In the geological timescale, the Tithonian is the latest age of the Late Jurassic Epoch and the uppermost stage of the Upper Jurassic Series. It spans the time between 149.2 ±0.7 annum and 143.1 ±0.6 (million years ago).[Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013; updated) The ICS International Chronostratigraphic Chart. Episodes 36: 199–204.] It is preceded by the Kimmeridgian and followed by the Berriasian (part of the Cretaceous).[See for a detailed version of the geologic timescale Gradstein et al. (2004)]
Stratigraphic definitions
The Tithonian was introduced in scientific literature by German stratigrapher
Albert Oppel in 1865. The name Tithonian is unusual in geological stage names because it is derived from
Greek mythology.
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 stage because the Tithonian finds itself hand in hand with the dawn of the Cretaceous.
The base of the Tithonian stage is at the base of the ammonite biozone of Hybonoticeras. A global reference profile (a GSSP) for the base of the Tithonian had in 2009 not yet been established.
The top of the Tithonian stage (the base of the Berriasian Stage and the Cretaceous System) is marked by the first appearance of small globular calpionellids of the species Calpionella, at the base of the Alpina Subzone .
Subdivision
The Tithonian is often subdivided into Lower/Early, Middle and Upper/Late substages or subages. The Late Tithonian is coeval with the
Portlandian Age of British stratigraphy.
The Tithonian stage contains seven ammonite biozones in the Tethys Ocean, from top to base:
Sedimentary environments
Sedimentary rocks that formed in the Tethys Ocean during the Tithonian include limestones, which preserve fossilized remains of, for example,
. The Solnhofen limestone of southern Germany, which is known for its fossils (especially
Archaeopteryx), is of Tithonian age.
Tithonian extinction
The later part of the Tithonian stage experienced an
extinction event.
It has been referred to as the
Tithonian extinction,
,
or
end-Jurassic extinction.
This event was fairly minor and selective, by most metrics outside the top 10 largest extinctions since the
Cambrian. Nevertheless, it was still one of the largest extinctions of the Jurassic Period, alongside the Toarcian Oceanic Anoxic Event (TOAE) in the
Early Jurassic.
Potential causes
Cooling and sea level fall
The Tithonian extinction has not been studied in great detail, but it is usually attributed to
habitat loss via a major marine regression (sea level fall).
There is good evidence for a marine regression in Europe across the Jurassic-Cretaceous boundary, which may explain the localized nature of the extinction.
On the other hand, there is no clear consensus on a correlation between sea level and terrestrial diversity during the Jurassic and Cretaceous. Some authors support a fundamental correlation (the so-called "common cause hypothesis"),
while others strongly voice doubts.
Sea level fall was likely related to the Tithonian climate, which was substantially colder and drier than the preceding Kimmeridgian stage. Northern coral reef ecosystems, such as those of the European Tethys, would have been particularly vulnerable to global cooling during this time.
Volcanism or asteroid impacts
Few Jurassic-Cretaceous boundary sections are precisely associated with carbon isotope anomalies.
Several
Arctic outcrops show a moderate (up to 5
Per mille) negative organic δ13C excursion in the middle part of the Tithonian. This excursion, sometimes called the Volgian Isotopic Carbon Excursion (VOICE), may be a consequence of volcanic activity.
The Tithonian stage saw the emplacement of the
Shatsky Rise, a massive
volcanic plateau in the
North Pacific. During the Late Jurassic and Early Cretaceous, numerous volcanic deposits can be found along the margin of Gondwana, which was beginning to fragment into smaller continents.
Three large Impact crater have been tentatively dated to the Tithonian: the Morokweng Impact Structure (South Africa, 80+ km diameter), Mjølnir crater (Barents Sea, 40 km diameter), and Gosses Bluff crater (Australia, 22 km diameter). These impacts would have caused local devastation, but likely had minimal impact on global ecosystems. Most volcanic events or extraterrestrial impacts in the Late Jurassic were concentrated around Gondwana, in contrast to the extinction event, which was centered on Laurasia ecosystems.
Sampling bias
It has been suggested that the putative extinction is a consequence of
Sampling bias. The Late Jurassic is packed with marine lagerstätten, exceptionally diverse and well-preserved fossil beds. A lack of earliest Cretaceous marine lagerstätten may appear as a loss of diversity, simply looking at the raw data alone.
Sampling bias may also explain apparent extinctions in terrestrial environments, which have a similar disconnect in fossil abundance. This is most obvious in sauropod-bearing deposits, which are abundant in the Late Jurassic and rare in the earliest Cretaceous.
Most studies relevant to the Tithonian extinction attempt to counteract sampling biases when estimating diversity loss or extinction rates.
Depending on the sampling method or the taxonomic group, the Tithonian extinction may still be apparent even once sampling biases are accounted for.
Impact on life
In 1986,
Jack Sepkoski argued that the Late Tithonian extinction was the largest extinction event between the end of the Triassic and the end of the Cretaceous. He estimated that a staggering 37% of genera died out during the Tithonian stage.
(1995) found a lower estimate, with the extinction of 5.6 to 13.3% of genera in the Tithonian. Proportional extinction was higher for continental genera (5.8–17.6%) than marine genera (5.1–6.1%).
Sepkoski (1996) estimated that about 18% of multiple-interval marine genera (those originating prior to the Tithonian) died out in the Tithonian.
Based on an updated version of Sepkoski's genera compendium, Bambach (2006) found a similar estimate of 20% of genera going extinct in the Late Tithonian.
Invertebrates
European
Bivalvia diversity is severely depleted across the J–K boundary.
However, bivalve fossils from the
Andes and
Siberia show little ecological turnover, so bivalve extinctions may have localized to the
Tethys Sea. Only a fraction of Jurassic
ammonite species survive to the Cretaceous, though extinction rates were actually lower in the late Tithonian relative to adjacent time intervals.
Moderate diversity declines have been estimated or observed in
Gastropoda,
Brachiopod,
Radiolaria,
Crustacean, and
Scleractinia Coral. This may have been related to the replacement of Jurassic-style
Coral reef by Cretaceous-style
Rudists reefs.
Reef decline was likely a gradual process, stretched out between the Oxfordian stage and the
Valanginian stage.
Marine vertebrates
Marine
Actinopterygii (ray-finned fishes) show elevated extinction rates across the Tithonian-Berriasian boundary. Most losses were quickly offset by substantial diversification in the Early Cretaceous. Sharks, rays, and freshwater fishes were nearly unaffected by the extinction.
Marine reptile were strongly affected by the Tithonian extinction.
It has long been suggested that Ichthyosauria and marine Teleosauroidea Crocodyliformes declined across the J–K boundary, with the latter group even going extinct.
Terrestrial vertebrates
On land,
Sauropoda dinosaur diversity was significantly reduced according to many
(but not all)
estimates.
Diplodocidae, basal
Macronaria, and
Mamenchisauridae took the brunt of the extinction,
though a few species of each group survived to the Early Cretaceous.
Conversely,
Rebbachisauridae and
Somphospondyli saw the opportunity to diversify in the Cretaceous.
also survived the extinction and even expanded into North America during the Early Cretaceous.
diversity declined through the entire Late Jurassic, with medium-sized predators such as
Megalosauridae being the hardest hit.
(particularly
Stegosauria) diversity saw a small drop across the J–K boundary. Theropod and ornithischian extinctions were notably less pronounced than in sauropods.
Most non-pterodactyloid Pterosaur perished by the end of the Jurassic.
Notes
Literature
-
; (2004): A Geologic Time Scale 2004, Cambridge University Press.
-
; 1865: Die Tithonische Etage, Zeitschrift der Deutschen Geologischen Gesellschaft, 1865: pp 535–558.
External links
