American Dinosaurs

Peter Dodson. Encyclopedia of Dinosaurs. Editor: Philip J Currie & Kevin Padian. Amsterdam: Academic Press, 1997.

The United States has a great diversity of dinosaurs spanning a wide stratigraphic range. Although the concept of dinosaur was born in England, it found fertile ground in the United States. The United States has more different kinds of dinosaurs than any other country by a wide margin. A recent tabulation based on data as of 1988 shows that the United States has 64 known genera of dinosaurs compared with 40 for Mongolia and 36 for China. Such figures rapidly become dated as new kinds from around the world are described. In 1993, for instance, four new dinosaurs were described from the United States: Shuvosaurus from Texas, Utahraptor from Utah, and Naashoibitosaurus and Anasazisaurus from New Mexico. In 1994, Mymoorapelta from Utah was added, and in 1995 the ceratopsids Einiosaurus andachelousaurus from Montana were formally described. Forthcoming are a theropod from the Early Cretaceous of Utah, an Ankylosaurian and an Ornithopod from Texas, and a basal ornithischian from New Mexico. Thus, growth of knowledge of new kinds of dinosaurs continues at least as rapidly in the United States as in China and at a greater rate than in Argentina or in Mongolia.

There are four fundamental reasons why the United States has so many different kinds of dinosaurs: stratigraphy, climate and geography, human resources, and history. Like Argentina and China, and unlike Canada and Mongolia, the United States has dinosaur bearing continental strata that span most of the stratigraphic interval in which dinosaurs may be expected from the Carnian stage of the Late Triassic to the Maastrichtian stage of the Late Cretaceous. The United States has large areas of outcrop in semiarid climates, principally in the west, where erosion is relatively unencumbered by vegetation, unlike Canada, England, or the eastern United States, for example. There is also a large corps of professional, commercial, and amateur dinosaur collectors in this country, all of whom contribute to ongoing discoveries.

The explicit history of dinosaur paleontology in the United States extends back to 1856, when Joseph Leidy applied names to a collection of teeth from the Judith River beds along the Missouri River in Montana that was sent to Philadelphia by Ferdinand Hayden. The four names are Deinodon, Trachodon, Paleoscincus, and Troodon. Unfortunately, the first three names are nomina dubia, as these teeth are diagnostic only at the family level (this being the rule for dinosaur teeth, making it generally unwise to name dinosaurs on that basis). Before this time, an interesting bone had turned up in Cretaceous deposits from Woodbury, New Jersey. Such material had been discussed as early as 1787 at the American Philosophical Society in Philadelphia, but dinosaurs had not yet been recognized scientifically, and the report was forgotten. (Donald Baird has proposed that a hadrosaur metatarsal in the collection of the Academy of Natural Sciences of Philadelphia is this specimen.)

The discovery and description of Hadrosaurus foulkii from Haddonfield, New Jersey, by Leidy in 1858 marks the first time that a major portion of a dinosaur skeleton, including foreand hind-limbs, had been found. This allowed Leidy to reconstruct Hadrosaurus as a biped, showing that the Owen? Hawkins reconstruction of Iguanodon, exhibited at the Crystal Palace since 1854, was incorrect. The reconstruction and exhibition of Hadrosaurus at the Academy of Natural Sciences in 1868 marked the first time that a dinosaur skeleton had ever been exhibited anywhere in the world. Casts of this specimen were exhibited at Princeton University Geology Museum, the Smithsonian, and the Field Columbian Museum in Chicago, but it was not until the first decade of the 20th century that other dinosaur skeletons were exhibited at the American Museum Of Natural History, Yale Peabody Museum, and the Smithsonian. E. D. Cope described a partial skeleton of the enigmatic theropod Laelaps (preoccupied; renamed dryptosaurus Marsh 1877) from New Jersey in 1868. Cope named the ceratopsids Agathaumas in 1872 and Polyonax in 1874 from Wyoming and Colorado, respectively, but these are nomina dubia based on fragmentary material. In 1876, he collected and named Monoclonius from the Judith River Formation of Montana, the first valid ceratopsid. Up to this point, dinosaur finds had been geographically widespread and generally of poor quality. Montana had produced the most dinosaurs up to this time, but most finds were not memorable.

In 1877, dinosaurs were discovered in abundance for the first time anywhere in the world at three separate localities: Cañon City and Morrison, both in Colorado, and Como Bluff, Wyoming. The beds proved to be of Late Jurassic age and have produced a remarkable fauna dominated by large sauropods, with stegosaurs also important; theropods and ornithopods were less abundant; recently an ankylosaur was reported. Intensive examination of the Morrison fauna waned after 1885. Renewed interest in the Morrison at the turn of the century, after Marsh and Cope had died, produced further sauropods (Brachiosaurus and Haplocanthosaurus) and the small theropod Ornitholestes. Beds of Triassic age were documented with the description of Coelophysis bauri by Cope in 1889. The study of beds of latest Cretaceous age began with the description of Triceratops Marsh 1889, followed by Torosaurus Marsh 1891, and then Tyrannosaurus(1905) and Ankylosaurus (1908) early in this century. Lancian hadrosaurine species were described by Marsh in 1890 and 1892, but the proper generic assignments (to Anatotitan and Edmontosaurus) were not recognized until much more recently. The United States lacks a major dinosaur fauna correlative with the Horseshoe Canyon Formation (early Maastrichtian) of Alberta, Canada, although the Two Medicine Formation of Montana, first studied by C. W. Gilmore beginning in 1914, contains an antecedent fauna, as do the Fruitland/Kirtland Formations of New Mexico and the Aguja Formation of Texas. A major fauna of Early Cretaceous age, very broadly correlative with the British Wealden fauna, was unknown in the United States until John Ostrom described the fauna of the Cloverly Formation of Wyoming and Montana in 1970. Lateral equivalents of the Cloverly (Cedar Mountain formationof Utah is partially equivalent; Trinity Group, TX) are now producing important specimens (Utahraptor; Proctor Lake ornithopod). Late Cretaceous dinosaurs from New Mexico began to be described in 1910. The Late Triassic is sparsely productive of dinosaurs, the rich deposits of Coelophysis being a conspicuous exception. There are Early Jurassic dinosaurs in Connecticut and the southwest; the Middle Jurassic is essentially unknown.

Primitive theropods are well represented, the most prominent being Coelophysis (known from scores of skeletons from the mass death assemblage at Ghost Ranch, NM), Dilophosaurus, and Ceratosaurus. Large theropods are represented by two principal taxa, the Allosauridae (Allosaurus) and the Tyrannosauridae. Tyrannosaurus now appears to be one of the most common large theropods. Good specimens of Albertosaurus are common in Canada but are very rare in the United States. The fossil record of maniraptorans in the United States is rather sparse, apart from the imperfect material of Ornitholestes and Coelurus. Deinonychus is the most important American maniraptoran, and recently the larger Utahraptor has been described. Ornithomimids are poorly represented but were surely present. Manymaniraptoran taxa are documented principally by teeth (e.g., Aublysodon, Paronychodon, and Ricardoestesia) and thus are in perilous condition taxonomically. No segnosaurs have been confirmed.

“Prosauropods” (basal Sauropodomorphs) are somewhat sparse in the American fossil record. Anchisaurus andammosaurus are the principal taxa, although Massospondylus has been reported from the Early Jurassic of Arizona. No sauropods of Early and Middle Jurassic age are known, but the Late Jurassic Morrison Formation contains a sauropod assemblage that is rivaled in quality, quantity, and diversity only by the correlative assemblages from China. For nearly a century, these sauropods presented the basis for understanding sauropods everywhere in the world. The important taxa Camarasauridae (Camarasaurus), Brachiosauridae (Brachiosaurus), and Diplodocidae (Diplodocus, Apatosaurus,and Barosaurus) were established on Morrison sauropods. The taxa Cetiosauridae (Haplocanthosaurus) and Titanosauridae (Alamosaurus) are known but are much less important here.

Basal ornithischians are poorly represented at present, but Technosaurus from Texas seems representative of such basal taxa. In addition, teeth of basal ornithischians have been documented in Late triassic sediments from Pennsylvania to Arizona. Scutellosaurus and Scelidosaurus are good basal thyreophorans. The Stegosauridae are magnificently characterized by Stegosaurus, but there is otherwise very low diversity of this family, in contrast to China. There are few basal ankylosaurians, but there are good representatives of the Nodosauridae (Sauropelta) and of the Ankylosauridae (Ankylosaurus), both taxa being established on American taxa. A very important recent discovery is that of the ankylosaur Mymoorapelta from the Morrison Formation of Colorado. The nodosaur Edmontonia is now reported from Alaska. For both families, there are more skulls than skeletons, with no complete skeletons in either taxa having yet been collected. Ornithopods are well represented in the United States. Hypsilophodontids are somewhat fragmentary (Othnielia and Orodromeus), although there are several good specimens of the enigmatic Thescelosaurus. Basal iguanodontians are also well represented by Dryosaurus and Tenontosaurus, the latter of which is particularly abundant and widespread with specimens being reported from Montana, Wyoming, Utah, Oklahoma, and Texas (possibly Maryland as well). Camptosaurus is an abundant American iguanodontian, and a fine skull of Iguanodon itself, named I. Lakotensis, has been described. Hadrosaurs are abundant in the United States, including both lambeosaurines and hadrosaurines. The former are represented only by Parasaurolophus from New Mexico and Utah and Hypacrosaurus from northern Montana. Hadrosaurines come from New Jersey (Hadrosaurus), Alabama (Lophorothon), New Mexico (Kritosaurus), and extensively from Wyoming, Montana, North Dakota, and South Dakota (Anatotitan and especially Edmontosaurus, which is one of the most abundant dinosaurs both in the United States and in the world). Edmontosaurus is also reported from the North Slope of Alaska. Pachycephalosaurs are principally represented bycrania, particularly of Pachycephalosaurus itself. Protoceratopsids are documented by a few incomplete specimens of Leptoceratops and by a specimen of Montanoceratops. Ceratopsids include both centrosaurines (Monoclonius) from Montana and chasmosaurines, especially Triceratops, from Wyoming, Montana, northdakota,South Dakota,andcolorado; Chasmosaurus from Texas; Torosaurus from Montana and South Dakota; and Pentaceratops from New Mexico. Ceratopsids are endemic to North America. Triceratops is among the most abundant of all dinosaurs. There is a fragmentary occurrence of Pachyrhinosaurus from the North Slope of Alaska.

Because dinosaurs are so diverse in the United States, it is tempting to think of this country as a center of evolution for worldwide faunas. This may not be so. In the Late Triassic, plateosaurids, common in Europe, Asia, and South America, are rare in the United States. Rare Early Jurassic sauropodomorphs have been found in Arizona and Connecticut. There are significant resemblances between Late Triassic Coelophysis of New Mexico and Early Jurassic Syntarsus of Zimbabwe and South Africa, but the resemblances between these relatively primitive theropods include few derived characters. There are essentially no Middle Jurassic beds in the United States to document the antecedents of the marvelous Late Jurassic sauropods, ornithopods, and stegosaurs of the Morrison Formation. Haplocanthosaurus may be presumed to be representative of the basal cetiosaurid radiation better documented in England, Europe, and South America. Brachiosaurus from Colorado has affinities with congeneric fossils from Tanzania. Other faunal elements having congeners in East Africa are Dryosaurus, probably Barosaurus, and less certainly Ceratosaurus and Allosaurus. It is significant that stegosaurs are much less diverse in the United States than they are in China, although Stegosaurusitself may be the most highly derived stegosaur. Camptosaurus is an important basal iguanodontian in the United States, with a sister species in the Middle Jurassic of England. In the Early Cretaceous, Tenontosaurus is an endemic ornithopod more basal than Camptosaurus. Although Iguanodon appears to have reached North America, it seems to have been uncommon there. Important new evidence suggests that Polacanthus from the Wealden of England also lived in Utah. In the Late Cretaceous, there is scant evidence for the titanosaurid sauropod fauna that dominated much of the world. It is postulated that Alamosaurus was a late migrant from South America, reintroducing sauropods which had been absent since the Early Cretaceous. There is evidence of faunal interchange with Asia based on similarities at the level of family and genus. A close relationship, possibly at the species level, of Tyrannosaurus with the asiatictarbosaurus is recognized. Other evidence for exchange is better documented by Canadian dinosaurs, notably the hadrosaurine saurolophus. Due to the relatively impoverished faunas of the Judith River Formation and dearth of early Maastrichtian dinosaurs in the United States, coupled with the relatively restricted area of late Maastrichtian strata in Alberta and Saskatchewan, faunal overlap between the United States and Canada is not as great as expected, and the greater diversity and completeness of specimens favors Canada. Although ceratopsids range from Alaska to Mexico, the only identifiable specimens of this family in Asia are teeth and horn core fragments from Uzbekistan. Mid-Cretaceous dinosaurs are found in Maryland, and Late Cretaceous dinosaurs are known from the eastern seaboard of the United States, from New Jersey to North Carolina, and also along the Gulf Coast and Mississippi embayment from Alabama to western Tennessee and Missouri. Few skeletons have been described (Hadrosaurus and Dryptosaurus from New Jersey), and the faunal relationship to dinosaurs in the West, across the Inland Sea, is not evident. Hadrosaurusappears to be a sister group of Kritosaurus from New Mexico and/or Gryposaurus from Alberta. Dryptosaurus is a nonarctometatarsalian of unclear relationship to any other theropod. It is claimed that there is a specimen ofalbertosaurus from Alabama, but it is undescribed. This would present the same biogeographic challenge thathadrosaurus presents. The mechanism of faunal exchange across a 1000- to 1500-km inland sea has yet to be elucidated.