Cenomanian-Turonian boundary event

The Cenomanian-Turonian boundary event, or the Cenomanian-Turonian extinction event, the Cenomanian-Turonian anoxic event (OAE 2), and referred also as the Bonarelli Event,[2] was one of two anoxic extinction events in the Cretaceous period. (The other being the earlier Selli Event, or OAE 1a, in the Aptian.[3]) The OAE 2 occurred approximately 91.5 ± 8.6 Ma,[4] though other estimates are given as 93–94 Ma.[5] The Cenomanian-Turonian boundary has recently been refined to 93.9 ± 0.15 Ma[6] There was a large carbon disturbance during this time period. However, apart from the carbon cycle disturbance, there were also large disturbances in the oxygen and sulfur cycles of the ocean.

The event brought about the extinction of the Pliosauridae, and most Ichthyosauria. Coracoids of Maastrichtian age were once interpreted by some authors as belonging to ichthyosaurs, but these have since been interpreted as plesiosaur elements instead.[7] Although the cause is still uncertain, the result starved the Earth's oceans of oxygen for nearly half a million years, causing the extinction of approximately 27 percent of marine invertebrates, including certain planktic and benthic foraminifera, mollusks, bivalves, dinoflagellates and calcareous nannofossils.[8] The global environmental disturbance that resulted in these conditions increased atmospheric and oceanic temperatures. Boundary sediments show an enrichment of trace elements, and contain elevated δ13C values.[9]

The Cenomanian and Turonian stages were first noted by D'Orbigny between 1843 and 1852. The global type section for this boundary is located in the Bridge Creek Limestone Member of the Greenhorn formation near Pueblo, Colorado, which are bedded with the Milankovitch orbital signature. Here, a positive carbon-isotope event is clearly shown, although none of the characteristic, organic-rich black shale is present. It has been estimated that the isotope shift lasted approximately 850 kyrs longer than the black shale event, which may be the cause of this anomaly in the Colorado type-section.[10] A significantly expanded OAE2 interval from southern Tibet documents a complete, more detailed, and finer-scale structures of the positive carbon isotope excursion that contains multiple shorter-term carbon isotope stages amounting to a total duration of 820±25 kyrs.[11]

The boundary is also known as the Bonarelli event because of 1- to 2-meter layer of thick black shale that marks the boundary and was first studied by Guido Bonarelli in 1891.[12] It is characterized by interbedded black shale, chert and radiolarian sands is estimated to span a 400,000-year interval. Planktic foraminifera do not exist in this Bonarelli level, and the presence of radiolarians in this section indicates relatively high productivity and an availability of nutrients.

One possible cause of this event is sub-oceanic volcanism, possibly the Caribbean large igneous province, with increased activity approximately 500,000 years earlier. During that period, the rate of crustal production reached its highest level for 100 million years. This was largely caused by the widespread melting of hot mantle plumes under the oceans at the base of the lithosphere. This resulted in the thickening of the oceanic crust in the Pacific and Indian Oceans. This volcanism would have sent large quantities of carbon dioxide into the atmosphere, leading to global warming. Within the oceans, the emission of SO2, H2S, CO2, and halogens would have increased the acidity of the water, causing the dissolution of carbonate, and a further release of carbon dioxide. When the volcanic activity declined, this run-away greenhouse effect would have likely been put into reverse. The increased CO2 content of the oceans could have increased organic productivity in the ocean surface waters. The consumption of this newly abundant organic life by aerobic bacteria would produce anoxia and mass extinction.[8] The resulting elevated levels of carbon burial would account for the black shale deposition in the ocean basins.[9]

System/
Period
Series/
Epoch
Stage/
Age
Age (Ma)
Paleogene Paleocene Danian younger
Cretaceous Upper/
Late
Maastrichtian 66.0 72.1
Campanian 72.1 83.6
Santonian 83.6 86.3
Coniacian 86.3 89.8
Turonian 89.8 93.9
Cenomanian 93.9 100.5
Lower/
Early
Albian 100.5 ~113.0
Aptian ~113.0 ~125.0
Barremian ~125.0 ~129.4
Hauterivian ~129.4 ~132.9
Valanginian ~132.9 ~139.8
Berriasian ~139.8 ~145.0
Jurassic Upper/
Late
Tithonian older
Subdivision of the Cretaceous system
according to the ICS, as of 2017.[1]

The δ13C isotope excursion

The positive δ13C isotope excursion found at the Cenomanian-Turonian boundary is one of the main carbon isotope events of the Mesozoic. It represents one of the largest disturbances in the global carbon cycle from the past 110 million years. This δ13C isotope excursion indicates a significant increase in the burial rate of organic carbon, indicating the widespread deposition and preservation of organic carbon-rich sediments and that the ocean was depleted of oxygen at the time.[13][14][15] Within the positive carbon isotope excursion, short eccentricity scale carbon isotope variability is documented in a significantly expanded OAE2 interval from southern Tibet.[11]

Large igneous provinces and their possible contribution

Several independent LIP events occurred around the time of OAE2. Within the time period from 90–95 million years ago, two separate LIP events occurred—the Madagascar and the Caribbean-Colombian. Trace metals such as chromium, scandium, copper and cobalt have been found at the C-T boundary and this suggests that an LIP could have been involved in the occurrence of the event.[16] The timing of the peak in trace metal concentration coincides with the middle of the anoxic event, suggesting that whilst the LIP may have occurred during but not have initiated the event. Other studies linked the lead isotopes of OAE-2 to the Caribbean-Colombian and the Madagascar LIPs.[17] A modeling study performed in 2011 also confirmed that it is possible that a LIP may have been initiated the event, as the model revealed that the peak amount of carbon dioxide degassing from volcanic LIP degassing could have resulted in more than 90% global deep ocean anoxia.[18]

Changes in oceanic biodiversity and its implications

The alterations in diversity of various marine invertebrate species such as calcareous nannofossils indicate a time when the oceans were warm and oligotrophic, in an environment with short spikes of productivity followed by long periods of low fertility. A study performed in the Cenomanian-Turonian boundary of Wunstorf, Germany, reveal the uncharacteristic dominance of a calcareous nannofossil species, Watznaueria, present during the event. Unlike the Biscutum species, which prefer mesotrophic conditions and were generally the dominant species before and after the C/T Boundary event; Watznaueria species prefer warm, oligotrophic conditions.[19]

At the time, there were also peak abundances of green algal groups Botryococcus and prasinophytes, coincident with pelagic sedimentation. The abundances of these algal groups are strongly related to the increase of both the oxygen deficiency in the water column and the total organic carbon content. The evidence from these algal groups suggest that there were episodes of halocline stratification of the water column during the time. A species of freshwater dinocyst—the Bosedinia was also found in the rocks dated to the time and these suggest that the oceans had reduced salinity.[20]

See also

References

  1. ^ Super User. "ICS - Chart/Time Scale". www.stratigraphy.org.
  2. ^ Cetean, Claudia G.; Balc, Ramona; Kaminski, Michael A.; Filipescu, Sorin (August 2008). "Biostratigraphy of the Cenomanian-Turonian boundary in the Eastern Carpathians (Dâmboviţa Valley): preliminary observations". Studia Universitatis Babes-Bolyai, Geologia. 53 (1): 11–23. doi:10.5038/1937-8602.53.1.2.
  3. ^ Li, Yong-Xiang; Bralower, Timothy J.; Montañez, Isabel P.; Osleger, David A.; Arthur, Michael A.; Bice, David M.; Herbert, Timothy D.; Erba, Elisabetta; Premoli Silva, Isabella (2008-07-15). "Toward an orbital chronology for the early Aptian Oceanic Anoxic Event (OAE1a, ≈120 Ma)". Earth and Planetary Science Letters. 271 (1–4): 88–100. Bibcode:2008E&PSL.271...88L. doi:10.1016/j.epsl.2008.03.055.
  4. ^ Selby, David; Mutterlose, Jörg; Condon, Daniel J. (July 2009). "U–Pb and Re–Os geochronology of the Aptian/Albian and Cenomanian/Turonian stage boundaries: Implications for timescale calibration, osmium isotope seawater composition and Re–Os systematics in organic-rich sediments". Chemical Geology. 265 (3–4): 394–409. Bibcode:2009ChGeo.265..394S. doi:10.1016/j.chemgeo.2009.05.005.
  5. ^ Leckie, R; Bralower, T.; Cashman, R. (2002). "Oceanic anoxic events and plankton evolution: Biotic response to tectonic forcing during the mid-Cretaceous" (PDF). Paleoceanography. 17 (3): 1–29. Bibcode:2002PalOc..17.1041L. doi:10.1029/2001pa000623.
  6. ^ Meyers, Stephen R.; Siewert, Sarah E.; Singer, Brad S.; Sageman, Bradley B.; Condon, Daniel J.; Obradovich, John D.; Jicha, Brian R.; Sawyer, David A. (January 2012). "Intercalibration of radioisotopic and astrochronologic time scales for the Cenomanian-Turonian boundary interval, Western Interior Basin, USA". Geology. 40 (1): 7–10. doi:10.1130/g32261.1. ISSN 1943-2682.
  7. ^ Sachs, Sven; Grant‐Mackie, Jack A. (March 2003). "An ichthyosaur fragment from the Cretaceous of Northland, New Zealand". Journal of the Royal Society of New Zealand. 33 (1): 307–314. doi:10.1080/03014223.2003.9517732.
  8. ^ a b "Submarine eruption bled Earth's oceans of oxygen". New Scientist. 16 July 2008. Retrieved 2018-05-09.(subscription required)
  9. ^ a b Kerr, Andrew C. (July 1998). "Oceanic plateau formation: a cause of mass extinction and black shale deposition around the Cenomanian–Turonian boundary?". Journal of the Geological Society. 155 (4): 619–626. Bibcode:1998JGSoc.155..619K. doi:10.1144/gsjgs.155.4.0619.
  10. ^ Sageman, Bradley B.; Meyers, Stephen R.; Arthur, Michael A. (2006). "Orbital time scale and new C-isotope record for Cenomanian-Turonian boundary stratotype" (PDF). Geology. 34 (2): 125. Bibcode:2006Geo....34..125S. doi:10.1130/G22074.1.
  11. ^ a b Li, Yong-Xiang; Montañez, Isabel P.; Liu, Zhonghui; Ma, Lifeng (March 2017). "Astronomical constraints on global carbon-cycle perturbation during Oceanic Anoxic Event 2 (OAE2)". Earth and Planetary Science Letters. 462: 35–46. doi:10.1016/j.epsl.2017.01.007. ISSN 0012-821X.
  12. ^ G. Bonarelli, Il territorio di Gubbio - Notizie geologiche, Roma 1891.
  13. ^ Nagm, Emad; El-Qot, Gamal; Wilmsen, Markus (December 2014). "Stable-isotope stratigraphy of the Cenomanian–Turonian (Upper Cretaceous) boundary event (CTBE) in Wadi Qena, Eastern Desert, Egypt". Journal of African Earth Sciences. 100: 524–531. Bibcode:2014JAfES.100..524N. doi:10.1016/j.jafrearsci.2014.07.023. ISSN 1464-343X.
  14. ^ Jenkyns, Hugh C. (March 2010). "Geochemistry of oceanic anoxic events: REVIEW". Geochemistry, Geophysics, Geosystems. 11 (3): n/a–n/a. doi:10.1029/2009GC002788.
  15. ^ Schlanger, S. O.; Arthur, M. A.; Jenkyns, H. C.; Scholle, P. A. (1987). "The Cenomanian-Turonian Oceanic Anoxic Event, I. Stratigraphy and distribution of organic carbon-rich beds and the marine δ 13 C excursion". Geological Society, London, Special Publications. 26 (1): 371–399. doi:10.1144/GSL.SP.1987.026.01.24. ISSN 0305-8719.
  16. ^ Ernst, Richard E.; Youbi, Nasrrddine (July 2017). "How Large Igneous Provinces affect global climate, sometimes cause mass extinctions, and represent natural markers in the geological record". Palaeogeography, Palaeoclimatology, Palaeoecology. 478: 30–52. Bibcode:2017PPP...478...30E. doi:10.1016/j.palaeo.2017.03.014.
  17. ^ Kuroda, J; Ogawa, N; Tanimizu, M; Coffin, M; Tokuyama, H; Kitazato, H; Ohkouchi, N (15 April 2007). "Contemporaneous massive subaerial volcanism and late cretaceous Oceanic Anoxic Event 2". Earth and Planetary Science Letters. 256 (1–2): 211–223. Bibcode:2007E&PSL.256..211K. doi:10.1016/j.epsl.2007.01.027. ISSN 0012-821X.
  18. ^ Flögel, S.; Wallmann, K.; Poulsen, C.J.; Zhou, J.; Oschlies, A.; Voigt, S.; Kuhnt, W. (May 2011). "Simulating the biogeochemical effects of volcanic CO2 degassing on the oxygen-state of the deep ocean during the Cenomanian/Turonian Anoxic Event (OAE2)". Earth and Planetary Science Letters. 305 (3–4): 371–384. Bibcode:2011E&PSL.305..371F. doi:10.1016/j.epsl.2011.03.018. ISSN 0012-821X.
  19. ^ Linnert, Christian; Mutterlose, Jörg; Erbacher, Jochen (February 2010). "Calcareous nannofossils of the Cenomanian/Turonian boundary interval from the Boreal Realm (Wunstorf, northwest Germany)". Marine Micropaleontology. 74 (1–2): 38–58. Bibcode:2010MarMP..74...38L. doi:10.1016/j.marmicro.2009.12.002. ISSN 0377-8398.
  20. ^ Prauss, Michael L. (April 2012). "The Cenomanian/Turonian Boundary event (CTBE) at Tarfaya, Morocco: Palaeoecological aspects as reflected by marine palynology". Cretaceous Research. 34: 233–256. doi:10.1016/j.cretres.2011.11.004. ISSN 0195-6671.

Further reading

2018 in paleobotany

This article records new taxa of plants that are scheduled to be described during the year 2018, as well as other significant discoveries and events related to paleobotany that occurred in the year 2018.

2019 in paleomalacology

This list 2019 in paleomalacology is a list of new taxa of ammonites and other fossil cephalopods, as well as fossil gastropods, bivalves and other molluscs that are scheduled to be described during the year 2019, as well as other significant discoveries and events related to molluscan paleontology that are scheduled to occur in the year 2019.

Caribbean large igneous province

The Caribbean large igneous province (CLIP) consists of a major flood basalt, which created this large igneous province (LIP). It is the source of the current large eastern Pacific oceanic plateau, of which the Caribbean-Colombian oceanic plateau is the tectonized remnant. The deeper levels of the plateau have been exposed on its margins at the North and South American plates. The volcanism took place between 139 and 69 million years ago, with the majority of activity appearing to lie between 95 and 88 Ma. The plateau volume has been estimated as on the order of 4 x 106 km³. It has been linked to the Galápagos hotspot.

Cenomanian

The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding age. Both age and stage bear the same name.

As a unit of geologic time measure, the Cenomanian age spans the time between 100.5 ± 0.9 Ma and 93.9 ± 0.8 Ma (million years ago). In the geologic timescale it is preceded by the Albian and is followed by the Turonian. The Upper Cenomanian starts approximately at 95 M.a.

The Cenomanian is coeval with the Woodbinian of the regional timescale of the Gulf of Mexico and the early part of the Eaglefordian of the regional timescale of the East Coast of the United States.

At the end of the Cenomanian an anoxic event took place, called the Cenomanian-Turonian boundary event or the "Bonarelli Event", that is associated with a minor extinction event for marine species.

Darwin Guyot

Darwin Guyot is a volcanic underwater mountain top, or guyot, in the Mid-Pacific Mountains between the Marshall Islands and Hawaii. Named after Charles Darwin, it rose above sea level more than 118 million years ago during the early Cretaceous period to become an atoll, developed rudist reefs, and then drowned, perhaps as a consequence of sea level rise. The flat top of Darwin Guyot now rests 1,266 metres (4,154 ft) below sea level.

Detailed logarithmic timeline

This timeline shows the whole history of the universe, the Earth, and mankind in one table. Each row is defined in years ago, that is, years before the present date, with the earliest times at the top of the chart. In each table cell on the right, references to events or notable people are given, more or less in chronological order within the cell.

Each row corresponds to a change in log(time before present) of about 0.1 (using log base 10). The dividing points are taken from the R′′20 Renard numbers. Thus each row represent about 21% of the time from its beginning until the present.

The table is divided into sections with subtitles. Note that each such section contains about 68% of all the time from the beginning of the section until now.

Eagle Ford Group

The Eagle Ford Group (also called the Eagle Ford Shale) is a sedimentary rock formation deposited during the Cenomanian and Turonian ages of the Late Cretaceous over much of the modern-day state of Texas. The Eagle Ford is predominantly composed of organic matter-rich fossiliferous marine shales and marls with interbedded thin limestones. It derives its name from outcrops on the banks of the West Fork of the Trinity River near the old community of Eagle Ford, which is now a neighborhood within the city of Dallas. The Eagle Ford outcrop belt trends from the Oklahoma/Texas border southward to San Antonio, westward to the Rio Grande, Big Bend National Park, and the Quitman Mountains of West Texas. It also occurs in the subsurface of East Texas and South Texas, where it is the source rock for oil found in the Woodbine, Austin Chalk, and the Buda Limestone, and is produced unconventionally in South Texas and the "Eaglebine" play of East Texas. The Eagle Ford was one of the most actively drilled targets for unconventional oil and gas in the United States in 2010, but its output had dropped sharply by 2015. By the summer of 2016, Eagle Ford spending had dropped by two thirds from $30 billion in 2014 to $10 billion, according to an analysis from the research firm, Wood Mackenzie. This strike has been the hardest hit of any oil fields in the world. The spending is, however, expected to increase to $11.6 billion in 2017. A full recovery is not expected any time soon.Fossils are relatively common in Eagle Ford rocks. Vertebrate fossils that have been found in the Eagle Ford include plesiosaurs, mosasaurs, teleost fish, and teeth from sharks and other fish. Invertebrate fossils found in the Eagle Ford include crustaceans, sea urchins, swimming crinoids, ammonites, oysters, inoceramid clams, and other gastropod shells

Flood basalt

A flood basalt is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Flood basalt provinces such as the Deccan Traps of India are often called traps, after the Swedish word trappa (meaning "stairs"), due to the characteristic stairstep geomorphology of many associated landscapes. Michael R. Rampino and Richard Stothers (1988) cited eleven distinct flood basalt episodes occurring in the past 250 million years, creating large volcanic provinces, lava plateaus, and mountain ranges. However, more have been recognized such as the large Ontong Java Plateau, and the Chilcotin Group, though the latter may be linked to the Columbia River Basalt Group. Large igneous provinces have been connected to five mass extinction events, and may be associated with bolide impacts.

Ichthyosaur

Ichthyosaurs (Greek for "fish lizard" – ιχθυς or ichthys meaning "fish" and σαυρος or sauros meaning "lizard") are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1840, although the term is now used more for the parent clade of the Ichthyosauria).

Ichthyosaurs thrived during much of the Mesozoic era; based on fossil evidence, they first appeared around 250 million years ago (Ma) and at least one species survived until about 90 million years ago, into the Late Cretaceous. During the early Triassic period, ichthyosaurs evolved from a group of unidentified land reptiles that returned to the sea, in a development similar to how the mammalian land-dwelling ancestors of modern-day dolphins and whales returned to the sea millions of years later, which they gradually came to resemble in a case of convergent evolution. Ichthyosaurs were particularly abundant in the later Triassic and early Jurassic periods, until they were replaced as the top aquatic predators by another marine reptilian group, the Plesiosauria, in the later Jurassic and Cretaceous periods. In the Late Cretaceous, ichthyosaurs were hard hit by the Cenomanian-Turonian anoxic event. Their last lineage became extinct for unknown reasons.

Science became aware of the existence of ichthyosaurs during the early nineteenth century, when the first complete skeletons were found in England. In 1834, the order Ichthyosauria was named. Later that century, many excellently preserved ichthyosaur fossils were discovered in Germany, including soft-tissue remains. Since the late twentieth century, there has been a revived interest in the group, leading to an increased number of named ichthyosaurs from all continents, with over fifty valid genera being now known.

Ichthyosaur species varied from one to over sixteen metres in length. Ichthyosaurs resembled both modern fish and dolphins. Their limbs had been fully transformed into flippers, which sometimes contained a very large number of digits and phalanges. At least some species possessed a dorsal fin. Their heads were pointed, and the jaws often were equipped with conical teeth that could help to catch smaller prey. Some species had larger, bladed teeth with which they could attack large animals. The eyes were very large, probably useful when deep diving. The neck was short, and later species had a rather stiff trunk. These also had a more vertical tail fin, used for a powerful propulsive stroke. The vertebral column, made of simplified disc-like vertebrae, continued into the lower lobe of the tail fin. Ichthyosaurs were air-breathing, warm-blooded, and bore live young. They may have had a layer of blubber for insulation.

Mesozoic

The Mesozoic Era ( or ) is an interval of geological time from about 252 to 66 million years ago. It is also called the Age of Reptiles and the Age of Conifers.The Mesozoic ("middle life") is one of three geologic eras of the Phanerozoic Eon, preceded by the Paleozoic ("ancient life") and succeeded by the Cenozoic ("new life"). The era is subdivided into three major periods: the Triassic, Jurassic, and Cretaceous, which are further subdivided into a number of epochs and stages.

The era began in the wake of the Permian–Triassic extinction event, the largest well-documented mass extinction in Earth's history, and ended with the Cretaceous–Paleogene extinction event, another mass extinction whose victims included the non-avian dinosaurs. The Mesozoic was a time of significant tectonic, climate, and evolutionary activity. The era witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses that would move into their current positions during the next era. The climate of the Mesozoic was varied, alternating between warming and cooling periods. Overall, however, the Earth was hotter than it is today. Dinosaurs first appeared in the Mid-Triassic, and became the dominant terrestrial vertebrates in the Late Triassic or Early Jurassic, occupying this position for about 150 or 135 million years until their demise at the end of the Cretaceous. Birds first appeared in the Jurassic (however, true toothless birds appeared first in the Cretaceous), having evolved from a branch of theropod dinosaurs. The first mammals also appeared during the Mesozoic, but would remain small—less than 15 kg (33 lb)—until the Cenozoic. The flowering plants (angiosperms) arose in the Triassic or Jurassic and came to prominence in the late Cretaceous when they replaced the conifers and other gymnosperms as the dominant trees.

Tetanurae

Tetanurae (/ˌtɛtəˈnjuːriː/ or "stiff tails") is a clade that includes most theropod dinosaurs, including megalosauroids, allosauroids, tyrannosauroids, ornithomimosaurs, maniraptorans, and birds. Tetanurans are defined as all theropods more closely related to modern birds than to Ceratosaurus and contain the majority of predatory dinosaur diversity. Tetanurae likely diverged from its sister group, Ceratosauria, during the late Triassic. Tetanurae first appeared in the fossil record by the Early Jurassic about 190 mya and by the Middle Jurassic had become globally distributed.The group was named by Jacques Gauthier in 1986 and originally had two main subgroups: Carnosauria and Coelurosauria, the clade containing birds and related dinosaurs such as compsognathids, tyrannosaurids, ornithomimosaurs, and maniraptorans. The original Carnosauria was a polyphyletic group including any large carnivorous theropod. Many of Gauthier's carnosaurs, such as tyrannosaurids, have since been re-classified as coelurosaurs or primitive tetanurans. Carnosauria has been reclassified as a group containing allosaurids that split from the Coelurosauria at the Neotetanurae/Avetheropoda node. Members of Spinosauroidea are believed to represent basal tetanurans.Tetanuran evolution was characterized by parallel diversification of multiple lineages, repeatedly attaining large body size and similar locomotor morphology. Cryolophosaurus has been claimed as the first true member of the group, but subsequent studies have disagreed on whether it is a dilophosaurid or tetanuran. Arcucci and Coria (2003) classified Zupaysaurus as an early tetanuran, but it was later placed as a sister taxon to the clade containing dilophosaurids, ceratosaurs, and tetanurans.Shared tetanuran features include a ribcage indicating a sophisticated air-sac-ventilated lung system similar to that in modern birds. This character would have been accompanied by an advanced circulatory system. Other tetanuran characterizing features include the absence of the fourth digit of the hand, placement of the maxillary teeth anterior to the orbit, a strap-like scapula, maxillary fenestrae, and stiffened tails. During the Late Jurassic and Early Cretaceous, large spinosaurids and allosaurs flourished but possibly died out in the northern hemisphere before the end of the Cretaceous, and were replaced as apex predators by tyrannosauroid coelurosaurs. At least in South America, carcharodontosaurid allosaurs persisted until the end of the Mesozoic Era, and died out at the same time the non-avian coelurosaurs.

Tropic Shale

The Tropic Shale is a Mesozoic geologic formation. Dinosaur remains are among the fossils that have been recovered from the formation, including Nothronychus graffami.

The Tropic Shale is a stratigraphic unit of the Kaiparowits Plateau of south central Utah. The Tropic Shale was first named in 1931 after the town of Tropic where the Type section is located. The Tropic Shale outcrops in Kane and Garfield counties, with large sections of exposure found in the Grand Staircase-Escalante National Monument.

Turonian

The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous epoch, or a stage in the Upper Cretaceous series. It spans the time between 93.9 ± 0.8 Ma and 89.8 ± 1 Ma (million years ago). The Turonian is preceded by the Cenomanian stage and underlies the Coniacian stage.At the beginning of the Turonian an anoxic event took place which is called the Cenomanian-Turonian boundary event or the "Bonarelli Event".

Western Interior Seaway anoxia

Three Western Interior Seaway anoxic events occurred during the Cretaceous in the shallow inland seaway that divided North America in two island continents, Appalachia and Laramidia (see map). During these anoxic events much of the water column was depleted in dissolved oxygen. While anoxic events impact the world's oceans, Western Interior Seaway anoxic events exhibit a unique paleoenvironment compared to other basins. The notable Cretaceous anoxic events in the Western Interior Seaway mark the boundaries at the Aptian-Albian, Cenomanian-Turonian, and Coniacian-Santonian stages, and are identified as Oceanic Anoxic Events I, II, and III respectively. The episodes of anoxia came about at times when very high sea levels coincided with the nearby Sevier orogeny that affected Laramidia to the west and Caribbean large igneous province to the south, which delivered nutrients and oxygen-adsorbing compounds into the water column.

Most anoxic events are recognized using the 13C isotope as a proxy to indicate total organic carbon preserved in sedimentary rocks. If there is very little oxygen, then organic material that settles to the bottom of the water column will not be degraded as readily compared to normal oxygen settings and can be incorporated into the rock. 13Corganic is calculated by comparing the amount of 13C to a carbon isotope standard, and using multiple samples can track changes (δ) in organic carbon content through rocks over time, forming a δ13Corganic curve. The δ13Corganic, as a result, serves as a benthic oxygen curve.

The excellent organic carbon preservation brought about by these successive anoxic events makes Western Interior Seaway strata some of the richest source rocks for oil and gas.

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