. Bibcode:1997JGR...10221645P. doi:10.1029/97JE01743. PMID 11541145.
  9. ^ Ocampo, Adriana; Vajda, Vivi; Buffetaut, Eric (2006). "Unravelling the Cretaceous–Paleogene (KT) Turnover, Evidence from Flora, Fauna and Geology". In Cockell, Charles; Gilmour, Iain; Koeberl, Christian (eds.). Biological Processes Associated with Impact Events. Springer. pp. 197–219. doi:10.1007/b135965. ISBN 978-3-540-25735-6.
  10. ^ Kring, DA (2003). "Environmental consequences of impact cratering events as a function of ambient conditions on Earth". Astrobiology. 3 (1): 133–152. Bibcode:2003AsBio...3..133K. doi:10.1089/153110703321632471. PMID 12809133.
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  12. ^ a b Renne, P. R.; Deino, A. L.; Hilgen, F. J.; Kuiper, K. F.; Mark, D. F.; Mitchell, W. S.; Morgan, L. E.; Mundil, R.; Smit, J. (2013). "Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary" (PDF). Science. 339 (6120): 684–687. Bibcode:2013Sci...339..684R. doi:10.1126/science.1230492. ISSN 0036-8075. PMID 23393261.
  13. ^ Keller, G, Adatte, T, Stinnesbeck, W, Rebolledo-Vieyra, Fucugauchi, JU, Kramar,U, & Stüben, D (2004). "Chicxulub impact predates the K-T boundary mass extinction". PNAS. 101 (11): 3753–3758. Bibcode:2004PNAS..101.3753K. doi:10.1073/pnas.0400396101. PMC 374316. PMID 15004276.CS1 maint: multiple names: authors list (link)
  14. ^ a b "Chicxulub". Earth Impact Database. Planetary and Space Science Centre University of New Brunswick Fredericton. Retrieved December 30, 2008.
  15. ^ Penfield, Glen. Interview: The Dinosaurs: Death of the Dinosaur. 1992, WHYY.
  16. ^ Schulte, P.; Alegret, L.; Arenillas, I.; et al. (2010). "The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary" (PDF). Science. 327 (5970): 1214–18. Bibcode:2010Sci...327.1214S. doi:10.1126/science.1177265. ISSN 0036-8075. PMID 20203042. Archived from the original (PDF) on December 9, 2011. Retrieved 9 December 2016.
  17. ^ Amos, Jonathan (May 15, 2017). "Dinosaur asteroid hit 'worst possible place'". BBC News – via
  18. ^ St. Fleur, Nicholas (17 November 2016). "Drilling Into the Chicxulub Crater, Ground Zero of the Dinosaur Extinction". The New York Times. Retrieved 4 November 2017.
  19. ^ Becker, Luann (2002). "Repeated Blows" (PDF). Scientific American. 286 (3): 76–83. Bibcode:2002SciAm.286c..76B. doi:10.1038/scientificamerican0302-76. PMID 11857903. Retrieved January 28, 2016.
  20. ^ Dinosaur-Killing Asteroid Triggered Lethal Acid Rain, Livescience, March 09, 2014
  21. ^ Hofman, C, Féraud, G & Courtillot, V (2000). "40Ar/39Ar dating of mineral separates and whole rocks from the Western Ghats lava pile: further constraints on duration and age of the Deccan traps". Earth and Planetary Science Letters. 180 (1–2): 13–27. Bibcode:2000E&PSL.180...13H. doi:10.1016/S0012-821X(00)00159-X.CS1 maint: multiple names: authors list (link)
  22. ^ a b Duncan, RA; Pyle, DG (1988). "Rapid eruption of the Deccan flood basalts at the Cretaceous/Tertiary boundary". Nature. 333 (6176): 841–843. Bibcode:1988Natur.333..841D. doi:10.1038/333841a0.
  23. ^ Alvarez, W (1997). T. rex and the Crater of Doom. Princeton University Press. pp. 130–146. ISBN 978-0-691-01630-6.
  24. ^ Mullen, L (October 13, 2004). "Debating the Dinosaur Extinction". Astrobiology Magazine. Retrieved 2007-07-11.
  25. ^ Mullen, L (October 20, 2004). "Multiple impacts". Astrobiology Magazine. Retrieved 2007-07-11.
  26. ^ a b Mullen, L (November 3, 2004). "Shiva: Another K–T impact?". Astrobiology Magazine. Retrieved 2007-07-11.
  27. ^ Chatterjee, S; Guven, N; Yoshinobu, A & Donofrio, R (2006). "Shiva structure: a possible K-Pg boundary impact crater on the western shelf of India" (PDF). Special Publications of the Museum of Texas Tech University (50). Retrieved 2007-06-15.
  28. ^ Chatterjee, S; Guven, N; Yoshinobu, A & Donofrio, R (2003). "The Shiva Crater: Implications for Deccan Volcanism, India-Seychelles rifting, dinosaur extinction, and petroleum entrapment at the KT Boundary". Geological Society of America Abstracts with Programs. 35 (6): 168. Retrieved 2007-08-02.
  29. ^ MacLeod, N.; Rawson, P.F.; et al. (1997). "The Cretaceous–Tertiary biotic transition". Journal of the Geological Society. 154 (2): 265–292. Bibcode:1997JGSoc.154..265M. doi:10.1144/gsjgs.154.2.0265. ISSN 0016-7649.
  30. ^ Liangquan, Li; Keller, Gerta (1998). "Abrupt deep-sea warming at the end of the Cretaceous". Geology. 26 (11): 995–8. Bibcode:1998Geo....26..995L. doi:10.1130/0091-7613(1998)026<0995:ADSWAT>2.3.CO;2.
  31. ^ Marshall, C. R.; Ward, PD (1996). "Sudden and Gradual Molluscan Extinctions in the Latest Cretaceous of Western European Tethys". Science. 274 (5291): 1360–1363. Bibcode:1996Sci...274.1360M. doi:10.1126/science.274.5291.1360. PMID 8910273.
  32. ^ Archibald, J. David; Fastovsky, David E. (2004). "Dinosaur Extinction". In Weishampel, David B.; Dodson, Peter; Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 672–684. ISBN 978-0-520-24209-8.
  33. ^ Ellis, J; Schramm, DN (1995). "Could a Nearby Supernova Explosion have Caused a Mass Extinction?". Proceedings of the National Academy of Sciences. 92 (1): 235–238. arXiv:hep-ph/9303206. Bibcode:1995PNAS...92..235E. doi:10.1073/pnas.92.1.235. PMC 42852. PMID 11607506.

External links

Further reading


Alamitophis is a genus of fossil snakes in the extinct family of Madtsoiidae. Its length is estimated at 80 centimetres (2.6 ft) and it probably fed on frogs, lizards, and small mammals. It is found in Australia (Tingamarra Fauna, after which A. tingamarra is named) and Argentina (Allen, La Colonia and Los Alamitos Formations, after which the genus is named).

Alvarez hypothesis

The Alvarez hypothesis posits that the mass extinction of the dinosaurs and many other living things during the Cretaceous–Paleogene extinction event was caused by the impact of a large asteroid on the Earth. Prior to 2013, it was commonly cited as having happened about 65 million years ago, but Renne and colleagues (2013) gave an updated value of 66 million years. Evidence indicates that the asteroid fell in the Yucatán Peninsula, at Chicxulub, Mexico. The hypothesis is named after the father-and-son team of scientists Luis and Walter Alvarez, who first suggested it in 1980. Shortly afterwards, and independently, the same was suggested by Dutch paleontologist Jan Smit.

In March 2010, an international panel of scientists endorsed the asteroid hypothesis, specifically the Chicxulub impact, as being the cause of the extinction. A team of 41 scientists reviewed 20 years of scientific literature and in so doing also ruled out other theories such as massive volcanism. They had determined that a 10–15 km (6–9 mi) space rock hurtled into earth at Chicxulub. For comparison, the Martian moon Phobos is 11 km (7 mi) and Mount Everest is just under 9 km (5.6 mi). The collision would have released the same energy as 100,000,000 megatonnes of TNT (4.2×1023 J), over a billion times the energy of the atomic bombs dropped on Hiroshima and Nagasaki.A 2016 drilling project into the peak ring of the crater strongly supported the hypothesis, and confirmed various matters that had been unclear until that point. These included the fact that the peak ring comprised granite (a rock found deep within the earth) rather than typical sea floor rock, which had been shocked, melted, and ejected to the surface in minutes, and evidence of colossal seawater movement directly afterwards from sand deposits. Crucially the cores also showed a near complete absence of gypsum, a sulfate-containing rock, which would have been vaporized and dispersed as an aerosol into the atmosphere, confirming the presence of a probable link between the impact and global longer-term effects on the climate and food chain.

Boltysh crater

The Boltysh crater or Bovtyshka crater is an impact crater in the Kirovohrad Oblast of Ukraine, near the village of Bovtyshka. The crater is 24 kilometres (15 mi) in diameter and its age of 65.17 ± 0.64 million years, based on argon dating techniques, is within error of that of Chicxulub crater in Mexico and of the Cretaceous–Paleogene boundary (K–Pg boundary). The Chicxulub impact is believed to have caused the mass extinction at the end of the Cretaceous period, which included the extinction of the dinosaurs. The Boltysh impact likely occurred several thousand years before Chicxulub, suggesting the extinction event may have been driven by multiple meteor strikes over an extended period of time about 65 million years ago.

Chicxulub impactor

The Chicxulub impactor ( CHEEK-shə-loob), also known as the K/Pg impactor and (more speculatively) as the Chicxulub asteroid, was an asteroid or other celestial body some 11 to 81 kilometres (7 to 50 mi) in diameter and having a mass between 1.0×1015 and 4.6×1017 kg, which struck the Earth at the end of the Cretaceous period, 66 million years ago, creating the Chicxulub crater. It impacted a few miles from the present-day town of Chicxulub in Mexico, after which the impactor and its crater are named. Because the estimated date of the object's impact and the Cretaceous–Paleogene boundary (K–Pg boundary) coincide, there is a scientific consensus that its impact was the Cretaceous–Paleogene extinction event which caused the death of the planet's non-avian dinosaurs and many other species.The impactor's crater is over 150 kilometres (93 miles) in diameter making it the one of the largest known impact craters on Earth.


Cimolestes ("Cretaceous robber") is a genus of basal, non-placental eutherians. Fossils have been found in North America, where they first appeared during the Late Cretaceous; they died out at the start of the Paleocene.They were once considered to be marsupials, then primitive placental mammals, but now are considered to be members of the order Cimolesta (which was named after the genus), outside of placental mammals proper. Before they were determined to be non-placental eutherians, the cimolestids were once considered the common ancestral group of the clades Carnivora and the extinct Creodonta.

Cimolestes in particular follows as the direct outgroup to Taeniodonta, indicating that the latter evolved from forms similar to it.

Climate across Cretaceous–Paleogene boundary

The climate across the Cretaceous–Paleogene boundary (K–Pg or formerly the K–T boundary) is very important to geologic time as it marks a catastrophic global extinction event. Numerous theories have been proposed as to why this extinction event happened including an asteroid known as the Chicxulub asteroid, volcanism, or sea level changes. While the mass extinction is well documented, there is much debate about the immediate and long-term climatic and environmental changes caused by the event. The terrestrial climates at this time are poorly known, which limits the understanding of environmentally driven changes in biodiversity that occurred before the Chicxulub crater impact. Oxygen isotopes across the K–T boundary suggest that oceanic temperatures fluctuated in the Late Cretaceous and through the boundary itself. Carbon isotope measurements of benthic foramifinera at the K–T boundary suggest rapid, repeated fluctuations in oceanic productivity in the 3 million years before the final extinction, and that productivity and ocean circulation ended abruptly for at least tens of thousands of years just after the boundary, indicating devastation of terrestrial and marine ecosystems. Some researchers suggest that climate change is the main connection between the impact and the extinction. The impact perturbed the climate system with long-term effects that were much worse than the immediate, direct consequences of the impact.

Cretaceous Research

Cretaceous Research is a bimonthly peer-reviewed scientific journal published by Elsevier. The journal focuses on topics dealing with the Cretaceous period and the Cretaceous–Paleogene boundary.


Cynodontosuchus is an extinct genus of baurusuchid mesoeucrocodylian. Fossils have been found from Argentina of Late Cretaceous age from the Bajo de la Carpa Formation (dating back to the Santonian) as well as the Pichi Picun Leufu Formation (dating back to the Coniacian and Santonian). Fossils also have been found in the Tiupampan Santa Lucía Formation of Bolivia.

Denver Formation

The Denver Formation is a geological formation that is present within the central part of the Denver Basin that underlies the Denver, Colorado, area. It ranges in age from latest Cretaceous (Maastrichtian) to early Paleocene, and includes sediments that were deposited before, during and after the Cretaceous-Paleogene boundary event.The formation is known for its paleontological resources, including dinosaur remains that are found in the Late Cretaceous part of the formation, and it includes aquifers that are important sources of water for the area.

Edmonton Group

The Edmonton Group is a Late Cretaceous (Campanian stage) to early Paleocene stratigraphic unit of the Western Canada Sedimentary Basin in the central Alberta plains. It was first described as the Edmonton Formation by Joseph Burr Tyrrell in 1887 based on outcrops along the North Saskatchewan River in and near the city of Edmonton. E.J.W. Irish later elevated the formation to group status and it was subdivided into four separate formations. In ascending order, they are the Horseshoe Canyon, Whitemud, Battle and Scollard Formations. The Cretaceous-Paleogene boundary occurs within the Scollard Formation, based on dinosaurian and microfloral evidence, as well as the presence of the terminal Cretaceous iridium anomaly.


Euclastes is an extinct genus of sea turtles that survived the Cretaceous–Paleogene mass extinction. The genus was first named by Edward Drinker Cope in 1867, and contains three species. E. hutchisoni, was named in 2003 but has since been reassigned to the genus Pacifichelys, while E. coahuilaensis named in 2009 was reassigned as Mexichelys coahuilaensis in 2010.

Fort Union Formation

The Fort Union Formation is a geologic unit containing sandstones, shales, and coal beds in Wyoming, Montana, and parts of adjacent states. In the Powder River Basin, it contains important economic deposits of coal, uranium, and coalbed methane.


Hainina is an extinct mammal genus from the latest Cretaceous to the Paleocene of Europe. Though small, it outsurvived the final dinosaurs.


Hoploparia is a genus of fossil lobster belonging to the family Nephropidae. The type species of this genus is Hoploparia longimana.

These epifaunal carnivores lived from the Jurassic to the Paleogene period (from 201.6 to 28.4 Ma). Fossils of this genus have been found in sediments of Europe, Argentina, Madagascar, Canada and United States.


Hyposaurus is a genus of extinct marine dyrosaurid crocodyliform. Fossils have been found in Paleocene aged rocks of the Maria Farinha Formation in Pernambuco, Brazil, Iullemmeden Basin in West Africa,

?Campanian–Maastrichtian (Late Cretaceous) Shendi Formation of Sudan and Maastrichtian (Late Cretaceous) through Danian (Early Paleocene) strata in New Jersey, Alabama and South Carolina. Isolated teeth comparable to Hyposaurus have also been found in Thanetian (Late Paleocene) strata of Virginia. It was related to Dyrosaurus, and is the only dyrosaurid known from the western hemisphere.

The priority of the species H. rogersii has been debated, however there is no sound basis for the recognition of more than one species from North America. The other North American species (i.e. H. fraterculus, H. ferox and H. natator) are therefore considered nomina vanum (i.e. empty names).


Palaeophiidae is an extinct family of marine snake belonging to the superfamily Alethinophidia.

Species within this genus lived from the Late Cretaceous to the Late Eocene, approximately from 70.6 to 33.9 million years ago.

Shiva crater

The Shiva Crater is a geologic structure, which is hypothesized by Sankar Chatterjee and colleagues to be a 500-kilometre (310 mi) diameter impact structure. This geologic structure consists of the Bombay High and Surat Depression. They lie beneath the Indian continental shelf and the Arabian Sea west of Mumbai, India. Chatterjee named this structure after Shiva, the Hindu god of destruction and renewal.

Trinidad Lake State Park

Trinidad Lake State Park is a state park 4 miles (6.4 km) west of Trinidad, Colorado, United States. The park protects Trinidad Lake, a dammed reservoir. There are hiking trails, and camping and boating opportunities. The park features historical attractions such as the coal mining ruins at Cokedale. An exposure of the Cretaceous–Paleogene boundary (K–Pg boundary) is visible in the southern part of the park.

A portion of the mountain route of the Santa Fe Trail runs through the park.


Yaminuechelys is an extinct genus of chelid turtle from Argentina. The genus first appeared during the Late Cretaceous, and then becomes extinct during the Late Paleocene.

Proposed K–Pg boundary craters
≥20 km diameter


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