130


Journal of Paleontology 90(1):102–132


Farquhar et al., 1989) in the Turney Ranch Formation, consistent with a semi-arid climate. Plant macrofossil evidence indicates that the broad flood-


plains of the Turney Ranch Fm. supported a taxodiaceous/ cupressaceous (or possibly cheirolepidiaceous) evergreen woodland. Documented permineralized plant fossils from the Lower Cretaceous of southern Arizona so far consist only of conifer material. Stoyanow (1949) mentioned specimens identified as Araucarioxylon from the Lower Cretaceous Lowell Formation in southern Arizona but provided no data on the features used to make that determination. All of the wood known from the Turney Ranch Formation is coniferous. In contrast, plant fossils from the Cedar Mountain Formation and the Dakota Formation (Albian-Cenomanian) in Utah and northern Arizona provide evidence of forested landscapes with a mix of conifers, small angiosperm trees, and tree ferns (Bailey, 1924; Andrews and Kern, 1947; Thayne et al., 1983; Thayn et al., 1985; Tidwell and Thayn, 1985). To the east, fusainized and silicified conifer wood from the Albian of Texas is clearly associated with the foliage and reproductive organs identified as Frenelopsis ramosissima (Axsmith and Jacobs, 2005). The F. ramosissima wood is anatomically similar to the Turney Ranch wood in sharing abietinean pitting, mostly uniserate rays, indistinct/absent growth rings, and the absence of resin canals, but the tracheid and ray cells in the Turney Ranch wood are smaller, and the estimated tree height is taller for the Turney Ranch specimens. The low diversity of trees compared to assemblages from further north, if not due to sampling bias, probably reflects the ability of these southern trees to tolerate the periodic flooding that characterizes alluvial floodplain settings. The lack of growth rings in sampled wood seems to suggest that year-round conditions of temperature and water availability that were favorable to growth; however, even seasonally dry tropical forests can include both evergreen and deciduous species that do not produce distinct growth rings in the present (Worbes, 1999) and during the Cretaceous (Peralta-Medina and Falcon-Lang, 2012). Despite the absence of growth rings in the wood, the small to medium diameter of the tracheids suggests the presence of drought resistance necessary to cope with a warm, variable climate (Wilson and Jackson, 2006). In sum, Turney Ranch Formation sedimentological,


isotopic, and floral data together provide evidence for strong sub-annual variability in rainfall; there may have been one long dry season or several shorter dry periods spread throughout the year. In turn, these geological and floral data are congruent with the bone histology of Sonorasaurus, which indicates that its growth was generally more sporadic and slower than that of other sauropods (see above; Klein and Sander, 2008). The holotype and only known specimen of Sonorasaurus died in an ‘inland’ environment in the categorization of Mannion and Upchurch (2011); the full range of paleoenvironmental habitats inhabited by the species requires further discoveries to establish (D’Emic and Foreman, 2012).


Conclusions


Sonorasaurus, formerly one of the most enigmatic sauropods from the Cretaceous of North America, is herein shown to be taxonomically valid, one of the last sauropod taxa known before


their Late Cretaceous North American hiatus, and one of the youngest members of the clade Brachiosauridae. Our investi- gation of its bone histology indicates that Sonorasaurus grew slowly and somewhat sporadically and likely only reached about half the adult body mass of the Late Jurassic brachio- saurids Giraffatitan or Brachiosaurus. This slow, irregular growth may have been related to its harsh environment. Our reinterpretation of the paleoenvironment of the Turney Ranch Formation that hosts the only known remains of Sonorasaurus based on new geologic and floral data suggests that it inhabited an inland, low relief evergreen woodland with highly variable precipitation. The presence of a brachiosaurid, an ornithopod similar to Tenontosaurus, and a carcharodontosaurine theropod suggests that the relative faunal homogeneity that characterized other Early Cretaceous North American basins to the north and east extended to southernmost Arizona as well.


Accessibility of supplemental data


Supplemental data deposited in Dryad data package: http://dx. doi.org/10.5061/dryad.51j0j


Acknowledgments Thanks to R. Thompson for field assistance and information regarding the Sonorasaurus quarry and Bisbee Group in general. Thanks to D. Colodner, S. Norman, and C. Aslan (ASDM) for collections access, assistance, information, and hospitality at the ASDM. Thanks to M. Pecha at the University of Arizona LaserChron Center for assistance. J. Carballido (Museo Edigio Feruglio) kindly provided additional anatomical information about Europasaurus. Thanks to K. M. Melstrom (University of Utah) for field and collections assistance, B. Howes (Macalester College) for lab assistance, Jonathan Wingerath (Smithsonian Institution, NMNH) for assistance mak- ing thin sections of the silicified wood, C. Seiffert, S. Shurtleff, J. Latimer, A. Troidl, and T. Arce for assistance with permits, and R. Hill (New York Institute of Technology) and C. Rubin and S. Pongkitwitoon (Stony Brook University) for use ofmicroscopy equipment. Thanks toW. R. Dickinson for discussions regarding the stratigraphy of the Turney Ranch and Cintura formations. Fieldwork was performed under permit numbers KE 29-1169191 and 2013-076ps. Thanks to reviewersM.Wedel and J. Fronimos and editor D. Varricchio for comments that improved the manu- script. Funding was provided by the Scott Turner Award of the University of Michigan and the National Geographic Foundation’s Committee for Research and Exploration (grant number 9252-13) toM.D.D.


References


Andrews, H.N., and Kern, E.M., 1947, The Idaho Tempskyas and associated fossil plants: Annals of the Missouri Botanical Garden, v. 34, p. 119–183.


Archibald, L.E., 1982, Stratigraphy and sedimentology of the Bisbee Group in the Whetstone Mountains, Pima and Cochise Counties, Southeastern Arizona: Unpublished Master’s Thesis, University of Arizona, 208 pp.


Archibald, L.E., 1987, Stratigraphy and sedimentology of the Bisbee Group in the Whetstone Mountains, southeastern Arizona: Arizona Geological Society Digest, v. 18, p. 273–282.


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