Ischium The left ischium is nearly complete, demonstrating the typical triradiate ornithopod form, although it shows multiple fractures along the shaft (Fig. 5.11). The ischium is 720 mm long, 256 mm from the pubic peduncle to the iliac peduncle, and ranges in width from 76 to 52 mm from the obturator process to the boot. The acetabulum is denoted by a gentle sloping, shallow concavity. The pubic peduncle is smaller than the iliac peduncle, similar to the ratios seen in Probactrosaurus, Bactrosaurus, and Eolambia (Godefroit et al., 1998; Kirkland, 1998; Norman, 2002; McDonald et al., 2012). The iliac peduncle is a broad (98 mm cranial width) and subrectangular process. The pubic peduncle is small (50 mm cranial width), narrow and rectangular. The craniomedial part of the proximal blade is weathered and the shape of the obturator process cannot be determined. A ridge extends dorsolaterally away from the obturator process down the length of the shaft, flattening distally. This tapering ridge is similar to the ischial ridge noted in Probactrosaurus, although more sublime (Norman, 2002). The shaft is straight for most of its length, with a gentle ventral deflection distally, and terminating in an ischial knob, or boot. The ischium fits within the Type 1 Gilmoreosaurus classification used by Brett-Surman and Wagner (2007).
Pubis A partial left pubis, 420 mm long as preserved, is primarily represented by the cranial pubic process, with the remainder shattered along the caudal shaft and acetabular margin (Fig. 5.11). The cranial pubic process is shallowly convex along the ventral margin and concave along the dorsal margin. The cranial pubic process is 180 mm in vertical height, twice that of the minimum constriction of the neck. It is laterally compressed and thins cranially with an axe-like cranial margin, similar to Eolambia (Kirkland, 1998; McDonald et al., 2012), however the margin is not completely preserved. The prepubic neck is relatively long, 12 cm and thin, and similar in form to that of Tethyshadros (Dalla Vecchia, 2009). The dorsal margin of the neck is thick and rounded, while the ventral margin is thin. Both the iliac and ischial peduncles are broken and missing, as is the acetabular margin. The pubis is morphologically similar to that of Tethyshadros and Eolambia (Kirkland, 1998; Dalla Vecchia, 2009; McDonald et al., 2012).
5.12. Right femur UTA-AASO-125. (A) posterior view; (B) anterior view. Arrows denote crocodyliform bite marks (see Noto et al., 2012).
Femur The femur is represented by a partial right element, but its smaller size relative to the other pelvic elements suggests it likely belongs to a different individual (UTA-AASO-125). The femur preserves the proximal third of the bone, including a partial femoral neck, the greater and cranial trochanters, and a small dorsal section of the fourth trochanter (Fig. 5.12). The medial surface is gently bowed, while the lateral surface is straight. The femoral head is missing, with only a fragmentary base indicating the neck was narrow and formed a saddle-shaped depression between the head and greater trochanter. The greater trochanter is craniocaudally elongate and mediolaterally compressed, with a convex dorsal surface, forming the majority of the proximolateral end of the femur. The greater and cranial trochanters were separated by a narrow vertical cleft. The cranial trochanter is weathered but shows it was small and occupied a craniolateral position on the femur. The fourth trochanter is missing, and its ventral extent truncated by a break in the shaft, so its exact shape cannot be determined. What is preserved shows it was positioned along the caudomedial surface of the shaft about a third of the way down, as opposed to about half way down the shaft as is seen in many hadrosauroids. The fourth trochanter was proximally narrow, probably forming a ridge as in Eolambia, Telmatosaurus, and Bactrosaurus (Weishampel et al., 1993; Godefroit et al., 1998; McDonald et al., 2012). Of note, this element preserves evidence of predatory behavior via two pit marks along the caudal shaft (Fig. 5.12A). The pit marks are round impressions in the bone surface attributed to the feeding behavior of a large crocodyliform, discussed in detail in Noto et al. (2012).
DISCUSSION
Taxonomic Affinity
The discovery of the AAS postcranial material adds significantly to what is known about the anatomy of basal hadrosauroids from the Woodbine Formation. This new material represents a non-hadrosaurid hadrosauroid of uncertain affinity pending further analysis. It can confidently be assigned to the Hadrosauroidea on the basis of an ischium with a shaft that is nearly straight (McDonald et al., 2010:character 64). It is excluded from Hadrosauridae by possession of a large, dish-shaped coracoid, dorsocranially oriented pseudoacromion process of the scapula, triangular postacetabular process, and odontoid fused to the body of the axis (Horner et al., 2004; Brett-Surman and Wagner, 2007). The remaining mixture of basal and derived characters in the AAS taxon highlights the mosaic nature of hadrosauroid evolution prior to the emergence of Hadrosauridae, as noted by Dalla Vecchia (2009).
The gentle slope of the axial neural spine is similar to the iguanodontians Iguanodon, Tenontosaurus, and Camptosaurus, and is a feature not seen in hadrosaurids. The axis of hadrosaurids typically has a rounded, crescent, or blade-like neural spine that expands outwards dorsally, as seen in Kritosaurus, Gryposaurus, and Edmontosaurus (Parks, 1920; Lull and Wright, 1942; Davies, 1983). In the dorsal ribs, the length of separation between the capitulum and tuberculum, extended rectangular shape, and gentle descending slope to the rib shaft are morphologically similar to Iguanodon bernissartensis, and not seen in hadrosaurids such as Edmontosaurus (Lambe, 1920; Lull and Wright, 1942; Norman, 1980; Campione, this volume). However, heart-shaped centra of proximal and middle caudal vertebrae is a feature seen in the Hadrosauridae (Horner et al., 2004).
In the appendicular skeleton, the scapula and coracoid are primitively iguanodontian, and are similar in form to Camptosaurus and Iguanodon and to the basal hadrosauroids Eolambia and Probactrosaurus (Norman, 1980, 1986, 2002, 2004; Carpenter and Wilson, 2008; McDonald et al., 2012). The ilium contains an unusual combination of traits, but is most morphologically similar to Bactrosaurus, Probactrosaurus, Cedrorestes, and Eolambia (Godefroit et al., 1998; Norman, 2002; Gilpin et al., 2007; McDonald et al., 2012). The postacetabular process of the ilium retains a primitive triangular shape similar to Camptosaurus, Cedrorestes, Eolambia, and Mantellisaurus (Gilpin et al., 2007; Paul, 2007; Carpenter and Wilson, 2008; McDonald et al., 2012), whereas the pubic peduncle is extremely small – a feature usually found in hadrosaurids (Horner et al., 2004). Another derived feature is the more cranial position of the suprailiac crest, positioned between the acetabulum and ischial peduncle, a trait UTA-AASO-2003 shares with Claosaurus and hadrosaurids (Brett-Surman and Wagner, 2007; Prieto-Márquez, 2011). In the femur the more proximal origin of the fourth trochanter is unlike many hadrosauroids and a condition also found in Telmatosaurus and Bactrosaurus, although this may be ontogenetic (Weishampel et al., 1993; Godefroit et al., 1998). UTA-AASO-2003 appears to be more derived than Probactrosaurus due to possessing a short pubic peduncle, development of an incipient “antitrochanter” on the suprailiac crest, and overall cranial orientation of the crest. Likewise it appears more primitive than Telmatosaurus in possessing an iguanodontian-grade pectoral girdle. Further work, including a comprehensive cladistic analysis, is necessary to test the generality of this hypothesis.
Comparison with Woodbine Ornithopods
The discovery of the AAS specimens widens the scope of knowledge on the Woodbine Formation ornithopods and corresponds well to previously discovered material; however, direct comparison with previously collected material is hampered by their fragmentary nature and lack of overlapping elements with the AAS specimen. The partial femur described by Lee (1997a), which was also found in the Lewisville Member, shares several features in common