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Journal of Paleontology 90(1):102–132
Gaussian distribution of the reported age uncertainty, of the five youngest grains is 136.3±4.1 (2σ) (Fig. 3.4). The pooling of the youngest population of grains minimizes the uncertainty based on single ablated grains and is the most robust means to estimate of the maximum depositional age (Dickinson and Gehrels, 2009).
The ~136Ma estimate for the maximum depositional age
of the Sonorasaurus quarry appears to be too old based on stratigraphic correlations of Cretaceous strata in southeastern Arizona, and it is likely that our samples simply failed to capture any younger volcanic grains that may have been in the fluvial system at the time. The Turney Ranch Formation was regarded as Albian-Cenomanian by Ratkevitch (1998) and Scarbourough (2000) on the basis a K/Ar minimum age of about 98 Ma. Dickinson and Lawton (2001) correlated the Mural Limestone in the Mule Mountains with the estuarine strata of the middle portion of the Shellenberger Canyon Formation in the Whetstone Mountains (Fig. 2). This represents the maximum extent of the Aptian-Albian (ca. 112 Ma) transgression in the area, and makes the overlying Shellenberger Canyon and Turney Ranch Formations in the Whetstone Mountains and the Cintura Formation in the Mule Mountains younger than this transgression. Dickinson et al. (2009) reported U/Pb ages from the Cintura Formation of northern Sonora, Mexico, which is possibly coeval to the Turney Ranch Formation or to the upper part of the underlying Shellenberger Canyon Formation (Ratkevitch, 1998; McCord and Gillette, 2005; Lucas and Heckert, 2005). These ages (108 and 111 Ma) were from detrital zircons thought to be derived from coeval volcanism to the west (Dickinson et al., 2009), suggesting a maximum age of Albian for the Cintura Formation. Thus, the Turney Ranch Formation is likely younger than ~110 Ma. Furthermore, while our non-random grain selection prohibits quantitative provenance analysis of the Turney Ranch Formation, the observed peaks younger than 285Ma and peaks at 1407Ma and 1675Ma are consistent with a sediment source from Upper to Middle Jurassic San Rafael Group eroded in the highlands to the north and west. This is similar to the Cintura Formation (Dickinson et al., 2009) and suggests the Turney Ranch Formation represents the continuation of the nonmarine depositional system that initiated after the Aptian-Albian transgression. Moreover, this age framework is in general agreement with late Albian-early Cenomanian age estimates for the Cintura Formation based on fossil bivalves (Scott, 2007; Scott et al., 2007; González-León et al., 2008). In sum, radiometric dates bracket the age of Sonorasaurus between ca. 98 and ca. 110 Ma; biostratigraphic evidence suggests that it lived sometime just before or after the Albian-Cenomanian boundary.
Description
The following description uses the nomenclature for vertebral laminae and fossae proposed by Wilson (1999, 2012) and Wilson et al. (2011), respectively. Traditional, ‘Romerian’ terminology is used (Wilson, 2006).
General remarks and taphonomy of the quarry.—Sonorasaurus thompsoni was found within an isolated, lenticular sandstone
unit in the lower portion of an 8.5-m thick succession of fluvi- ally deposited beds (Scarborough, 2000). The fluvial succession is underlain and overlain by thick overbank deposits dominated by red paleosols. The fluvial succession itself is composed of laminated and cross-bedded sandstones containing rounded green claystone clasts and plant debris interbedded with green to maroon mudstone lenses (sampled for detrital zircons) (Scar- borough, 2000). Scarborough (2000) argues this succession represents deposition along a meander bend and successive scour and fill events.We agree, but note that the scour surfaces (i.e., sandstones with mudstone rip-up clasts) likely represent channel reoccupation of the area after a local avulsion or lateral migration of the channel away from the area. Ratkevitch (1997b) depicts a roughly bimodal (approximately N-S and E-W) orientation for the bones in the quarry, but neither the methods used to create the map nor pictures of the quarry are available. It appears that the fossils did not exhibit a uniform orientation. Many of the long bones are broken into two or more pieces, and processes of the vertebrae are almost invariably broken off. Broken surfaces are not sharp, indicating that breakage occurred before or during deposition. We found no clear examples of desiccation flaking of the bone, insect traces, puncture marks, tooth drags, or other diagnostic features of significant scavenging on the remains (contra Ratkevitch, 1998). Overall, the variable orientation of the fossils, particu- larly long elements, presence of vegetation debris, and the lack of evidence of scavenging on the bone suggest relatively rapid burial after death and moderate transport that broke many of the bones.
Dorsal vertebrae.—All of the dorsal vertebrae are fragmentary and come from the posterior part of the series based on the absence of parapophyses on the centrum or near the neurocen- tral junction. All are missing most of their neural arches and parts of their centra (Fig. 9), but are preserved enough to show that neurocentral junctions were fused. Of the many dorsal vertebral fragments recovered at the Sonorasaurus quarry, parts of two are still articulated (ASDM 500-104). The dorsal ver- tebrae are somewhat deformed, and only one preserves its entire posterior centrum (Fig. 9.2, 9.3), which is deeply concave and measures about 27cm×21 cm. Centrum length is not preserved in any dorsal vertebra, but likely would have been in the range of 25–30 cm based on what is preserved. The centra are excavated by large lateral pneumatic fossae
and foramina (pleurocoels) that ramify into 2–5 cm diameter chambers but do not permeate the entire centrum or neural arch (Fig. 9). This type of pneumaticity represents the camerate to semicamellate condition (sensu Wedel, 2003). The neural arch is set anteriorly on a very elongate centrum—about one-third of the vertebral length occurs posterior to the base of the centropostzy- gapophyseal lamina (cpol). This condition resembles that of the middle dorsal vertebrae of Brachiosaurus (Fig. 9, Riggs, 1903) and Giraffatitan (Janensch 1950), in contrast to the more symmetrically placed neural arches of Camarasaurus (Osborn and Mook, 1921), Cedarosaurus (Tidwell et al., 1999), Tasta- vinsaurus (Canudo et al., 2008), Ligabuesaurus (MDD personal observation, 2010) and Sauroposeidon (D’Emic and Foreman, 2012). Several well-developed laminae connect the centra to their neural arches, including a centroprezygapophyseal lamina (cprl),
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