Hadrosaurus, Leidy, 1858

Prieto-Márquez, Albert, Weishampel, David B. & Horner, John R., 2006, The dinosaur Hadrosaurus foulkii, from the Campanian of the East Coast of North America, with a reevaluation of the genus, Acta Palaeontologica Polonica 51 (1), pp. 77-98 : 88-89

Is Hadrosaurus Gryposaurus and/or Kritosaurus ?

During the last three decades, Hadrosaurus has been synonymized ( Baird and Horner 1977; Brett−Surman 1979), compared ( Davies 1983), distinguished ( Ostrom 1961; Weishampel and Horner 1990), and closely related ( Brett−Surman 1975, 1989; Wagne 2001) to Gryposaurus or Kritosaurus . Because clarifying the taxonomy of Kritosaurus and Gryposaurus lies beyond the scope of this paper, we will follow Horner et al. (2004) in regarding Gryposaurus as comprising three species, G. notabilis ( Lambe, 1914) , G. incurvimanus ( Parks, 1920a) and G. latidens ( Horner, 1992) , and Williamson (2000) in restricting Kritosaurus to K. navajovius [ Brown, 1910; synonymized in Wiliamson (2000) with Naashoibitosaurus ostromi and Anasazisaurus horneri ].

Carpenter (1982) and Davies (1983) found Hadrosaurus to be different from Gryposaurus on the basis of dental papillation. However, Coombs (1988) pointed out that isolated teeth are not useful for diagnosing hadrosaurid lower taxa because features such as dental papillation and tooth size vary along the tooth row. Thus, features seen within both dentary and maxillary H. foulkii teeth, such as well−developed papillae, indicate nothing regarding their taxonomic identity at generic and specific levels. Still, G. latidens (AMNH 5465) dentary teeth, as well as those from a juvenile Gryposaurus sp. (ROM 1939), have remarkably wider crowns than H. foulkii . This great crown breadth is consistent also throughout the dentary and maxilla. Thus, this condition indicates that at least H. foulkii is not G. latidens . However, the teeth of H. foulkii , G. incurvimanus (TMP 80−22−1), and K. navajovius (AMNH 5799, as seen in Brown 1910) cannot be distinguished regarding the crown morphology of the enameled surface, except in the greater development of papillae in the former. But because this is also the case between H. foulkii and other taxa (e.g., M. peeblesorum, OTM F 138), teeth neither support nor refute the synonymy of Hadrosaurus and Gryposaurus .

The ANSP maxillary fragments are too incompletely preserved to allow discrimination from other hadrosaurid maxillae. The dorsoventral thickness of the dorsal border on the medial side of ANSP 9203, adjacent to the dorsal margin of the alveoli, comprises 25% of the total dorsoventral depth of the maxillary fragment. This feature indicates that in the middle region of the complete maxilla the line of alveolar foramina would be located within the dorsal third or fourth of the maxilla, ventral to the dorsal process. This condition is also present in species of Gryposaurus (e.g., MOR 478−5−28−8−1), as well as in K. navajovius (NMMNH P−16106), and also in other genera such as, for example, Prosaurolophus (MOR 454−6−24−6−2), Saurolophus (AMNH 5221) and Edmontosaurus (CMN 2289) . Thus, these specimens neither support nor refute a potential synonymy of Hadrosaurus and Gryposaurus .

Davies (1983) concluded that Hadrosaurus is different from Gryposaurus on the basis of comparisons of the humeri, femora, ilia, caudal vertebrae, and metatarsals of ANSP 10005 with those featured in illustrations of Gryposaurus ( G. incurvimanus in Parks 1920b, ROM 4514 in Davies thesis, but today catalogued as ROM 764; and G. notabilis in Pinna 1979, specimen MCSNM v345). Davies (1983) contrasted the different angulation of the neural spine of the caudal vertebrae of Gryposaurus with the one preserved in ANSP 10005. However, as correctly pointed out by Wagner (2001), the angulation of the neural spines in caudal vertebrae varies along the series, so that more distal vertebrae have more inclined neural spines. Thus, Davies (1983) may have compared vertebrae from different locations in the tail of these taxa. Davies also provided several length ratios between appendicular elements, such humerus/radius, femur/tibia, and hindlimb/forelimb. The values of these ratios are very similar between H. foulkii and Gryposaurus .

The coracoid of ANSP 10005 is too incompletely preserved to allow precise correspondence with those in other taxa. Still, when compared with Gryposaurus (AMNH 5465 and ROM 764), the glenoid in H. foulkii appears broader transversely. However, preservational mediolateral crashing (e.g., present in AMNH 5465) is likely to account for this difference.

The humerus of H. foulkii differs from that in species of Gryposaurus and K. navajovius in having a deltopectoral crest that comprises less than 50%% of the length of the element (as correctly stated by Davis [1983]), whereas in the other taxa the deltopectoral crest extends to at least 50% or more of the length of bone (e.g., ROM 764, MOR 399 and 478, NMMNH P−16106, AMNH 5350 and 5465). Furthermore, in H. foulkii , the craniolateral corner of the crest is smooth and rounded, whereas in many Gryposaurus humeri this region is very angular and orthogonal with respect to the humeral shaft (e.g., AMNH 5350 and 5465, ROM 764). Thus, it is unlikely that the humerus of ANSP 10005 could belong to a species of Gryposaurus .

Both the radius and the ulna of ANSP 10005, especially the former, have thicker shafts relative to their length than that of G. incurvimanus (e.g., ROM 764), G. notabilis (CMN 2278) , and some specimens of G. latidens (e.g., AMNH 5465; but not MOR 478, which, although it is mediolaterally crashed, is similar in proportions to ANSP 1005). Howerver, robustness is expected to be variable among the individuals of a given hadrosaurid taxon (e.g., in B. canadensis , compare the much more robust appendicular elements of MOR 794 relative to MOR 1071, even though they are the same length).

For the femur, Davies (1983) noticed differences in the morphology of the fourth trochanter between ANSP 10005 and G. incurvimanus (ROM 764). However, the caudal margin of the fourth trochanter in ANSP 10005 is incompletely preserved. Comparisons of the femur, tibia, fibula, metatarsals II and IV, and phalanx III1 of ANSP 10005 with those of several Gryposaurus specimens (ROM 764, AMNH 5350, 5465, ROM 2278, MOR 478), as well as K. navajovius (NMMNH P−16106) only revealed differences in robustness. For example, the tibia in H. foulkii has a thicker mid shaft for its length than the one in G. incurvimanus (ROM 764). Likewise, the fibulae of G. latidens (AMNH 5465) is proportionally longer and its shaft begins to thin more distally than in ANSP 10005. Again, our current understanding of the variation of these elements among hadrosaurids do not allow to discriminate whether these differences are due to ontogeny, individuals of the same age being more or less robust, differences in growth rates, taxonomy, or a combination of all these factors.

Regarding the pelvic girdle of Hadrosaurus , the pubic fragment is too incomplete to be compared with other hadrosaurids. Davies (1983) pointed out that the supracetabular process of the ilium of G. incurvimanus (ROM 764) is more robust and ventrally deflected than in ANSP 10005. Nevertheless, in ANSP 10005 the lateral margin of the supraacetabular process is missing. The depression on the dorsal margin of the ilium at the level of this process is similar in both specimens. In G. latidens (AMNH 5465) the supraacetabular process is craniocaudally wider than in H. foulkii . However, the degree of development of the supraacetabular process varies among hadrosaurids (e.g., Edmontosaurus ilia in CMN 2289, ROM 867 and 801). In ANSP 10005 the caudal protuberance of the ischial peduncle appears more pronounced than in ROM 764, but the feature is eroded in the last specimen. In G. latidens (MOR 478, AMNH 5465) the preacetabular process is proximally shallower than in ANSP 10005. However, in ANSP 10005 this region is heavily reconstructed with plaster, which probably contributes to the apparent depth of the proximal region of the preacetabular process. However, the thickness of the preacetabular process as it curves cranioventrally in ANSP 10005 is unaffected by the reconstruction. In this regard, the ilium of G. latidens (AMNH 5465) differs from ANSP 10005 in having a deeper preacetabular at two two thirds of its length. Thus, in view of these differences it is likely, but not certain, that the ilium of ANSP 10005 does not belong to a species of Gryposaurus .

Whereas in the ischium of ANSP 10005 the shaft curves upward, in G. incurvimanus (ROM 764) the ischial shaft is subhoritzontal and has a very slight ventral deflection distally. We have not observed any other ischia unequivocally referable to species of Gryposaurus that we could use for more comparisons.

In conclusion, the synonymy of Hadrosaurus with Gryposaurus is not supported by the present osteological data, primarily from differences in the humerus and ilium. It is more likely that ANSP 10005 represents a separate taxon, probably at the generic level. Still, this matter could only be settled when more complete cranial material is found for Hadrosaurus and when a more exhaustive understanding of the meaning (taxonomic, ontogenetic, polymorphic) of the variation of postcranial elements in hadrosaurid dinosaurs is achieved.