The sauropod hiatus is a period in the North American fossil record for most of the Late Cretaceous noted for its lack of sauropod remains. It may represent an extinction event, possibly caused by competition with ornithischian herbivores, habitat loss from the expansion of the Western Interior Seaway, or both. Alternatively, it has been argued that the hiatus represents a decrease in inland deposits that would have effectively preserved the animals, creating the illusion of an extinction. The sauropod hiatus ended shortly before the end of the Cretaceous, with the appearance of Alamosaurus, most likely an immigrant from South America, in the southern parts of North America.
History of the concept
Definite evidence of Late Cretaceous sauropods in North America was first discovered in 1922, when Charles Whitney Gilmore described Alamosaurus sanjuanensis. The term "sauropod hiatus" was coined by researchers Spencer G. Lucas and Adrian P. Hunt in 1989 to describes how fossils of the clade become scarce in western North America near the beginning of the Late Cretaceous. Another gap in the North American sauropod record was noted in the Early Cretaceous, from the Berriasian to the Barremian, but this gap was partially filled by the discovery of fragmentary remains of a Camarasaurus-like sauropod from the Lakota Formation in 2014. The fossil record of sauropods in this part of the Early Cretaceous is poor globally, not only in North America.
Background
Around the end of the Early Jurassic, sauropods, a clade of large, long-necked dinosaurs, established themselves as the "dominant group" of large herbivores in land-based environments, possibly following a global warming event. They achieved global distribution, becoming a significant and diverse component of Mesozoic ecosystems. Sauropods were highly successful in North America during the Late Jurassic; the Morrison Formation of the western United States preserves one of the best-known of all sauropod faunas, with 24 recognized sauropod species. The fossil record of North American sauropods is poor for the first half of the Early Cretaceous, but later in the Early Cretaceous, several lineages of sauropod are known from North America, including turiasaurs such as Moabosaurus, brachiosaurids such as Cedarosaurus, and somphospondyls such as Astrophocaudia.
Beginning of the hiatus
The North American sauropod hiatus began during the Cenomanian, the first age of the Late Cretaceous. Several sauropods are known from the upper Albian or lower Cenomanian of North America. The brachiosaurid Abydosaurus and indeterminate sauropod teeth have been found in the Mussentuchit Member, with the maximum age of the Abydosaurus fossils calculated to be 104.46±0.95 Ma. The age of the brachiosaurid Sonorasaurus is not well-constrained, but probably was between 98 and 110 Ma. Sauropod bones have been reported from the Dakota Formation. Remains possibly referable to Sauroposeidon have been found in strata with a maximal age calculated to be 104.4 Ma.
The species epithet of Sauroposeidon proteles means "perfected before the end" in reference to its status as one of the last North American sauropods before the sauropod hiatus.
Possible causes
The cause of the sauropod hiatus has been a subject of debate. Lucas and Hunt proposed two hypotheses: a lack of properly preserved remains or a genuine extinction followed by re-population from South America. In the first scenario—rejected by Lucas and Hunt themselves but later further explained by Thomas Lehman in 2001—as sea levels rose, the sauropods became restricted to upland environments that were not as well represented in fossil record as coastal ones, before eventually migrating back to lowland areas as they became more suitable. In the second, the North American sauropods became extinct, and those that appear near the end of the Cretaceous represent immigrants from South America via land bridges, belonging to a group known as titanosaurians.
In 2011, Philip D. Mannion and Paul Upchurch re-evaluated the sauropod hiatus, taking numerous factors into account. They argued that the event represented sampling bias, as the sauropods' preferred inland habitat is not as well represented in the fossil record as coastal regions, also claiming that the existence of Cenomanian-aged Sonorasaurus thompsoni shortened the gap to the Turonian–early Campanian.