Dicraeosauridae

Dicraeosaurids, drawn to scale.

Dicraeosaurids are a part of Diplodocoidea and are the sister group to Diploidocidae. In the past two decades, the known diversity of the group has doubled.[5] However, the classification of Suuwassea as a dicraeosaurid is not universally agreed upon.[5][7] Some phylogenetic analyses have found Suuwassea to be a basal diplodocoid instead of a dicraeosaurid.[7] One 2015 analysis has even found Dyslocosaurus as a member of Dicraeosauridae.[9] A 2016 reappraisal of Amargatitanis has placed it into the Dicraeosauridae, as well.[10] In 2018 a new genus, Pilmatueia, was described.[11]

Size comparison of the most complete members of the sauropod family Dicraeosauridae

Dicraeosaurids are differentiated from their sister group, diplodocids, and from most sauropods by their relatively small body size and short necks.[12] Dicraeosaurids are advanced sauropods within the monophyletic clade Neosauropoda, which is generally characterized by gigantism. The relatively small body size of dicraeosaurids make them an important outlier relative to other taxa in Neosauropoda.[13]

There have been several different proposed phylogenies of Dicraeosauridae and the intra-group cladistics are not resolved. Suuwassea is variably positioned as either a basal dicraeosaurid or a basal diplodocoid. The most recently published phylogeny by Tschopp et al. (2015) is as follows:[9]

Tschopp includes Dyslocosaurus and Dystrophaeus as dicraeosaurids, two genera traditionally not considered to be part of Dicraeosauridae. The specimens of Dystrophaeus viamelae are highly fragmentary, with only a few bones available for study including an ulna, partial scapula, partial dorsal vertebrae, a distal radius, and some metacarpals. Dyslocosaurus polyonychius also has extremely limited fossil evidence that only includes appendicular elements, and the position of it in Tschopp's phylogeny is therefore considered "preliminary".[9]

Several studies, however, do not include even Suuwassea in Dicraeosauridae, such as Sereno et al. (2007);[12] and JD Harris (2006).[8] Other studies, however, recover Suuwassea as a basal dicraeosaurid, including Whitlock (2010)[5] and Salgado et al. (2006).[14]

Dicraeosaurid specimens have been found in three continents - Africa, South America, and North America. The distribution of species is primarily Gondwonan, with the exception of the North American Suuwassea. The presence of Suuwassea in North America is unique among dicraeosaurids, therefore making the proper taxonomic classification of Suuwassea essential. The group likely first diverged from the diplodocids in the middle Jurassic in North America and subsequently dispersed into Gondwana, with the most diversity in East Africa and South America.[5] Amargasaurus was the latest surviving dicraeosaurid genus, living into the Early Cretaceous period.[5]