Tithonian
Tithonian | |||||||||||||||||||
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In the Ma and 145.0 ± 4 Ma (million years ago). It is preceded by the Kimmeridgian and followed by the Berriasian (part of the Cretaceous).[2]
Stratigraphic definitionsThe Tithonian was introduced in scientific literature by German stratigrapher Laomedon of Troy and fell in love with Eos, the Greek goddess of dawn. His name was chosen by Albert Oppel for this stratigraphical stage because the Tithonian finds itself hand in hand with the dawn of the Cretaceous.[3]
The base of the Tithonian stage is at the base of the GSSP or golden spike ) 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 alpina, at the base of the Alpina Subzone . SubdivisionThe 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 domain, from top to base:
Sedimentary environmentsSedimentary rocks that formed in the Tethys Ocean during the Tithonian include limestones, which preserve fossilized remains of, for example, Solnhofen limestone of southern Germany, which is known for its fossils (especially Archaeopteryx ), is of Tithonian age.
Tithonian extinctionThe later part of the Tithonian stage experienced an extinction event.[4][5] It has been referred to as the Tithonian extinction,[6][7][8] Jurassic-Cretaceous (J–K) extinction,[4][5][9] or end-Jurassic extinction.[10][11] 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.[7][12] Potential causesCooling and sea level fallThe 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).[6] There is good evidence for a marine regression in Europe across the Jurassic-Cretaceous boundary, which may explain the localized nature of the extinction.[13][8][11] 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"),[11] while others strongly voice doubts.[14] 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.[5]
Volcanism or asteroid impactsFew Jurassic-Cretaceous boundary sections are precisely associated with carbon isotope anomalies. Three large 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 Laurasian ecosystems.[5]
Sampling biasIt has been suggested that the putative extinction is a consequence of sampling biases. 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.[17][18] 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.[18] Most studies relevant to the Tithonian extinction attempt to counteract sampling biases when estimating diversity loss or extinction rates.[14][5] Depending on the sampling method or the taxonomic group, the Tithonian extinction may still be apparent even once sampling biases are accounted for.[5][19] Impact on lifeIn 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.[20] Benton (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%).[21] Sepkoski (1996) estimated that about 18% of multiple-interval marine genera (those originating prior to the Tithonian) died out in the Tithonian.[7] 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.[22] InvertebratesEuropean gastropods, brachiopods, radiolarians, crustaceans, and scleractinian corals. This may have been related to the replacement of Jurassic-style coral reefs by Cretaceous-style rudist reefs.[5] Reef decline was likely a gradual process, stretched out between the Oxfordian stage and the Valanginian stage.[25]
Marine vertebratesMarine actinopterygians (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.[26] Xenopsaria, namely elasmosaurids and leptocleidians.[4] This turnover of marine reptile faunas may be a consequence of the turnover of reefs and marine fishes, which would have benefited generalized predators more than specialists.[5]
It has long been suggested that ichthyosaurs and marine teleosauroid crocodyliforms declined across the J–K boundary, with the latter group even going extinct.[27][29][30] More recent finds suggest that ichthyosaurs diversity remained stable or even increased in the Early Cretaceous.[10][4][5] Early Cretaceous ichthyosaur fossils are rare enough that this hypothesis is still a matter of debate.[11] European teleosauroids did indeed suffer total extinction,[31] but teleosauroids as a whole survived into the Early Cretaceous in other parts of the world.[32][33][34] Metriorhynchoids, the other major group of marine crocodyliforms, were not strongly affected by the Tithonian extinction.[31]
Terrestrial vertebratesOn land, sauropod dinosaur diversity was significantly reduced according to many[35][36][11][5][19] (but not all)[18][37] estimates. Diplodocids, basal macronarians, and mamenchisaurids took the brunt of the extinction,[5] though a few species of each group survived to the Early Cretaceous.[38][39][40] Conversely, rebbachisaurids and somphospondyls saw the opportunity to diversify in the Cretaceous.[5] Turiasaurs also survived the extinction and even expanded into North America during the Early Cretaceous.[9] Theropod diversity declined through the entire Late Jurassic, with medium-sized predators such as megalosaurids being the hardest hit.[11][5] Ornithischian (particularly stegosaur) diversity saw a small drop across the J–K boundary. Theropod and ornithischian extinctions were notably less pronounced than in sauropods.[36][11] Most non-pterodactyloid pterosaurs perished by the end of the Jurassic.[11] Practically no earliest Cretaceous sites are known to preserve pterosaur fossils, so the precise timing of non-pterodactyloid extinctions is very uncertain.[17] Coastal and freshwater crocodyliforms experienced high extinction rates across the J–K boundary, preceding a significant diversification of more terrestrially-adapted metasuchians in the Cretaceous.[29][30][5] Coastal and freshwater turtle diversity also declined, at least in Europe.[11][30] Many tetrapod groups saw strong (albeit gradual) ecological turnover through the J-K boundary. These groups include lissamphibians, lepidosaurs, choristoderes, and mammaliaforms.[11] ReferencesNotes
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