Hadrosaurus foulkii, , 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 : 78-80

Hadrosaurus foulkii

In his first publication on H. foulkii, Leidy (1858 , reprinted in 1859) used the relative size of the different regions of the vertebral column of living crocodiles and iguanas to estimate that his new dinosaur was probably 25 feet (8 m) in length. He also noted the resemblance of this animal’s teeth and vertebrae with those of Iguanodon , a more basal taxon from England. More importantly for paleobiological inferences, Leidy also pointed out the great disparity in length between the forelimb and hindlimb in H. foulkii and suggested that this dinosaur browsed on foliage in a kangaroo−like posture, using its hindlimbs and tail as a tripodal support. At the same time, he regarded it as most probably an amphibious animal with fresh−water habits. The presence of H. foulkii in marine sediments suggested that its remains were transported by rivers to be deposited at the bottom of the Cretaceous sea.

Before the discovery of H. foulkii , hadrosaurid dinosaurs were only known from several isolated teeth discovered in the badlands of Montana ( Trachodon mirabilis Leidy,1856a ) and from two caudal vertebrae and a phalanx from the Lance Formation of Nebraska ( Thespesius occidentalis Leidy, 1856b ). Still, after the discovery of H. foulkii , so little was known about dinosaurs on either side of the Atlantic before the great discoveries of the late 19th and 20th centuries, that H. foulkii was much discussed and figured for several decades after its original description ( Leidy 1868a; 1870; Cope 1868, 1869, 1874, 1883; Hawkins 1874, 1875; Lydekker 1888). For example, Leidy returned to H. foulkii in a monograph on the Cretaceous reptiles of the United States ( Leidy 1865), which summarized the available information on the geology of the Upper Missouri, New Jersey, Alabama, Texas, and elsewhere in the U.S. Crocodilians, plesiosaurs, mosasaurs, turtles, and dinosaurs were compared with extinct and living forms from the Old and New World. H. foulkii and two other dinosaurs [ Astrodon johnstoni and Tomodon horrificus —later to be renamed Diplotomodon by Leidy (1868b)]—were described. This contribution represented Leidy’s most detailed and comprehensive osteology of H. foulkii . Comparisons were made not only with modern iguanas and Iguanodon in the context of their relationship to herbivory, but also with the teeth of Trachodon (Edmontosaurus) . Without having the benefit of knowing that all hadrosaurids had a dental battery, Leidy argued for the close apposition of each functional tooth and its successors. But beyond these claims about its teeth, Leidy made no effort to infer anything about the paleobiology of H. foulkii . Gone was his earlier speculation about body posture, locomotion, and habitat first introduced in his report of 1858.

In a summary of the then known reptiles from the Mesozoic and Tertiary strata of New Jersey, Cope (1868) devoted a highly speculative section to Hadrosaurus . Cope first provided a brief description of the general anatomical proportions of the animal and estimated its total length in 28–30 feet (8.4–9 m), claiming that on land this dinosaur walked at times using only its hindlimbs, flexed at the knees, while trailing its tail. Cope also suggested that it wandered in salt lagoons and used its tail, hindlimbs, and pelvic bones as support to reach upward to the foliage, employing its forelimbs to draw the food to its mouth. Cope even commented on the facial expression of Hadrosaurus , pointing out that the “exposure on each side [of] the upper jaw of several rows of shiny teeth...would give [the animal] a somewhat grinning physiognomy...” ( Cope 1868: 737). Interestingly, it appears that Cope already contemplated the hypothesis that Hadrosaurus lacked muscular cheeks, as has been suggested more recently for ornithischian dinosaurs in general ( Papp and Witmer 1998).

In his extensive synopsis of fossil amphibians and reptiles of North America, Cope (1869) erected Hadrosauridae to include Hadrosaurus and provided additional osteological details on appendicular elements of H. foulkii , with comparisons with other dinosaurs known at that time such as Iguanodon , Hylaeosaurus , Laelaps , and Scelidosaurus . In particular, Cope reconstructed the pelvic girdle of H. foulkii , identifying a previously unrecognized fragment of pubis. However, Cope misplaced the ischium and pubis, reversing their orientation and attaching the former to the pubic peduncle of the ilium and the later to the ischiadic peduncle ( Cope 1869: 95, fig. 29). Likewise, Cope reported on the discovery of a fragment of the proximal scapula (Cope 1860: 92, fig. 27). We have been unable to locate this fragment, but Cope’s illustration shows an element that certainly may represent the scapula. This partial bone is missing its glenoid and coronoid articulations, but includes the more ventral portion of the acromial process and the ventral half of the blade. Cope also included descriptions of other Hadrosaurus species, as well as the erection of additional hadrosaurid taxa from the Cretaceous of the East Coast of North America, such as O. immanis and H. tripos (see below).

Around the same time, Leidy (1868a) again compared the dentition of H. foulkii with that of T. mirabilis and considered both forms as probably only specifically separated, contemplating the possibility that Trachodon might be cogeneric with Hadrosaurus . However, in his 1869 monograph, Cope synonymized Trachodon and Thespesius with Hadrosaurus and included T. mirabilis under this last. Two years later, Leidy rejected Cope’s synonymy of Hadrosaurus with Thespesius , and, while recognizing the similar tooth morphology in these animals (and Trachodon ), he emphasized caudal vertebral differences that he thought could be used to distinguish both taxa ( Leidy 1870).

In a later paper, Cope (1874) listed several fragmentary remains from Cretaceous strata of the Western United States under Hadrosaurus , including the two caudal vertebrae and phalanx of the type of T. occidentalis of Leidy (1856b) and the type of Claosaurus agilis , from the Niobrara Formation of Western Nebraska ( Marsh 1872; Carpenter et al. 1995). Waterhouse Hawkins later published a few remarks on the correct identification and articulation of “pseudo−clavicles” as pelvic elements (his “ischiatic bones”) in H. foulkii , drawing comparisons with those of the ostriches and rheas ( Hawkins 1874, 1875).

In 1883, Cope revisited H. foulkii and compared it with other hadrosaurids, particularly with the skull and dentition of a complete Edmontosaurus skeleton [AMNH 5730, referred by Cope to Diclonius mirabilis , but later regarded as the type of Anatoitan copei by Brett−Surman (1990)]. Again, Cope speculated on the paleobiology of these animals, concluding that Hadrosaurus and its relatives would fed on soft vegetation (as well as fishes without bony scales) because he regarded their dentitions as too weak to process tree branches and harder plant material. A few years later, Lydekker (1888) synonymized the genus Hadrosaurus with Trachodon on the basis of their dentitions, echoing Leidy (1868a) while providing a brief description of the teeth of the former.

In their monograph on North American hadrosaurids, Lull and Wright (1942) reexamined the anatomy and relationships of H. foulkii , as well as that of more poorly known hadrosaurid species that had been discovered along the East Coast since the 1850s. Although they paraphrased much of Leidy’s earlier descriptions, Lull and Wright attempted to assimilate H. foulkii into the relationships of the many North American hadrosaurids that had been discovered up to that time. First asserting that “the generic distinction of Hadrosaurus rests on an insecure basis owing to the absence of a skull” ( Lull and Wright 1942: 138), they provided skeletal evidence that it was “a flat−headed” form (i.e., a hadrosaurine) on the basis of the morphology of its ischium, forelimb proportions, and shape of the teeth. Lull and Writght compared H. foulkii with better−known hadrosaurids, particularly Anatosaurus annectens (now Edmontosaurus annectens ; Rozhdestvensky 1968; Weishampel and Horner 1990; Horner et al. 2004). They regarded these two forms as distinct, based on differences in the morphology of the caudal vertebrae, middle dorsal spines, ilia, and teeth, as well as the remoteness in time and space between them. Finally, they considered the other East Coast species that had been referred to or had been closely allied with H. foulkii up to that time. Two of these, Ornithotarsus immanis and H. tripos , were named by Cope in his comprehensive study of the extinct non−mammalian tetrapods of North America ( Cope 1869).

O. immanis was based on a partial tibia, fibula, astragalus, and calcaneum from the Monmouth Formation near Keyport, New Jersey, while H. tripos consisted of caudal vertebrae from the Black Creek Formation of Sampson County, North Carolina. A year later, H. minor was erected by Marsh (1870) for several dorsal vertebrae thought to come from the Navesink Formation near Barnsboro, New Jersey. Finally, Cope (1871) described H. cavatus , known only from several caudal vertebrae from near this same locality. Clearly, all of these taxa were based on poor material (considering that many hadrosaurid species were now known from complete skulls and sometimes complete skeletons) and Lull and Wright (1942) regarded each to be established on an insecure basis. Shortly after the publication of Lull and Wright (1942), Colbert (1948) described new material from the Navesink Formation of New Jersey, which he referred to H. minor .

In his detailed analysis of the cranial anatomy of North American hadrosaurids, Ostrom (1961) recognized the incomplete nature of the holotype of H. foulkii in the context of being the type genus of Hadrosauridae and Hadrosaurinae , particularly that the former does not allow referral to either flat−headed, solid−crested or hollow−crested forms based on direct cranial data. However, Ostrom opted for the conservative solution of assuming that H. foulkii was a hadrosaurine.

Baird and Horner (1977) provided a brief statement about H. foulkii and other Late Cretaceous dinosaurs from the East Coast in a brief abstract. Their reappraisal of these early finds includes referral of O. immanis and perhaps H. cavatus to H. foulkii . More importantly, they suggested that Gryposaurus ( Kritosaurus in their abstract), from the Western Interior of North America, was a junior synonym to Hadrosaurus , although they provided no characters to support this idea. They also thought that Colbert’s (1948) specimen of H. minor might be referable to Edmontosaurus , as E. minor .

Since then, most studies including H. foulkii have concentrated in comparing this taxon with Gryposaurus , some of them concluding that these taxa are different ( Carpenter 1982; Davies 1983), whereas others regarded these hadrosaurids as closely related ( Brett−Surman 1975, 1989; Wagner 2001) or synonymous ( Brett−Surman 1979) (see below for details on this matter). In their review of Hadrosauridae, Weishampel and Horner (1990) regarded H. foulkii as a valid taxon and as a “gryposaur” closely related to taxa such as Gryposaurus notabilis and Aralosaurus tuberiferus . However, no synapomorphies between H. foulkii and species of Gryposaurus were provided. Likewise, O. immanis was synonymized with H. foulkii . More recently, Horner et al. (2004) considered the possibility that H. foulkii would have a basal unresolved relationship within Euhadrosauria (see Weishampel et al. 1993), based on relative decrease in tooth size, the presence of a single strong carina on dentary teeth, and the possession of a distally unexpanded shaft (a condition suggested by Lull and Wright 1942).