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This taxon has a body form that is distinct from its sympatric contemporary B. dawsoni: it is smaller in body length overall, less robust, and has a much more lightly built vertebral column with slender, tall neural spines along the dorsal, sacral, and caudal regions.

Craniodental Anatomy A crushed and distorted, yet almost complete, dentary (NHMUK R1831; Fig. 2.4) demonstrates that this bone was more slender than the specimen (NHMUK OR28660) that can be attributed to B. dawsoni, and another referred specimen (NHMUK R1834; Norman, in press) indicates that the anterior part of the dentary is deflected ventrally (contra McDonald et al., 2010). Several tooth crowns are preserved in NHMUK R1831 (Norman, 2010, 2011b, in press) and are quite distinct in surface details from those referred to B. dawsoni. Whereas the enameled face of the crown is shield shaped and fringed with mammilate, ledge-shaped denticles (as in B. dawsoni), the ridge pattern seen on the lingual face of the crown differs considerably. There is a prominent primary ridge running the length of the crown distal (posterior) to the midline, and mesial (anterior) to this is a series of strand-like minor ridges that extend down the remainder of the crown for varying distances.

Vertebrae Most characteristically, the dorsals develop remarkably long, backwardly inclined, narrow spines that in life should have given the appearance of a tall midline ridge (Fig. 2.5). The dorsals differ markedly from those of B. dawsoni. The caudals do not exhibit the low (squat), inclined angular form seen in B. dawsoni; in contrast, they appear to have compressed, tall centra and support equally elongate, narrow spines. Middle and posterior caudals display the gradual loss of the caudal rib and become lower and more apparently elongate, eventually developing angular (hexagonal) cross sections; they do not seem to show the deeply concave articular surfaces seen in B. dawsoni.

Girdles and Limbs The pectoral girdle differs only in size from that of B. dawsoni: most of the elements appear similar in general shape and the sternals are hatchet shaped. The forelimb resembles that of B. dawsoni in the shape of each element of upper and lower arm, but there is a clear difference in the shape of the characteristic pollex spine (thumb-spike). Whereas in B. dawsoni the pollex is short, laterally compressed and bluntly truncated, that of H. fittoni appears to be tall, laterally compressed and pointed (triangular in lateral aspect; Fig. 2.6); this thumb-spike resembles Mantell’s (1827) classic “nasal horn.” The manus (Fig. 2.6) resembles that described in Iguanodon bernissartensis (Norman, 1980) in the relative shape and proportions of each digit, although overall it seems to have rather shorter digits than might have been expected, judged by the dimensions of the associated radius and ulna.

2.6. Hypselospinus cf. fittoni, NHMUK R1831 (R1832/R1833). Reconstructed antebrachium and manus in lateral view. Abbreviations: mcI/mcIII, metacarpals; MCB, metacarpo-carpal block; ol, ossified ligaments; PO, pollex ungual; RA, radius; UL, ulna. Scale bar equals 10 cm (from Norman, in press).

The pelvis comprises an ilium with a narrow, untwisted, elongate preacetabular process with a low, curved medial ridge; the deep, central portion of the ilium is flat and has a relatively compressed dorsal edge (with, at most, a slight lateral expansion on its dorsal margin above and behind the ischiadic peduncle); the postacetabular process tapers (as upper and lower borders converge) to form a blunt, rounded transverse bar; below the latter is a low-vaulted brevis fossa, demarcated laterally by the presence of a ridge (Norman, 2010:fig. 5). This iliac morphology is clearly distinct from that seen in B. dawsoni (Norman, 2011b:fig. 8), which has a transversely thick and axially twisted preacetabular process, a broad dorsal edge to the main blade, a deep postacetabular process that is inflected medially toward its ventral edge but has no lateral ridge, and a brevis shelf that is either absent or very reduced in extent (NHMUK R3788). The pubis (Fig. 2.7A) is incomplete but has a deep and slightly upwardly curved, parallel-sided, prepubic process with an anterior tip that appears to be moderately dorsoventrally expanded. In contrast to the pubis of B. dawsoni, the pubic shaft is cylindrical. The ischium (Fig. 2.7B) has a robust, curved (J-shaped) shaft that appears to be twisted along its length and ends in an enlarged anteriorly expanded “boot”; the proximal external surface of the shaft bears a flattened, scarred facet adjacent to the flap-like obturator process (obt) is positioned close to the proximal end of the shaft and offered mechanical support to the pubic shaft.

The hindlimb, as in the case of Barilium, is not known from good-quality articulated material; it differs little in morphology from what is known in B. dawsoni (Fig. 2.8). Hindlimb material of this taxon was first illustrated by Lydekker (1889). There is a prominent crested fourth trochanter that probably terminated in a marginally pendent tip that does not resemble that seen in camptosaurs (contra Lydekker, 1889).

2.7. Hypselospinus cf. fittoni, NHMUK R811. (A) pubis partial (right, this is a reversed image) in lateral view; (B) ischium complete (left) in lateral view. Abbreviations: ac, acetabular margin; ap, anterior blade of the pubis; ib, ischial “boot”; il.p, iliac peduncle; obt, obturator process; obt.c, obturator channel; pp, pubic peduncle; p.pu, posterior ramus of the pubis. Scale bar equals 10 cm (from Norman, in press).