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Mamenchisauridae is a family of large, quadrupedal, herbivorous sauropodomorph dinosaurs. The family is characterized as having incredibly long cervical regions, with several members of Mamenchisauridae having necks that comprise over half their body length. undefined Cladistic analysis has placed Mamenchisauridae in the derived clade Eusauropoda and describes the group as more closely related to Neosauropoda than to Shunosaurus. The family flourished mainly in the Jurassic period, but there is evidence suggesting it lived through the Early Cretaceous period. Specimens belonging to Mamenchisauridae have been found exclusively on the Asian and African continents. It is hypothesized that mamenchisaurids were adapted for both shoreline and forest environments, and that the family contained the largest dinosaur species in China.

Characteristics
Mamenchisauridae was defined by Chinese paleontologists Young and Zhao in 1972 based on the discovery of a skeleton they named Mamenchisaurus hochuanensis. This skeleton is considered to be the type specimen of this family and has several autapomorphic skeletal features that distinguish Mamenchisauridae from other sauropod families. According to their definition, mamenchisaurids are characterized by 19 elongated cervicals, 4 fused sacrals, and 16 procoelous caudals with lateral processes, among other features. Huang et. al further described the unique skeletal features of this family in 2014. They identified an outward torsion of the distal humerus, as well as cranial processes on both the distal humerus and proximal ulna.

Phylogenetics
Mamenchisauridae belongs to the derived clade Eusauropoda, or “true sauropods”, which is nested within the larger suborder of Sauropodomorpha. The phylogenetic placement of Mamenchisauridae is relatively uncontested, although the phylogeny of some species within the family are unresolved.

The cladogram below was developed by Young & Zhao in their 1972 publication that first defined Mamenchisauridae.

The cladogram below follows that published in a 1998 paper by Wilson & Sereno concerning phylogeny of sauropods. In this diagram, the family Mamenchisauridae is represented by Omeisaurus, a genus that falls within this group.

The cladogram below follows that published by Huang et. al in a 2014 paper that defined additional members of Mamenchisauridae.

Paleogeography
Mamenchisaurids flourished between the Early Jurassic and Early Cretaceous periods. Fossils of Tonganosaurus have been dated to the Early Jurassic period. Mamenchisaurus fossils from the Shaximiao Formation date between 170.3-163.5 Ma, Chuanjiesaurus fossils date between 166.1-163.5 Ma , and Eomamenchisaurus fossils from the Zhanghe Formation date between 175.6-161.2 Ma – all of which fall in the Middle-Late Jurassic periods. An indeterminate cervical vertebra from the Phu Kradung Formation in Thailand suggests that mamenchisaurids lived into the Early Cretaceous.

Mamenchisaurids have been found primarily, but not exclusively, in China. Fossils of Wamweracaudia extend Mamenchisauridae into Africa, while mamenchisaurid remains found in the Itat Formation extend the group into Siberia.

Habitat
Young & Zhao concluded that Mamenchisaurus was adapted to both aquatic and terrestrial environments, and likely lived and ate along the shoreline. Paleontologist Gregory S. Paul theorized that Omeisaurus was adapted for and lived in dense forests. He also hypothesized that the genera within Mamenchisauridae were adapted to a wide range of habitats.

Physiology
Young & Zhao estimated the Mamenchisaurus specimen they discovered to be 22 meters long and weigh 45 tons in life. At the time of their paper’s publication, their specimen represented the largest dinosaur species found in China. Griebeler et al. performed long-bone histology on an unnamed mamenchisaurid SGP 2006/9 in a larger paper studying the lifespan of sauropods. They estimated their individual to have weighed 27.6 tons, reached maturity at 20 years, and died at 31 years.