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Ornithopoda: the tuskers, antelopes, and "mighty ducks" of the Mesozoic

Ornithopods had it all. Some had tusks projecting from the corners of their mouths, some had spikes on their thumbs, some had more teeth than just about any other kind of animal, some sported hollow crests atop their heads, and all had long tails that projected straight back from their hips. Many must have had the grace of a running antelope.

Ornithopods had one of the longest reigns of all dinosaur groups, lasting for most of the Mesozoic. From the Early Jurassic, when they first show up in the fossil record, until the end of the Cretaceous, when all went extinct, ornithopods evolved as one of the most diverse clades among dinosaurs, boasting nearly 100 species at present count. In so doing, they also managed to reach all of the regions of the globe, from Asia, Europe, and North America in the north to South America, Australia, and Antarctica in the south.

Some ornithopods have become so well known that they border on the famous. Iguanodon, for example, a founder member of Sir Richard Owen's original 1842 Dinosauria, is an Early Cretaceous ornithopod known from isolated skeletal parts to huge quantities of skeletons, throughout Europe as well as from North America and Asia.

But even better known than Iguanodon are the hadrosaurids, a group of ornithopods whose fossil record extends back some 90 million years prior to the close of the Cretaceous. Commonly called "duck-billed" dinosaurs (Figure 10.1), hadrosaurids are known from extremely abundant fossil remains, ranging from piles of bones in single-species bonebeds, to fully articulated skeletons, to isolated elements. The preservation of their remains can be spectacular, including skin impressions and ossified tendons, and delicate bones such as sclerotic rings, stapes, and hyoid bones. Paleontologists also have at their hands the remains of hadrosaurid eggs and growth series that range from hatchlings through juveniles and "teenagers" to adults. Finally, hadrosaurid footprints and trackways abound in some parts of the world. Thus these dinosaurs, as well as many other kinds of ornithopods, offer a smorgasbord of information about their anatomy, biology, and evolution.

Ornithopods presently fall into four groups that are nested within each other. Most primitive are heterodontosaurids, best known from Heterodontosaurus itself. The remaining ornithopods are called euor-nithopods ("true ornithopods"), which primitively include such forms as Hypsilophodon, Orodromeus, and Thescelosaurus. Nested still higher within euornithopods are iguanodontians, formed of Tenontosaurus, Dryosaurus, Iguanodon (naturally), and hadrosaurids, among other taxa. Nearly all hadrosaurids can be grouped into those with hollow crests atop their heads (lambeosaurines) and those that have solid crests or are flat-headed (hadrosaurines).

Figure 10.1. Edmontosaurus, a Late Cretaceous hadrosaurid ornithopod from the Western Interior of North America, looking at you, over its left shoulder and to the right.

HadrosauridHeterodontosaurus Skeleton
Figure 10.2. Right lateral view of the skull and skeleton of (a) Heterodontosaurus, (b) Hypsilophodon, and (c) Maiasaura.

In body size, ornithopods run the gamut. Early in their history, they were generally small (ranging from 1 to 2 m in length) and only iguano-dontians attained great body size; one of the species of Iguanodon and the vast majority of hadrosaurids ranged upward of 10-12 m. Not all iguanodontians were towering; Dryosaurus was a comparatively tiny 3-4 m in length. Youngsters of course were smaller still: hatchling material Orodromeus, Dryosaurus, and a few hadrosaurids suggest individ-

uals of approximately 30 cm length (Figure 10.2). An ever-increasing abundance of hatchling and juvenile specimens of other taxa are also providing considerable information on the ways and rates that dinosaurs grow - information that a generation ago was only dreamed about, but now is at our fingertips.

Because of their abundance and often-spectacular preservation, ornithopods have been the subject of considerable research. They have figured widely in studies of Mesozoic paleoecology, the evolution of vertebrate herbivory, and archosaurian phylogeny. They have even begun to play a role in our understanding of dinosaurian reproduction and life history strategies. All of these subjects will be touched on in this chapter.

Ornithopods ranged from near the paleoequator to such high-paleo-latitude occurrences as the north slopes of Alaska, the Yukon, and Spitsbergen in the Northern Hemisphere, and Seymour Island, Antarctica, and the southern coast of Victoria, Australia in the Southern Hemisphere (Figures 10.3 and 10.4). Local conditions in these regions varied widely, such that we can safely assume that the many ornithopods lived in quite diverse habitats, with variable climates. For example, the Lower Jurassic sediments of southern Africa from which Heterodontosaurus and its relatives have been recovered are indicative of a semi-arid, probably seasonal (wet-dry) climate, altogether quite inhospitable.

Ornithopods elsewhere in the world and at other times are known from a vast array of terrestrial depositional environments, ranging from

Heterodontosaurus

Figure 10.3. Global distribution of Heterodontosauridae and basal Euornithopoda.

Ornithopod lives and lifestyles

Going their way

Figure 10.3. Global distribution of Heterodontosauridae and basal Euornithopoda.

Global Dinosaur Distribution

upper coastal-plain deposits, to lower coastal-plain channels and overbank deposits, to delta-plain sediments. Several ornithopods, hadrosaurids mostly, are known from islands and even rare marine occurrences, where they are thought to represent the remains of bloated carcasses swept out to sea.

Some degree of habitat partitioning among ornithopods has been reported; that is, several genera or species appear to have divided the available ecospace into domains that do not overlap with each other. For example, J. R. Horner (Museum of the Rockies, Bozeman, Montana) has noted that, in western North America, there are many hadrosaurine taxa that tend to be found in near-marine deltaic sediments. In contrast, other hadrosaurines, but more especially the vast majority of lambeo-saurine taxa, are restricted to lower coastal-plain sediments that were deposited inland from these near-marine environments. Finally, other hadrosaurines - specifically Maiasaura - have been found nowhere else but in upper coastal-plain sediments and are therefore thought to be endemic to these environments.

Ornithopods varied widely in their anatomy and geographical distribution, and hence in the way they presumably carried out their daily lives (see Box 10.1). These animals are now thought to be predominantly bipedal terrestrial animals, although with different agility and speed. As might be expected, many of the smaller forms - heterodontosaurids, basal euornithopods, and a few iguanodontians - must have been fast running, although they also may have adopted a quadrupedal stance when foraging or standing still. Some of the larger ornithopods, such as

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