Sonorasaurus thompsoni, Ratkevich & Scarborough, 1998

D, Michael D. & Emic, 2012, The early evolution of titanosauriform sauropod dinosaurs, Zoological Journal of the Linnean Society 166 (3), pp. 624-671 : 650

SONORASAURUS THOMPSONI ( RATKEVITCH, 1998)

Sonorasaurus was originally described as a brachiosaurid and is important because of its Albian–?Cenomanian age, which would be on a par with the youngest known North American sauropods before the start of the ‘sauropod hiatus’ ( Lucas & Hunt, 1989; Ratkevitch, 1998). Sonorasaurus is represented by a somewhat fragmentary partial skeleton, which includes presacral and caudal vertebrae and some limb elements. Sonorasaurus is a titanosauriform on the basis of semicamellate presacral vertebral pneumaticity, middle caudal vertebrae with neural arches set on the anterior half of the centrum, anterior- middle caudal vertebrae with posteriorly projecting transverse processes, metacarpal I with an undivided distal condyle that is perpendicular to the shaft, metacarpals with reduced or absent distal articular facets, and a fibula that lacks a corrugated subtriangular proximal scar (Appendix 3).

Brachiosaurid affinities are supported for Sonorasaurus on the basis of metatarsal IV bevelled distally and metatarsal IV with a medial embayment on proximal end (Appendix 3). Previous hypotheses for brachiosaurid affinities for Sonorasaurus were based on its elongate forelimb bones ( Ratkevitch, 1998), but more recent discoveries have shown that similarly elongate limb bones are present in taxa that are here resolved as basal somphospondylans (e.g. Ligabuesaurus , Sauroposeidon ) and basal titanosaurs (e.g. Andesaurus , Malawisaurus ). Further data are needed to firmly establish the affinities of Sonorasaurus .