D’Emic et al.—Revision of the sauropod dinosaur Sonorasaurus
Fibula.—The right fibula is nearly complete, subequal in length to the radius and about four times the length of the central metatarsals (Table 2). The shaft is straight with a D-shaped cross section, and the proximal and distal ends are orthogonal to the
shaft in mediolateral views (Fig. 20). The proximal half of the fibula expands gently on its anterior side but does not bear a well-developed anterior crest as in some titanosauriforms (e.g., Erketu, Euhelopus, Neuquensaurus, Wilson and Upchurch, 2009). The proximal end has a shallow medial fossa. Just above the midshaft, the lateral end bears a muscle scar composed of two bulbous ridges as in most sauropods (e.g., Camarasaurus, Ostrom and McIntosh, 1966; Giraffatitan, Janensch, 1961, Neuquensaurus, Salgado et al., 2005). The distal end is beveled slightly upwards laterally and it expands abruptly as a dorsoventrally thick shelf in the medial direction (Fig. 20). The distal end of the fibula is parallelogram-shaped in distal view.
Pes.—Four right metatarsals were recovered from the Sonor- asaurus quarry (Fig. 21); they are identified as metatarsals I, II, IV, and V based on comparisons with other sauropods (e.g., Giraffatitan, Cedarosaurus, Gobititan). Eusauropod meta- tarsals are oriented subhorizontally (Wilson and Sereno, 1998; Carrano, 2005) and will be described as such here. Metatarsal I is the shortest element, followed by metatarsal V; metatarsals II and IV are longer and subequal in length (Table 2). No collateral ligament pits or medial/lateral tubercles are present on the distal ends or shafts of the metatarsals. Metatarsal I has a stout shaft with weakly flaring proximal
and distal ends. The proximal end is D-shaped, pointed dorsally, and longer dorsoventrally than transversely. The proximal (astragalar) articular face is beveled about 30 degrees relative to the long axis of the bone in both transverse (downwards medially) and dorsoventral (downward ventrally) senses. The distal end is undivided distally but bears two subequally developed condyles posteriorly, and is strongly beveled upward medially. A posterolateral process is present, although it is weaker than that observed in some diplodocoids (McIntosh et al., 1992) or other sauropods such as Giraffatitan (D’Emic, 2012). No tubercles are found on the medial and/or lateral face of metatarsal I, in contrast to the situation in Cedarosaurus and Venenosaurus. Metatarsal I of Sonorasaurus is more gracile than that bone in either of the two latter genera (Figs. 21–23). Metatarsal II is subrectangular in proximal view, with flat
dorsal and medial faces and a curved lateral face for articulation with metatarsal
III.Aweak fossa is present on the ventral face of the proximal end as in some other sauropods (e.g., Ligabue- saurus, Gobititan; M.D.D. personal observation). The distal end of metatarsal II is beveled upward medially and is weakly divided. Metatarsal IV is more elongate than the other metatarsals. The proximal face is somewhat kidney-shaped, with a medial embayment as in some other titanosauriforms (D’Emic et al., 2011). Its proximal margin is roughly orthogonal to the bone; in contrast, its distal end is strongly beveled upwards medially (ca. 20 degrees), unlike the condition in most sauropods, but similar to the condition in brachiosaurids (D’Emic, 2012). The lateral margin of the shaft is nearly straight, whereas the medial margin is strongly curved. As in the other metatarsals (and metacarpals), the medial part of the distal end of the bone is more strongly developed than the lateral part.
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The dorsal face of the distal end is flat, whereas the ventral face is divided. Metatarsal V is funnel–shaped and dorsoventrally compressed as in other saurischians. Metatarsal V is autapomorphically elongate compared to other sauropods with the exception of Cedarosaurus, which has a rod-like (i.e., unexpanded) fifth metatarsal (D’Emic, 2013). The proximal end forms a sharp angle across its face transversely in dorsoventral views, unlike the flat proximal ends of the other metatarsals. The area medial to this sharp bend represents a facet for articulation with metatarsal IV. The proximal end flares to over three times the midshaft breadth, whereas the distal end expands modestly to form a bulb. This is very different from the narrow proximal end of metatarsalVin Cedarosaurus (Fig. 21). The lateral curvature of metatarsal V is tighter than the medial curvature. On the dorsal face of the midshaft, a slight roughened area might indicate the insertion of a ligament or extensor tendon; this roughened area corresponds to a raised welt in some other sauropods (e.g., Gobititan).
Phalanges.—Several phalanges were preserved in the holotypic quarry (Fig. 24); their disarticulation and poor preservation makes it difficult to identify them. The ungual phalangeal frag- ments are too poorly preserved to certainly tell if they belong to the manus or pes. The non-ungual phalanges likely belong to the pes based on the lack of distal flaring that would be present in manual phalanges, weak collateral ligament pits, and the overall proportions of the elements when compared with other sauropods (Fig. 24). The phalangeal formula is unknown. The non-ungual phalanges are broader transversely than
they are dorsoventrally tall or proximodistally long (Table 2). Their proximal faces are gently cupped whereas their distal faces are convex and weakly divided into two condyles; this division is deeper ventrally. In proximal view the phalanges are D-shaped, with a flat ventral edge and a rounded dorsal edge whose apex is offset medially. The medial halves of the elements are more robust and have flat faces, whereas the lateral halves taper in dorsoventral and proximodistal senses. One phalanx (Fig. 24; ASDM 500-283) likely represents phalanx I.1 based on its fit with metatarsal I and comparisons with the referred pes of Cedarosaurus originally described by Gallup (1989; see D’Emic, 2013 for description). This bone resembles the other non–ungual phalanges in its proximal profile, but distally it is subtriangular, with an upturned, laterally facing lip for articulation with the ungual phalanx, posteriorly divided condyles, and a tapering medial lip that is visible as a small tab in dorsal view. A small fossa on the lateral face of the bone is interpreted as a small collateral ligament pit (Fig. 24). A similar fossa is present on the lateral side of a more distal phalanx as well (ASDM 500-279).
Dorsal ribs.—Many fragments of dorsal rib shafts are catalo- gued under ASDM 500, but none show the derived plank-like cross section of the anterior dorsal ribs of titanosauriforms (Wilson, 2002). The absence of this feature could be genuine or it could be the result of only middle-posterior ribs being preserved.
Growth and body size.—Hill and Ratkevitch (1996) studied the bone histology of some fragments from the Sonorasaurus
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