Editing Jurassic (section)

==== Late Jurassic ====
The Oxfordian is named after the city of [[Oxford]] in England and was named by Alcide d'Orbigny in 1844 in reference to the [[Oxford Clay]]. The base of the Oxfordian lacks a defined GSSP. [[William Joscelyn Arkell|W. J. Arkell]] in studies in 1939 and 1946 placed the lower boundary of the Oxfordian as the first appearance of the ammonite ''[[Quenstedtoceras|Quenstedtoceras mariae]]'' (then placed in the genus ''[[Vertumniceras]]''). Subsequent proposals have suggested the first appearance of ''[[Cardioceras|Cardioceras redcliffense]]'' as the lower boundary.<ref name="Ogg-2012" /><ref name="jurassic.stratigraphy.org" />

The village of [[Kimmeridge]] on the coast of [[Dorset]], England, is the origin of the name of the Kimmeridgian. The stage was named by Alcide d'Orbigny in 1842 in reference to the [[Kimmeridge Clay]]. The GSSP for the base of the Kimmeridgian is the Flodigarry section at [[Staffin|Staffin Bay]] on the [[Isle of Skye]], [[Scotland]],<ref>{{Cite journal|last=BARSKI|first=Marcin|date=2018-09-06|title=Dinoflagellate cyst assemblages across the Oxfordian/Kimmeridgian boundary (Upper Jurassic) at Flodigarry, Staffin Bay, Isle of Skye, Scotland – a proposed GSSP for the base of the Kimmeridgian|url=http://dx.doi.org/10.5604/01.3001.0012.4594|journal=Volumina Jurassica|volume=XV|issue=1|pages=51–62|doi=10.5604/01.3001.0012.4594|doi-broken-date=1 November 2024 |s2cid=133861564|issn=1731-3708}}</ref> which was ratified in 2021. The boundary is defined by the first appearance of ammonites marking the boreal Bauhini Zone and the subboreal Baylei Zone.<ref name="jurassic.stratigraphy.org" />

The Tithonian was introduced in scientific literature by Albert Oppel in 1865. The name Tithonian is unusual in geological stage names because it is derived from [[Greek mythology]] rather than a place name. [[Tithonus]] was the son of [[Laomedon]] of [[Troy]] and fell in love with [[Eos]], the Greek goddess of [[dawn]]. His name was chosen by Albert Oppel for this [[Stratigraphy|stratigraphical]] stage because the Tithonian finds itself hand in hand with the dawn of the Cretaceous. The base of the Tithonian currently lacks a GSSP.<ref name="Ogg-2012" /> The working definition for the base of the Tithonian is the first appearance of the ammonite genus ''[[Gravesia (ammonite)|Gravesia]]''.<ref name="jurassic.stratigraphy.org" />

The upper boundary of the Jurassic is currently undefined, and the Jurassic–Cretaceous boundary is currently the only system boundary to lack a defined GSSP. Placing a GSSP for this boundary has been difficult because of the strong regionality of most biostratigraphic markers, and lack of any [[Chemostratigraphy|chemostratigraphic]] events, such as [[isotope]] excursions (large sudden changes in [[Stable isotope ratio|ratios of isotopes]]), that could be used to define or correlate a boundary. [[Calpionellid]]s, an enigmatic group of [[plankton]]ic [[protist]]s with urn-shaped calcitic [[Test (biology)|tests]] briefly abundant during the latest Jurassic to earliest Cretaceous, have been suggested to represent the most promising candidates for fixing the Jurassic–Cretaceous boundary<ref>{{Cite journal|last=WIMBLEDON|first=William A.P.|date=2017-12-27|title=Developments with fixing a Tithonian/Berriasian (J/K) boundary|url=https://www.researchgate.net/publication/321670503|journal=Volumina Jurassica|volume=15 |issue=1 |pages=107–112|doi=10.5604/01.3001.0010.7467|doi-broken-date=2024-11-14 |issn=1731-3708}}</ref> In particular, the first appearance ''[[Calpionella|Calpionella alpina]],'' co-inciding with the base of the eponymous Alpina subzone, has been proposed as the definition of the base of the Cretaceous.<ref>{{Cite journal|last1=Wimbledon|first1=William A.P.|last2=Rehakova|first2=Daniela|last3=Svobodová|first3=Andrea|last4=Schnabl|first4=Petr|last5=Pruner|first5=Petr|last6=Elbra|first6=Tiiu|last7=Šifnerová|first7=Kristýna|last8=Kdýr|first8=Šimon|last9=Frau|first9=Camille|last10=Schnyder|first10=Johann|last11=Galbrun|first11=Bruno|date=2020-02-11|title=Fixing a J/K boundary: A comparative account of key Tithonian–Berriasian profiles in the departments of Drôme and Hautes-Alpes, France|url=https://www.sav.sk/index.php?lang=sk&doc=journal-list&part=article_response_page&journal_article_no=18100|journal=Geologica Carpathica|volume=71|issue=1|doi=10.31577/GeolCarp.71.1.3|s2cid=213694912|doi-access=free|bibcode=2020GCarp..71..1.3W }}</ref> The working definition for the boundary has often been placed as the first appearance of the ammonite ''[[Strambergella jacobi]],'' formerly placed in the genus ''[[Berriasella]]'', but its use as a stratigraphic indicator has been questioned, as its first appearance does not correlate with that of ''C. alpina''.<ref>{{Cite journal|last1=Frau|first1=Camille|last2=Bulot|first2=Luc G.|last3=Reháková|first3=Daniela|last4=Wimbledon|first4=William A.P.|last5=Ifrim|first5=Christina|date=November 2016|title=Revision of the ammonite index species Berriasella jacobi Mazenot, 1939 and its consequences for the biostratigraphy of the Berriasian Stage|url=http://linkinghub.elsevier.com/retrieve/pii/S0195667116301057|journal=Cretaceous Research|language=en|volume=66|pages=94–114|doi=10.1016/j.cretres.2016.05.007|bibcode=2016CrRes..66...94F }}</ref>