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.^[1] 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^[2] 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 Cenomanian is coeval with the Woodbinian of the regional timescale of the Mexican Gulf and the Eaglefordian of the regional timescale of the US eastcoast.

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.

Stratigraphic definitions

The Cenomanian was introduced in scientific literature by French palaeontologist Alcide d'Orbigny in 1847. Its name comes from the New Latin name of the French city of Le Mans (département Sarthe), Cenomanum.

The base of the Cenomanian stage (which is also the base of the Upper Cretaceous series) is placed at the first appearance of foram species Rotalipora globotruncanoides in the stratigraphic record. An official reference profile for the base of the Cenomanian (a GSSP) is located in an outcrop at the western flank of Mont Risou, near the village of Rosans in the French Alps (département Hautes-Alpes, coordinates: 44°23'33"N, 5°30'43"E). The base is, in the reference profile, located 36 meters below the top of the Marnes Bleues Formation.^[3]

The top of the Cenomanian (the base of the Turonian) is at the first appearance of ammonite species Watinoceras devonense.

Important index fossils for the Cenomanian are the ammonites Calycoceras naviculare, Acanthoceras rhotomagense and Mantelliceras mantelli.

Sequence stratigraphy and palaeoclimatology

The late Cenomanian represents the highest mean sea-level observed in the Phanerozoic eon, the past six hundred million years (approximately one hundred and fifty meters above present day sea-levels). A corollary is that the highlands were at all time lows, so the landscape on Earth was one of warm broad shallow seas inundating low lying land areas on the precursors to today's continents. What few lands rose above the waves were made of old mountains and hills, upland plateaus, all much weathered. Tectonic mountain building was minimal and most continents were isolated by large stretches of water. Without highlands to brake winds, the climate would have been windy and waves large, adding to the weathering and fast rate of sediment deposition.

Palaeontology

Belemnites

Ankylosauria

Ankylosaurs of the Cenomanian
Taxa	Presence	Location	Description	Images
Acanthopholis	Albian or Aptian to Cenomanian	Upper Greensand Group, Cambridgeshire, England	A nodosaurid with an armor of oval plates set almost horizontally into the skin, with spikes protruding from the neck and shoulder area, along the spine. Its size has been estimated to be in the range of 3 to 5.5 meters (10 to 18 ft) long and approximately 380 kilograms (840 lb) in weight.	Acanthopholis
Animantarx	Cenomanian to Turonian	Cedar Mountain Formation, Utah, USA	thought of as a nodosaurid ankylosaur, although its precise relationships within that family are uncertain
Nodosaurus		Wyoming, Kansas, USA	A nodosaurid ankylosaur about 4 to 6 meters (13 to 20 feet) long with bony dermal plates covering the top of its body. It may have had spikes along its side as well. It had four short legs, five-toed feet, a short neck, and a long, stiff, clubless tail.
Stegopelta	Late Albian to early Cenomanian	Frontier Formation, Wyoming, USA	A poorly known genus of nodosaurid
Tsagantegia		Baynshiree Svita Formation, Dzun-Bayan, Mongolia	An ankylosaurid known from the remains of its skull
Zhejiangosaurus		Chaochuan Formation, Zhejiang, China	Nodosaurid
Zhongyuansaurus		Ruyang, Henan, China	Nodosaurid

Ornithopoda

Plesiosauria

Plesiosaurs of the Cenomanian
Taxa	Presence	Location	Description	Images
Plesiopleurodon		Belle Fourche Shale, Wyoming, USA	A pliosaurid characterized by a moderately long symphysis bearing 8 pairs of teeth, teeth that are nearly circular in cross-section and which are smooth on the outer surface (except near the base), ribs of the neck vertebrae being singled-headed (double-headed in Jurassic pliosaurs), and a long slender interpectoral bar on the coracoid	Plesiopleurodon

Pterosauria

Sauropoda

Sauropods of the Cenomanian
Taxa	Presence	Location	Description	Images
Andesaurus		Candeleros Formation, Argentina
Argentinosaurus		Río Limay Formation, Neuquén, Argentina		Argentinosaurus
Qiaowanlong		China

Theropoda

Theropods of the Cenomanian
Taxa	Presence	Location	Description	Images
Bahariasaurus		Bahariya Oasis, Egypt; Niger		Carcharodontosaurus
Carcharodontosaurus		Morocco; Niger
Deltadromeus		Morocco
Giganotosaurus		Rio Limay, Argentina
Mapusaurus		Huincul Formation, Argentina
Oxalaia		Brasil
Siamosaurus		Thailand
Siats	Cenomanian	Mussentuchit Member, Cedar Mountain Formation, Utah, USA	An 11-12 meter neovenatorid carcharodontosaur known from juvenile remains. Youngest known allosauroid known from North America
Sigilmassasaurus		Tafilalt, Morocco
Spinosaurus		Bahariya Oasis, Egypt; Tunisia; Morocco
Unenlagia		Comahue, Argentina
unnamed enantiornithine bird[4]		Nammoura, Ouadi al Gabour, Lebanon

References

Notes

1. ↑ See for a detailed geologic timescale Gradstein et al. (2004)
2. ↑ International Commission on Stratigraphy. "International Stratigraphic Chart". Archived from the original on 2008-05-29. Retrieved 2008-06-17. 
3. ↑ The GSSP for the Cenomanian was established by Kennedy et al. (2004)
4. ↑ Vecchia, F. M. D.; Chiappe, L. M. (2002). "First avian skeleton from the Mesozoic of northern Gondwana". Journal of Vertebrate Paleontology 22 (4): 856. doi:10.1671/0272-4634(2002)022[0856:FASFTM]2.0.CO;2. 

Literature

• Gradstein, F.M.; Ogg, J.G. & Smith, A.G.; 2004: A Geologic Time Scale 2004, Cambridge University Press.
• Kennedy, W.J.; Gale, A.S.; Lees, J.A. & Caron, M.; 2004: The Global Boundary Stratotype Section and Point (GSSP) for the base of the Cenomanian Stage, Mont Risou, Hautes-Alpes, France, Episodes 27, pp. 21–32.

External links

• GeoWhen Database - Cenomanian
• Late Cretaceous timescale, at the website of the subcommission for stratigraphic information of the ICS
• Stratigraphic chart of the Lower Cretaceous (including the Cenomanian), at the website of Norges Network of offshore records of geology and stratigraphy
• Cenomanian Microfossils: 20+ images of Foraminifera

Coordinates: 44°23′33″N 5°30′43″E / 44.39250°N 5.51194°E / 44.39250; 5.51194