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


macronarians, and Chubutisaurus + more deeply nested somphospondylans (decay index =2), Macronaria, Titano- sauriformes, and Somphospondyli (decay index=3), and Lithostrotia and Saltasauridae (decay index=5). Only one step is required to position Sonorasaurus as a more deeply nested brachiosaurid (or the sister taxon of any more deeply nested brachiosaurid), whereas two steps are required to position it as the sister taxon to Europasaurus or as the most basally branching brachiosaurid. Four steps are required to move Sonorasaurus outside Brachiosauridae, and 9 are required to position it as the sister taxon to the roughly contemporaneous North American taxon Sauroposeidon. Under DELTRAN, five synapomorphies support the monophyly of the Brachiosauridae (characters 5, 15, 40, 105, 121), 13 support the monophyly of brachiosaurids aside from Europasaurus (characters 2, 3, 4, 9, 10, 18, 36, 53, 56, 78, 81, 118, 112), one supports the mono- phyly of Giraffatitan and Sonorasaurus (character 120), and two support the five-way polytomy of deeply nested brachio- saurids (characters 79 and 93).


Discussion


Phylogenetic relationships of Sonorasaurus.—As perhaps one of the youngest sauropod taxa before the start of their Late Cretaceous North American hiatus, the phylogenetic relation- ships of Sonorasaurus are important for understanding the number of sauropod clades affected by their extirpation (D’Emic and Foreman, 2012). Anatomical information perti- nent to the phylogenetic affinities of Sonorasaurus thompsoni is discussed below, based on the analysis presented herein and with the original author of each character cited. The dorsal vertebrae of Sonorasaurus do not display the sub-centimeter, pervasive pneumaticity that is characteristic of somphospondy- lan sauropods (Wilson and Sereno, 1998), but instead have camerate to semicamellate pneumaticity (Wedel, 2003) as in basal titanosauriforms (e.g., Giraffatitan, Janensch, 1950). The dorsal vertebrae of Sonorasaurus also possess a hyposphene- hypantrum, suggesting affinities outside of deeply nested titanosaurs (Salgado et al., 1997). Its caudal vertebrae are not procoelous, again suggesting affinities outside of derived titanosaurs. The limbs of Sonorasaurus are gracile, as in brachiosaurids (Janensch, 1961), some basal somphospondy- lans (Bonaparte et al., 2006, D’Emic, 2012), and some titano- saurs (Gomani, 2005). The high metacarpal-to-radius ratio suggests affinities within Macronaria, and the undivided and orthogonally oriented distal end of metacarpal I (Wilson, 2002) and the anteroposteriorly compressed pubic peduncle of the ilium (D’Emic, 2012) suggest placement within Titanosaur- iformes. The straight shaft and absence of an anterior crest on the proximal fibula suggest affinities outside or at the base of Somphospondyli (D’Emic, 2012). Abruptly posteriorly expan- ded postzygapophyses are only found in Sonorasaurus, Giraffatitan, Europasaurus, Cedarosaurus, Lusotitan,and some specimens of Camarasaurus (this analysis). A ‘shoulder’ on the last several anterior caudal vertebrae (character 122) is found in Sonorasaurus, Giraffatitan, Lusotitan, Andesaurus,and some specimens of Camarasaurus, and is optimized in one part of the tree as a synapomorphy of brachiosauridsmore deeply nested than Europasaurus (this analysis). The medial embayment of the


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proximal end of metatarsal IV suggests titanosauriform affinities (D’Emic, 2012). The bevel of the distal end of metatarsal IV is a feature shared only with brachiosaurids (e.g., Abydosaurus, Giraffatitan;D’Emic, 2012). Strongly posteriorly expanded middle dorsal vertebral centra are only found in Sonorasaurus, Giraffatitan,and Atlasaurus. The large amount of missing data present in Brachiosauridae and its disjunct anatomical pattern suggest caution in interpreting the lower-level relationships recoveredwithin the clade. In other words, although Sonorasaurus can safely be regarded as a brachiosaurid, its lower-level affinities require additional data to firmly establish.


Interpretation of the paleoenvironment of Sonorasaurus.—Our interpretation of the paleoenvironment of the Turney Ranch Formation is that of a well-drained, semi-arid alluvial plain subject to variable precipitation events. Previous studies (Archibald, 1982; 1987) argued for estuarine or coastal influ- ence on deposition; however, we view this interpretation as problematic. First, no marine fossils have been recovered from the Turney Ranch Formation, nor have definitive marine trace fossils (Archibald, 1982; 1987). Indeed, there is a growing appreciation for the degree to which fully terrestrial strata can be significantly bioturbated (Hasiotis, 2002) and the presence of intense bioturbation in a localized area does not necessitate marine or estuarine conditions. Furthermore, previous reports of an Albian-Cenomanian marine interval at the top of the Turney Ranch Formation (Inman, 1987; Dickinson et al., 1989), transitional into the Fort Crittenden Formation are incorrect. Reassessment of the transition instead suggests a purely fluvial origin based on various sedimentary structures, internal scour surfaces, freshwater invertebrate fossils, and abundant pedo- genic carbonate nodules (William R. Dickinson, personal communcation 2014). Upper plane bed laminations are a com- mon occurrence within fully fluvial deposits and particularly common in fluvial systems subject to flashy, seasonal discharge (Fielding, 2006). Additionally, bar clinoforms do not display mud drapes on the foreset laminae (i.e., inclined heterolithic strata) commonly attributed to tidal influences on river flow patterns. The coarse-grained sandstone and conglomeratic units


with terrestrial signals. The majority of δ18O values fall within the range of other pedogenic carbonates of the Early and Late Cretaceous (White et al., 2001; Foreman et al., 2011). If we assume similar fractionation against 12C between soil organic matter and pedogenic carbonate (~15‰) as observed in modern soils (Cerling and Quade, 1993; Koch, 1998), most Turney Ranch Formation δ13C values fall within the predicted range (−7‰to −11‰accounting for a difference in in the δ13Cvalues between modern and mid-Cretaceous atmospheric CO2) if the landscape was dominated by C3 vegetation. Higher δ18O and δ13C values may indicate evaporation within the soil system and water stress to the plants (Cerling and Quade, 1993;


that entomb petrified logs combined with common scour-and- fill patterns within the fluvial systems (Archibald, 1982; this study) also support a flashy discharge regime for Turney Ranch fluvial systems. Ubiquitous development of pedogenic carbonate nodules and hardpan layers occur in modern environments where mean annual rainfall is less than 1.0 m/year and is variable (Cerling, 1984). Furthermore, all stable isotope results are firmly consistent


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