Dicraeosauridae

Dicraeosauridae is a family of diplodocoid sauropods who are the sister group to Diplodocidae. Dicraeosaurids are a part of the Flagellicaudata, along with Diplodocidae. Dicraeosauridae includes genera such as ''Amargasaurus'', ''Suuwassea'', ''Dicraeosaurus'', and ''Brachytrachelopan''. Specimens of this family have been found in North America, Asia, Africa, and South America. Their temporal range is from the Early or Middle Jurassic to the Early Cretaceous. Few dicraeosaurids survived into the Cretaceous, the youngest of which was ''Amargasaurus''.

The group was first described by German paleontologist Werner Janensch in 1914 with the discovery of ''Dicraeosaurus'' in Tanzania. Dicraeosauridae are distinct from other sauropods because of their relatively short neck size and small body size.

The clade is monophyletic and well-supported phylogenetically with thirteen unambiguous synapomorphies uniting it. They diverged from Diplodocidae in the Mid-Jurassic, as evidenced by the diversity of dicraeosaurids in both South America and East Africa when Gondwana was still united by land. However, there is some disagreement among paleontologists on the phylogenetic placement of ''Suuwassea'', the only genus of the Dicraeosauridae to be found in North America. It has been characterized as a basal dicraeosaurid by some and a member of the Diplodocidae by others. The placement of ''Suuwassea'' within Dicraeosauridae or Diplodocidae has substantial biogeographic implications for the evolution of Dicraeosauridae.

Classification

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. However, the classification of ''Suuwassea'' as a dicraeosaurid is not universally agreed upon. Some phylogenetic analyses have found ''Suuwassea'' to be a basal diplodocoid instead of a dicraeosaurid. One 2015 analysis has even found ''Dyslocosaurus'' as a member of Dicraeosauridae. A 2016 reappraisal of ''Amargatitanis'' has placed it into the Dicraeosauridae, as well. In 2018 a new genus, '' Pilmatueia'', was described.

Dicraeosaurids are differentiated from their sister group, diplodocids, and from most sauropods by their relatively small body size and short necks. 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.

Phylogeny

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 phylogeny published by Tschopp and colleagues in 2015 is as follows:

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".

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

Paleobiology

Feeding behavior

As sauropods, dicraeosaurids are obligate herbivores. Due to their relatively small necks and skull shape, it has been deduced that dicraeosaurids and diplodocids primarily browsed close to the ground or at mid height. Among the dicraeosurids, only ''Dicraeosaurus'' has well-preserved dentition. This makes it difficult for paleontologists to make definitive statements about Dicraeosauridae feeding behavior compared to diplodocid feeding behavior. However, compared to its known relatives, ''Dicraeosaurus'' is unique in that it has an equal number of teeth in the upper and lower jaw, though teeth in the lower jaw are replaced more slowly.

Anatomy

Dicraeosaurids are characterized by their relatively small body size, short necks, and long neural spines.Daniela Schwarz-Wings, 10th Annual Meeting of the European Association of Vertebrate Paleontologists. Royo-Torres, R, Gascó, F, and Alcalá, L., ''coord.'' ¡''Fundamental!'' 20: 1-290. 2012. They are 10–13 meters in body length. They share thirteen unambiguous synapomorphies including dorsal vertebrae without pleurocoels, the presence of a ventrally directed prong on the squamosal, and a subtriangular-shaped dentary symphysis.

Distribution and evolution

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. ''Amargasaurus'' was the latest surviving dicraeosaurid genus, living into the Early Cretaceous period.

Sources

{{Taxonbar, from=Q133812

Flagellicaudata

Early Jurassic first appearances
Early Cretaceous extinctions
Prehistoric dinosaur families