Hettangian

The Hettangian is the earliest age or lowest stage of the Jurassic period of the geologic timescale. It spans the time between 201.3 ± 0.2 Ma and 199.3 ± 0.3 Ma (million years ago).^[1] The Hettangian follows the Rhaetian (part of the Triassic period) and is followed by the Sinemurian.^[2]

In European stratigraphy the Hettangian is a part of the time span in which the Lias was deposited. An example is the British Blue Lias, which has an upper Rhaetian to Sinemurian age. Another example is the lower Lias from the Northern Limestone Alps where well-preserved but very rare ammonites, including Alsatites, have been found.

Stratigraphic definitions

The Hettangian was introduced in the literature by Swiss palaeontologist, Eugène Renevier, in 1864. The stage takes its name from Hettange-Grande, a town in north-eastern France, just south of the border with Luxembourg on the main road from Luxembourg City to Metz.

The base of the Hettangian stage (which is also the base of the Lower Jurassic series and the entire Jurassic system) is defined as the place in the stratigraphic column where fossils of the ammonite genus Psiloceras first appear. A global reference profile (a GSSP) for the base was defined 2010 at the Kuhjoch in the Karwendel in western Austria.^[3] The top of the Hettangian stage (the base of the Sinemurian) is at the first appearances of ammonite genera Vermiceras and Metophioceras.

Biostratigraphy

The Hettangian contains three ammonite biozones in the Tethys domain:

• zone of Schlotheimia angulata
• zone of Alsatites liasicus
• zone of Psiloceras planorbis

Palaeontology

†Ammonites

Alsatites proaries

At the end of the Triassic period, the ammonites died out almost entirely. During the Hettangian, however, the "Neoammonites" developed relatively quickly, so that even in the middle Hettangian a large number of genera and species existed.

†Ichthyosaurs

Ichthyosauria of the Hettangian
Taxa	Presence	Location	Description	Images
Ichthyosaurus	From Hettangian to Sinemurian	Belgium, England, Germany	Among the best known ichthyosaur genera, was smaller than most of its relatives, measuring 2 m (6 ft 7 in) in length.	Ichthyosaurus Temnodontosaurus
Temnodontosaurus	From Hettangian to Toarcian	England, Germany	The first Ichthyosaur, T.platyodon discovered by Mary Anning from the Blue Lias at Lyme Regis. Species of Temnodontosaurus were large, exceeding 12 meters (39 feet) in length.

Mammaliaformes

†Ornithischians

Ornithischians of the Hettangian
Taxa	Presence	Location	Description	Images
Abrictosaurus	From Hettangian to Sinemurian	Upper Elliot Formation, Lesotho and Cape Province, South Africa	Considered the most basal member of the family Heterodontosauridae	Abrictosaurus Heterodontosaurus Scelidosaurus
Fabrosaurus	From Hettangian to Sinemurian	Lesotho	As the only fossil known of it is a partial jawbone with three teeth, Fabrosaurus is a nomen dubium.
Heterodontosaurus		Upper Elliot Formation, Cape Province, South Africa	A small, fleet-footed ornithischian that reached a maximum size of about 3 feet. The hand of Heterodontosaurus had five fingers, two of which seem to be opposable, allowing Heterodontosaurus to grasp and manipulate food. Another interesting feature is the specialization of teeth which gave rise to the animal's name. At the front of the jaw beside the beak were small teeth likely used for chopping off leaves and stems.
Lanasaurus	From Hettangian to Sinemurian	Upper Elliot Formation, Orange Free State, South Africa	A heterodontosaurid ornithischian
Lycorhinus	From Hettangian to Sinemurian	Upper Elliot Formation, Cape Province, South Africa	A small herbivore dinosaur despite long canines it sported in its jaws; due to this unique characteristic it is very clearly allied to Heterodontosaurus
Scelidosaurus	From Hettangian to Sinemurian	Charmouth, West Dorset, England; Kayenta Formation, Arizona, USA	A genus of quadrupedal, lightly plated, herbivorous dinosaur about 4 meters (13 feet) long
Scutellosaurus		Arizona, USA	A bipedal armored herbivore, was around 1.3 m (4 ft 3 in) long, 0.5 m (20 in) tall at the hips, and weighed 10 kg (22 lb).
Stormbergia	From Hettangian to Sinemurian	Stormberg Series Formations, South Africa, Lesotho	A primitive ornithischian, about 2 meters long. The proportions of the posterior limbs show differences with those of Lesothosaurus, and reveal a lower ability to run

†Plesiosaurs

Plesiosauria of the Hettangian
Taxa	Presence	Location	Description	Images
Macroplata	Hettangian	Harbury, Warwickshire, UK		Macroplata Atychodracon
Atychodracon	Hettangian	Barrow-upon-Soar, Leicestershire, England	A genus of sauropterygian carnivorous reptile. It was about 7 m long.

See also

• Triassic-Jurassic extinction event
• Komlosaurus carbonis

References

Notes

Literature

• Gradstein, F.M.; Ogg, J.G. & Smith, A.G.; 2004: A Geologic Time Scale 2004, Cambridge University Press.
• Renevier, E.: Notices géologiques et paléontologiques sur les Alpes Vaudoises, et les régions environnantes. I. Infralias et Zone à Avicula contorta (Étage Rhaetien) des Alpes Vaudoises Bulletin de la Société Vaudoise des Sciences Naturelles 8, p. 39-97. (French)

External links

• GeoWhen Database - Hettangian
• Lower Jurassic timescale, at the website of the subcommission for stratigraphic information of the ICS
• Stratigraphic chart of the Lower Jurassic, at the website of Norges Network of offshore records of geology and stratigraphy