The later part of the Tithonian stage experienced an
extinction event. It has been referred to as the
Tithonian extinction,
Jurassic-Cretaceous (J–K) 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). 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. Several
Arctic outcrops show a moderate (up to 5
‰) 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. Most studies relevant to the Tithonian extinction attempt to counteract sampling biases when estimating diversity loss or extinction rates.
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.
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%). Sepkoski (1996) estimated that about 18% of multiple-interval marine genera (those originating prior to the Tithonian) died out in the Tithonian.
Invertebrates European
bivalve diversity is severely depleted across the J–K boundary.
Marine vertebrates '' Marine
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.
Marine reptiles were strongly affected by the Tithonian extinction. Among
plesiosaurs, only a few species of
Pliosauridae and
Cryptoclididae persisted, and they too would die out in the Early Cretaceous. Conversely, the Tithonian extinction acted as a trigger for a Cretaceous diversification event for plesiosaurs in the clade
Xenopsaria, namely
elasmosaurids and
leptocleidians. More recent finds suggest that ichthyosaurs diversity remained stable or even increased in the Early Cretaceous. but teleosauroids as a whole survived into the Early Cretaceous in other parts of the world.
Metriorhynchoids, the other major group of marine crocodyliforms, were not strongly affected by the Tithonian extinction. estimates.
Diplodocids,
basal macronarians, and
mamenchisaurids took the brunt of the extinction, Conversely,
rebbachisaurids and
somphospondyls saw the opportunity to diversify in the Cretaceous.
Turiasaurs also survived the extinction and even expanded into North America during the Early Cretaceous.
Theropod diversity declined through the entire Late Jurassic, with medium-sized predators such as
megalosaurids being the hardest hit.
Ornithischian (particularly
stegosaur) diversity saw a small drop across the J–K boundary. Theropod and ornithischian extinctions were notably less pronounced than in sauropods. Most non-pterodactyloid
pterosaurs perished by the end of the Jurassic. Practically no earliest Cretaceous sites are known to preserve pterosaur fossils, so the precise timing of non-
pterodactyloid extinctions is very uncertain. 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. Coastal and freshwater turtle diversity also declined, at least in Europe. Many tetrapod groups saw strong (albeit gradual) ecological turnover through the J-K boundary. These groups include
lissamphibians,
lepidosaurs,
choristoderes, and
mammaliaforms. == References ==