The first hadrosaur finds did not include much skull material. Hadrosaur teeth have been known since the 1850s (
Joseph Leidy's
Trachodon), and a few fragments of teeth and jaws were among the bones named
Hadrosaurus by Leidy in 1858. (The skeletal mount made for
Hadrosaurus by
Benjamin Waterhouse Hawkins included a speculative
iguana-like skull) Leidy had enough skeletal material to make other inferences about the
paleobiology of hadrosaurs, though. Of particular importance was the unequal lengths of the forelimbs and hindlimbs. He interpreted his new animal as a
kangaroo-like animal that browsed along rivers, using its forelimbs to manipulate branches. which revealed for the first time the complex hadrosaur tooth battery. Cope immediately drew attention to the
anterior part of the skull, which was drawn out, long, and wide. He compared it to that of a
goose in side view, and to a
short-billed spoonbill in top view. Additionally, he noted the presence of what he interpreted as the remnants of a dermal structure surrounding the beak. Significantly, Cope regarded his
Diclonius as an amphibious animal consuming soft water vegetation. His reasoning was that the teeth of the lower jaw were weakly connected to the bone and liable to break off if used to consume terrestrial food, and he described the beak as weak as well. Unfortunately for Cope, aside from misidentifying several of the bones of the skull, by chance the lower jaws he was studying were missing the walls supporting the teeth from the inside; the teeth were actually well-supported. While Cope anticipated publishing a full report with illustrations, he never did so, and instead the first accurate illustrated description of a hadrosaur skull and skeleton would be produced by his great rival,
Othniel Charles Marsh. While Marsh corrected several anatomical errors, he retained Cope's postulated diet of soft plants.
Lull and Wright (1942) The early study of hadrosaurid dietary adaptations and feeding behavior was summarized in a 1942
monograph by
Richard Swann Lull and
Nelda Wright. Unlike previous authors, they moved away from soft water plants as the major part of the diet, but retained the interpretation of an amphibious lifestyle. They drew attention to the extensive development of the hadrosaurid dental batteries, and compared their dental equipment to that of horses, noting the advantage the dinosaurs had in continual replacement of teeth. However, they found the purpose of the dental batteries uncertain: hadrosaur jaws were unlike those of any modern reptiles, and there did not appear to be an evolutionary pressure on hadrosaurids like
grasses were for horses. Lull and Wright eliminated the soft plants as the primary choice of diet, and eliminated grasses on the grounds that the beak was unlike that of grazing birds like geese, and that the quantity of available grasses appeared insufficient to feed hadrosaurids. Instead, they proposed
equisetaleans (horsetails) as the major food source, as these plants existed in the same times and places as hadrosaurids, are known to be rich in
starch, and contain abrasive
silica which would necessitate teeth that could be replaced. Softer land and water plants were proposed as secondary foods. Lull and Wright found that their proposed feeding ecology was comparable to that of a modern
moose, which browses on trees and feeds on water plants in wetlands. They further interpreted the complex anatomy of hadrosaurid snouts and nasal passages as adaptations to feeding underwater, like moose. Lull and Wright added a new element to hadrosaurid feeding by proposing the presence of muscles analogous to mammalian
cheek muscles, which would hold in food chopped by the teeth. These muscles would be attached on bony ridges present on the upper and lower jaws. The authors interpreted the action of the jaws as limited to simple up–and–down motions, finding forward–backward motion unlikely based on skull articulation. The vertical motion would cut food into short lengths, and the pieces would be retained by the cheeks. To manipulated the food in the cheeks, the authors inferred the presence of a well-developed tongue.
Ostrom (1964) and reception The general preexisting consensus on hadrosaurid paleobiology was challenged in 1964 by
John Ostrom, who found little evidence to support either a diet of aquatic plants or an amphibious lifestyle. Unlike previous depictions, he interpreted hadrosaurids as terrestrial foragers that browsed on land plants, not aquatic plants. Like Lull and Wright, he drew attention to the robust dental batteries, and found that hard, resistant foods were the most likely diet (such as woody, silica–rich, or fibrous materials). Unlike Lull and Wright, he interpreted hadrosaur jaws as using a complex rodent–like forward–backward grinding motion, and did not comment on the possibility of cheeks. Drawing on an older proposal made during study of a hadrosaur specimen with a preserved beak, he noted the possibility that the animals stripped leaves and shoots from branches by closing the beak over branches and pulling back. A terrestrial diet was also supported by the 1922 gut content study, which found conifer needles and twigs, seeds, and fruits inside the specimen. There was also more circumstantial evidence for terrestrial feeding. Ostrom found that hadrosaurid skeletal anatomy indicated that the animals were well–adapted to move on land, and were well–supported by
ossified tendons along the
vertebral spines, which would have hindered swimming. He also reported that aquatic plant pollen was rare in the rock units hadrosaurids are known from, which indicates that aquatic plants were uncommon. ==1984 hadrosaurid chewing hypothesis==