Nemegtosauridae

Nemegtosaurids
Temporal range: Early Cretaceous - Late Cretaceous, 117–66 Ma
Skeleton of a juvenile Rapetosaurus krausei, Field Museum of Natural History
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Order: Saurischia
Suborder: †Sauropodomorpha
Clade: †Titanosauria
Clade: †Lithostrotia
Family: †Nemegtosauridae
Barrett & Upchurch, 1995
Subgroups

See text.

Nemegtosauridae is a family of probably titanosaurian sauropod dinosaurs based originally on two late Cretaceous Mongolian species known only from their diplodocid-like skulls: Nemegtosaurus and Quaesitosaurus. Authorities disagree as to the relationship of these two genera with other sauropods, and indeed whether they even constitute a valid family. More recently, additional sauropods have been added to this group, and cladistic methodology has been used to shed further light on the evolutionary relationships of these animals. It is named after the Nemegt Basin in Mongolia, where some of its fossils were found.

History of classification

McIntosh 1990 included both these animals in the family Diplodocidae, subfamily Dicraeosaurinae, as they resemble the skull of Dicraeosaurus, although differing in certain details. Although the skull of Nemegtosaurus was found in the same formation as the headless skeleton of Opisthocoelicaudia, McIntosh rejects the possibility that they belong to the same animal, as Nemegtosaurus is diplodocid-like, but Opisthocoelicaudia camarasaur-like.

Upchurch 1995 placed the two genera in their own family, without giving a cladistic definition. However, the name first appeared in print a few weeks earlier in a paper by Barrett and Upchurch.

Wilson in 2002 referred the two genera to the family Nemegtosauridae, and on the basis of cladistic analysis, transferred them from the Diplodocoidea to the Titanosauria

Apesteguia (2004), in a paper describing a new Patagonian sauropod, Bonitasaura salgadoi, may have been the first to properly define the taxon, although without the use of cladistic analysis: the stemclade consisting of all titanosaurs more closely related to Nemegtosaurus than to Saltasaurus. He argued for a close relationship between Nemegtosaurus, Quaesitosaurus, Rapetosaurus, and Bonitasaura and referred to the previous phylogenetic analysis and use of Nemegtosauridae by Wilson (2002).

Upchurch et al. 2004 returned the Nemegtosauridae to the Diplodocoidea, and defined the former as the stem clade consisting of all Diplodocoidea closer to Nemegtosaurus than to Diplodocus. The only known species were Nemegtosaurus itself and Queasitosaurus.

Curry Rogers 2005, in one of the most complete cladistic analysis of the Titanosauria to date, returns both Nemegtosaurus and Quaesitosaurus to the titanosaurs, but rejects the validity of both the family Nemegtosauridae and the clade concepts given under that name. Quaesitosaurus is placed in the Saltasaurinae and Nemegtosaurus in a new, unnamed "Rapetosaurus clade" (which, under ICZN rules, would, if named, be termed subfamily Nemegtosaurinae or tribe Nemegtosaurini, depending on its position). She places Opisthocoelicaudia in a separate clade, the Opisthocoelicaudiinae. All three clades are included in the Saltasauridae (= Titanosauridae).

Unaware of Apesteguia's work, Wilson (2005) proposed a definition identical to his and further clarified the anatomy and relationships of the central genus Nemegtosaurus. He thus also defines the Nemegtosauridae as a stem-based clade that includes all titanosaurs more closely related to Nemegtosaurus than to Saltasaurus. He also suggests that Opisthocoelicaudia may eventually be shown to be a junior synonym of Nemegtosaurus.

Phylogeny

The cladogram below follows Zaher et al. (2011).[1]

Lithostrotia 

Malawisaurus
Nemegtosauridae 

Nemegtosaurus

Rapetosaurus

Tapuiasaurus

Isisaurus

Diamantinasaurus


Saltasauridae 
Opisthocoelicaudiinae 

Alamosaurus

Opisthocoelicaudia



Saltasaurinae 

Neuquensaurus

Saltasaurus








References

  1. ↑ Hussam Zaher, Diego Pol, Alberto B. Carvalho, Paulo M. Nascimento, Claudio Riccomini, Peter Larson, Rubén Juarez-Valieri, Ricardo Pires-Domingues, Nelson Jorge da Silva Jr., Diógenes de Almeida Campos (2011). "A Complete Skull of an Early Cretaceous Sauropod and the Evolution of Advanced Titanosaurians". PLoS ONE 6 (2): e16663. doi:10.1371/journal.pone.0016663. PMC 3034730. PMID 21326881.

Sources

External links

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