EUHELOPODIDAE (Romer, 1956)

D, Michael D. & Emic, 2012, The early evolution of titanosauriform sauropod dinosaurs, Zoological Journal of the Linnean Society 166 (3), pp. 624-671 : 643-644

EUHELOPODIDAE

The name ‘Euhelopodidae’ was originally employed to describe a clade containing Euhelopus and some Jurassic East Asian forms ( Romer, 1956; Upchurch, 1995). Although the name ‘Euhelopodidae’ has been applied to clades in some studies (e.g. Upchurch, 1995; Upchurch, 1998), it has never received a definition using phylogenetic nomenclature; only its content has been described or pointed to by labelling a cladogram. This content varies by phylogenetic analysis; Mamenchisaurus , Omeisaurus , Shunosaurus , and Euhelopus have all been considered members (see review in Wilson, 2002). The name ‘Euhelopodidae’ is not currently in use ( Wilson & Upchurch, 2009). Most of the fluidity in euhelopodid membership is because of the conflicting placement of Euhelopus in different phylogenies. For example, Wilson & Sereno (1998) and Wilson (2002) recovered it as the sister taxon of Titanosauria, whereas Upchurch (1998) and Upchurch et al. (2004) recovered it as a non-neosauropod. Recent restudy and rescoring of the data matrices of Wilson (2002) and Upchurch et al. (2004) favoured the conclusions of the former study, that Euhelopus is closely related to titanosaurs ( Wilson & Upchurch, 2009).

Herein Euhelopodidae is defined using phylogenetic nomenclature as a stem-based taxon comprising all sauropods more closely related to Euhelopus zdanskyi than Neuquensaurus australis (see Table 1 for phylogenetic nomenclature). I have chosen to define and employ Euhelopodidae herein (rather than coin and define a novel name) because (1) the name with its old definition has been in disuse for a decade; (2) coining new names instead of using old ones proliferates nomenclature, which should be avoided if possible; (3) the name does carry some of the original intended meaning with its new definition (i.e. Euhelopus - like, Asian sauropods). Regarding the last point, in this analysis, a previously unrecognized group of six Early-middle Cretaceous East Asian taxa is recovered: Qiaowanlong , Erketu , Daxiatitan , Euhelopus , Phuwiangosaurus , and Tangvayosaurus . Likewise, several fragmentarily represented taxa that were not included in this analysis seem to have affinities with these taxa based on the presence of synapomorphies recovered in this analysis (see ‘Fragmentarily represented taxa’ below).

Usually the six taxa recovered as euhelopodids in this analysis have been recovered as basal somphospondylans or basal titanosaurs when considered in cladistic analyses previously (e.g. You et al., 2008; Ksepka & Norell, 2010; Suteethorn et al., 2010), but features novel to this study suggest their monophyly (see Appendices 2, 3). Excluding these fragmentarily represented, basal taxa (e.g. Erketu , Qiaowanlong ) from the analysis tends to increase Bremer support for more derived euhelopodid clades. New discoveries or more complete descriptions may provide character scores that support a more derived position for basal forms such as Qiaowanlong or Erketu . Euhelopodid monophyly is supported by two unambiguous synapomorphies: (1) bifid cervical vertebrae and (2) cervical vertebrae with thick, subhorizontal epipophyseal– prezygapophyseal lamina. Nine additional synapomorphies support Euhelopodidae under ACCTRAN ( Tables 5, 6).

Qiaowanlong was originally described as a brachiosaurid, a position refuted by Ksepka & Norell (2010), Mannion & Calvo (2011), and this analysis. The early identification of Sauroposeidon as a brachiosaurid is likely to have contributed to the original description of Qiaowanlong as such, because most comparisons in its original description were focused on Sauroposeidon ( You & Li, 2009) . Three steps are required to position Qiaowanlong within Brachiosauridae according to this analysis, and a Templeton test rejects such a position (N = 9, P = 0.004). The position of Erketu is likewise supported by two synapomorphies, and the position of more derived euhelopodids is supported by a suite of nine features, including prong-like epipophyses, ‘trifid’ posterior cervical and anterior dorsal neural spines, and a low, pointed preacetabular process of the ilium.

Tangvayosaurus and Phuwiangosaurus are sister taxa within derived Euhelopodidae , in contrast to various studies that have suggested that these taxa are basal titanosaurs ( Allain et al., 1999; Upchurch et al., 2004; Canudo et al., 2008; Carballido et al., 2011a). Ten and three steps are required to place Phuwiangosaurus and Tangvayosaurus within the Titanosauria, respectively. Templeton tests reject the titanosaur affinities of both genera ( Phuwiangosaurus : N = 47, P = 0.0001; Tangvayosaurus : N = 23, P <0.0001).