Dinosaur Classics: Leidy’s Dinosaur Inventory

Contrary to a snarky review, this monograph is one of the most important works ever published in the history of vertebrate paleontology

Part of Plate XII from Leidy's Cretaceous Reptiles of the United States, showing some vertebrae from Hadrosaurus.
Part of Plate XII from Leidy's Cretaceous Reptiles of the United States, showing some vertebrae from Hadrosaurus. Feedloader (Clickability)

By the time the Smithsonian Contributions to Knowledge series published Joseph Leidy’s monograph Cretaceous Reptiles of the United States in 1865, dinosaurs were already famous. The English anatomist Richard Owen had coined the term “Dinosauria” more than two decades earlier, and South London’s “Dinosaur Court” was a popular destination. But paleontologists knew relatively little about North American dinosaurs. The infamous “Bone Wars” of the late 19th century—which would yield fossil celebrities like Stegosaurus, Triceratops, and Allosaurus—had not yet begun, and naturalists had no idea of just how many spectacular dinosaurs would be found in the American West.

Though all the fossils Leidy described were from North America, his monograph could have been called “Cretaceous Reptiles of New Jersey (And a few tidbits from elsewhere).” Many of the fossils within the report’s pages were found in the dark, wet marl of southwestern New Jersey. They included seagoing crocodylians, enormous marine lizards called mosasaurs and most importantly, the partial skeleton of Hadrosaurus foulkii. The remains of this herbivorous dinosaur made up the first partial dinosaur skeleton to be found in the United States, and within three years Hadrosaurus would become the first dinosaur to have its skeleton fully reconstructed. (Frustratingly, this sole skeleton of Hadrosaurus may have been even more complete, but the farmer whose land the dinosaur was found on, John E. Hopkins, gave away a number of bones which may have belonged to the Hadrosaurus before he knew the scientific significance of what he had accidentally turned up.)

Hadrosaurus has since been overshadowed by other dinosaurs, but at the time, its discovery was one of the most important finds in the history of paleontology. The skeleton Leidy described, though incomplete, showed that at least some dinosaurs had shorter forelimbs than hindlimbs and had a starkly different bodyplan from the weird, almost mammal-like designs Richard Owen had proposed two decades before. The close similarity between the bones of Hadrosaurus and Iguanodon from Europe, especially, appeared to indicate that dinosaurs were quite different from earlier representations of them—a notion confirmed through the discovery of a partial skeleton in 1866 belonging to a dinosaur now recognized as a tyrannosaur and named Dryptosaurus. The two dinosaurs from the New Jersey marl—Hadrosaurus and Dryptosaurus—were more bird-like in proportion and form, and therefore some paleontologists of the era supposed that, like birds, dinosaurs had active lifestyles.

Leidy’s monograph was historically significant for another reason. In addition to the fossils he described from New Jersey and other Eastern states, Leidy also mentioned a handful of fossils collected from sites further west by the young geologist Ferdinand Vandiveer Hayden. This was the man who would later gain fame for exploring the Rocky Mountains and the area that would become Yellowstone National Park in the years after the American Civil War, but when he began collaborating with Leidy, Hayden was a 24-year-old student with an itch to explore the American badlands.

Hayden was an irrepressible and energetic field naturalist—scientific lore holds that he earned the nickname “Man Who Picks Up Stones Running” from the Sioux for the rapidity with which he collected fossils—and during his first trip into the field in 1853 he collected a few dinosaur teeth and bones from sites along the Missouri River. He sent these along to Leidy for description. The paltry lot included scraps of other hadrosaurs—to which Leidy applied the now-discarded names Trachodon and Thespesius—and a misidentified tooth which paleontologist John Bell Hatcher would later recognize as the first piece of a horned dinosaur ever described. Despite the fact that he visited some of the most dinosaur-rich formations in the west, Hayden was not especially impressed with what he found. Writing to Leidy about the Judith River Formation—a formation which has yielded many fine specimens of Late Cretaceous dinosaurs—Hayden said “I find that the Bad Lands of the Judith are scarcely less interesting than those of the White river,” referring to a geologically younger slice of time known to contain the impressive skeletons of prehistoric mammals. Even though Hayden was right in the middle of dinosaur country, he just was not lucky enough to find more than a few scraps.

Leidy’s monograph was not meant to be an interpretative document. Even though Charles Darwin’s theory of evolution by means of natural selection had sparked a great deal of interest in evolution following the 1859 publication of On the Origin of Species, Leidy did not attempt to place the fossils he was describing in an evolutionary context. Cretaceous Reptiles of the United States was meant to form a base of knowledge from which to extend investigations and observations. Not everyone appreciated Leidy’s choice to describe rather than interpret. A scathing, anonymous review—signed simply “H”—in London’s Geological Magazine read, “Altogether we must, while expressing our thankfulness for the memoir, such as it is, say that it is the least able contribution to palaeontology that we remember. Its best praise is that it contains no quackery; its worst condemnation is that it contains no science.”

As historian Keith Thomson points out in his book The Legacy of the Mastodon, H’s criticism of Leidy was unfair and cruel. Leidy perceived his role to be a describer and observer of prehistoric life—the point of Cretaceous Reptiles of the United States was to create an inventory of what had been found and communicate those findings in detail. More than that, though, Thomson points out that vertebrate paleontology in North America was still a young science. Naturalists had only just begun to discover the different species which existed and sort out the geology of the layers they were entombed in. Without this basic knowledge—the sort Leidy was attempting to accumulate—any attempts at theorizing or interpreting the implications of the fossils for the pattern of evolution would be marred by a weak understanding of what actually existed within the North American formations. If the strata of New Jersey had been as extensively mapped and understood “as that for the Paris basin or English Wealden,” Thomson asserts, “the comments by ‘H’ about the lack of analysis would have been appropriate.” Given the young state of American paleontology, they were not.

Contrary to the snarky comments of H, Leidy’s monograph is one of the most important works ever published in the history of vertebrate paleontology. Given the fact that the original Hadrosaurus site has been paved over and very few Cretaceous fossil sites in New Jersey remain accessible to paleontologists, especially, the work is an indispensable catalog of what once lived in the Garden State and surrounding area. Cretaceous Reptiles of the United States is a dinosaur classic.


Leidy, J. 1865. Cretaceous Reptiles of the United States. Smithsonian Contributions to Knowledge 14:1-193

Thomson, K. 2008. The Legacy of the Mastodon. New Haven: Yale University Press. pp. 126-144

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