Читать книгу Hadrosaurs - David A. Eberth - Страница 94

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.