D’Emic et al.—Revision of the sauropod dinosaur Sonorasaurus


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D’Emic, 2012), stating that the “only character differentiating Sonorasaurus fromBrachiosaurus is its temporal location.” Later cladistic and comparative studies have varied in their support for brachiosaurid affinities for Sonorasaurus. For example, Sonorasaurus was found to lie outside Brachiosauridae in one of the analyses presented by Royo-Torres (2009); instead it was hypothesized to be amember of ‘Laurasiformes’ alongwith other taxa traditionally considered to be brachiosaurids, including Cedarosaurus and Venenosaurus (D’Emic, 2012). In contrast to the results of Royo-Torres (2009), D’Emic (2012) identified Sonorasaurus as a brachiosaurid but noted that further study was necessary to firmly establish its affinities. Mannion et al. (2013) resolved Sonorasaurus in a variety of positions within Titanosauriformes, including within Brachiosauridae or various clades within Somphospondyli depending on how the data were analyzed. In none of these analyses was the recovered phylogenetic position of Sonorasaurus strongly supported. The labile phylogenetic position of Sonorasaurus among various studies suggests that any hypothesis of relationship based on currently published data is weak. This is likely because these hypotheses have been based on little information, because the original description (i.e., the data used by most researchers) only figured some of the bones of the holotype. This prompted our redescription of the osteology of Sonorasaurus along with a reconsideration of its phylogenetic affinities, as presented below.


Sonorasaurus is also important because of its potentially


Figure 1. Locality information for Sonorasaurus thompsoni:(1) Location of Sonorasaurus thompsoni (black star) and other Aptian–Cenomanian sauropod body fossil sites (gray stars) in the western United States (modified from D’Emic, 2013; D’Emic and Foster, in press); (2) Simplified geologic map of southeastern Arizona showing pre-Quaternary exposures (inside of solid black lines), extent of Cretaceous sedimentary rocks (gray shaded areas), and location of fossil site (black star) (modified from Inman, 1987).


The formal description of Sonorasaurus was followed up by


further anatomical (Curtice, 2000), geological (Scarborough, 2000), and phylogenetic (e.g., Royo-Torres, 2009; D’Emic, 2012; Mannion et al., 2013) studies. Wilson (2002) recognized Sonorasaurus to be a member of the clade uniting Jobaria plus Neosauropoda, whereas Upchurch et al. (2004) considered it to be a nomen dubium. Curtice (2000:87) described strong similar- ity between Sonorasaurus and Brachiosaurus (then including Giraffatitan brancai as Brachiosaurus brancai;see Taylor, 2009;


young geological age, possibly representing the last sauropod taxon before the ‘sauropod hiatus,’ the ca. Cenomanian- Campanian absence of the clade from the North American record (see D’Emic and Foreman, 2012 and references therein). Based on regional correlations and assumptions about sedimentation rates, Ratkevitch (1997a:213) suggested that “the Turney Ranch Formation should overlap the Lower/Upper (Albian/Cenomanian) Cretaceous boundary” and placed its bounds between 110 and 76 Ma. Later, Ratkevitch (1998) and Scarborough (2000) narrowed Sonorasaurus’ hypothesized age range to closer to the Early-Late Cretaceous boundary, with the latter author favoring a late Albian (105–100 Ma) age. Mannion and Upchurch (2011:532) followed these authors, stating that Sonorasaurus was “late Albian–early Cenomanian” in age, using it as one piece of evidence to discount the reality of the “sauropod hiatus” in North America as an artefact of poor sampling. Mannion and Upchurch (2011) suggested that the sauropod hiatus was an artifact of the paleoenvironmental preferences of pre- and post-hiatus sauropods and differential preservation of certain paleoenvironments (i.e., ‘inland’ versus ‘coastal’) during the hiatus. Sonorasaurus also represents one of the last purported brachiosaurid sauropods worldwide, highlighting the need to establish its geologic age with more certainty. Like its geologic age, the published understanding of


the paleoenvironment of Sonorasaurus is ambiguous—for example, Ratkevitch (1998:79) described the Turney Ranch Formation as “fluvial and possibly estuarine…”. Perhaps due to this uncertainty, Sonorasaurus and its paleoenvironment were not included in the numerical analyses of sauropod environmental preferences by Mannion and Upchurch (2011:supplemental information), despite its importance as one of or the youngest


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