Ornithischia

Introduction

The Ornithischia are a very diverse group of herbivorous dinosaurs that include the hadrosaurs, ceratopsians, pachycephalosaurs, ankylosaurs, and stegosaurs. These species are variously noted for their armor-plating, bony skull outgrowths, and duck-billed mouths.

The Ornithischia are basically defined as all dinosaurs more closely related to Triceratops than to birds (or more specifically to Passer domesticus). The group includes all herbivorous dinosaurs, with the exception of the generally much larger sauropods, such as Brachiosaurus, Diplodocus, and Apatosaurus. Although some basal ornithischians, such as Pisanosaurus, probably existed as early as the Late Triassic (235 million years ago), they were very uncommon. They did, however, reach great diversity and distribution during the Late Jurassic and Late Cretaceous. Ornithischians lasted until the end of the Cretaceous (66 million years ago) when one of the world's mass extinction events wiped out all non-avian dinosaurs and 50 percent of all other animal life.

Fast Facts

Pronunciation: The term Ornithischia (or-ni-THIS-kee-a) was first introduced by paleontologist Harry Seeley in 1888 and means “bird-hipped.”

Time Period: Late Triassic (235 million years ago) to Late Cretaceous (65.5 million years ago)

Size: 60 cm to 16.5 m (2–55 ft) in length

Weight: 0.5 kg to 23 tons (1–46,000 lb)

Diet: Herbivorous

Location: Worldwide

Lifespan: 30 years of age or more

Homologous Traits

The herbivorous bird-hipped Ornithischia is one of the two divisions of Dinosauria, the other being the lizard-hipped Saurischia. Ornithischia is a monophyletic group, which means that all members are descended from a sole common ancestor, and all descendants are members of the group. Members of the monophyletic group also all possess the same evolutionarily advanced traits, known as synapomorphies.

Ornithischian dinosaurs share the traits that define archosaurs—specifically four limbs, oviparous reproduction, backbones, and upper and lower temporal skull openings—as well as those traits that define dinosaurs—which include upright posture, modified fourth and fifth digits on the hands, three-toed feet, specialized crests on the humerus and tibia bones, and a femur with a ball-like head at one end.

Although ornithischians are a diverse group of dinosaurs, with more than thirty synapomorphies, there are quite a few diagnostic features. Perhaps the most definitive trait is that from which they obtain their name. Though ornithischians are not the ancestors of birds, they are defined by their bird-hipped pelvis, in which the pubis and ischium (two of the three bones that make up the pelvis) both point backward as is seen in modern-day birds; this is an example of what is known as “convergent evolution.” Another diagnostic feature between the two dinosaurian groups is the presence of small skull openings between the eye socket and the nostrils, which are seen on ornithischian skulls but not on saurischian skulls.

Ornithischian dinosaurs are also defined by the presence of an unpaired predentary bone found at the front of the lower jaw, which became particularly pronounced in the parrot-billed ceratopsians and the duck-billed hadrosaurs. In addition, Ornithischians are defined by a toothless snout tip, a narrow palpebral “eyelid” bone located outside the eye socket, roughly leaf-shaped cheek teeth, a jaw positioned below the row of upper teeth, at least five sacral vertebrae, and hardened tendons above the pelvis area to stiffen the backbone.

Although not possessed by all ornithischian dinosaurs, this group is perhaps most famous for their ornate physical features, such as frills, horns, armor, and dermal plates, such as the back plates seen on Stegosaurus or the cranial horns seen on Triceratops.

Evolutionary Divergences

Ornithischian dinosaurs were not common during the Late Triassic (235–201 million years ago) and remained relatively rare until the Late Jurassic (163–145 million years ago) and their peak in the Late Cretaceous (100–66 million years ago).

Early basal forms of ornithischian dinosaurs found in the Late Triassic and Early Jurassic were relatively small, but continuing diversification over many millions of years saw the evolution of much different morphology. By the Late Jurassic, the group possessed an amazing array of body plans and sizes, as seen in the large horned quadrupedal stegosaurs and armored ankylosaurs, as well as the small, swift, predominately bipedal ornithopods. This diversification continued through the Cretaceous with the appearance of the duck-billed hadrosaurs, the horned ceratopsians, the large iguanodonts, and the thick-headed pachycephalosaurs.

Despite having a similar herbivorous diet, one of the most significant differences between the ornithischian and saurischian dinosaurs was that ornithischians evolved increasingly complex and specialized jaws and teeth. In particular, the ceratopsians and hadrosaurs evolved complex and densely packed teeth and ever more sophisticated digestive systems, which allowed them to benefit from the increasingly varied plant life seen in the Late Cretaceous. Unlike the sauropod dinosaurs, the development of specialized dentition made it possible for some ornithischian species, specifically the ornithopods, to grind and chew their food.

Likewise, studies of dinosaur eggs, nests, trackways, and bone structures have shown that smaller ornithischians probably evolved a warm-blooded, or endothermic, metabolism similar to mammals. This is supported by the discovery of small theropods in China that show a covering of feathers, presumably for insulation in cold environments that could not support cold-blooded animals. Large dinosaurs, such as sauropods, may have been more efficient as ectotherms, similar to most modern reptiles. Although research has increasingly supported the theory that dinosaurs were active and endothermic rather than sluggish creatures as originally imagined, much debate continues over the issue, and it remains an active area of study. A new theory emerged in 2024, revealing that warm-blooded dinosaurs first roamed the planet earlier than previously reported—approximately 180 million years ago during the Early Jurassic period—which led to new information about dinosaurs' habits, including their distribution across the earth.

Creatures in This Group

Ornithischia are one of the two orders of Dinosauria, predominantly classified by the arrangement of their hips. Basal ornithischian dinosaurs split to form a single Jurassic species known as Lesothosaurus and the Genasauria clade. Lesothosaurus is considered the most basal ornithischian dinosaur, with all other species falling within the Genasauria clade. The phylogenetic definition of Genasauria is Ankylosaurus magniventris, Stegosaurus stenops, Parasaurolophus walkeri, Triceratops horridus and Pachycephalosaurus wyomingensis, and their most recent common ancestor and all descendants. This clade is separated from Lesothosaurus based on their deep-set rows of teeth, differently shaped mandibles, and a reduction in their mandible opening (foramen). Genasauria is currently divided into the Thyreophora (“shield bearer”) and Cerapoda (“horn-foot”), which is further divided into the Ornithopoda (“bird-foot) and Marginocephalia (“fringe heads”).

The Thyreophora ornithischian dinosaurs include the armor-plated quadrupedal herbivores Stegosauria and Ankylosauria. Stegosauria includes such species as the Late Jurassic Tuojiangosaurus from Asia, Kentrosaurus from Africa, and Stegosaurus from America. Stegosaurs possessed small heads, elongated snouts, and tapered beak-like mouths. All stegosaur species, with the exception of Huayangosaurus, possessed noticeably shorter forelimbs than hind limbs, as well as arched backs along which ran vertical bony plates in two parasagittal rows. The ankylosaurians were closely related to the stegosaurs and are also noted for their bony osteoderm armor.

Cerapoda ornithischian dinosaurs include the Ornithopoda and Marginocephalia, which is, in turn, comprised of Ceratopsia and Pachycephalosauria. Ornithopods were a highly successful group of dinosaurs during the Late Jurassic to Late Cretaceous period, and they dominated areas of Asia, North America, and Europe. They include the duck-billed hadrosaurids, iguanodonts, and heterodontosaurids and are distinguished by having no armor. Marginocephalia dinosaurs are noted for the bony frill at the back of the skull and include the parrot-beaked ceratopsians, such as the Late Cretaceous Triceratops from western North America, and the bony-headed pachycephalosaurians, such as the Late Cretaceous Pachycephalosaurus from North America.

Ecology

Many paleontologists believe the long-necked sauropods were high browsers and fed on tree-top foliage (although recent evidence related to neck anatomy has suggested that this may not be the case). Unlike many sauropods, however, the herbivorous ornithischians were low browsers and actively grazed on low-lying vegetation, such as ferns, horsetails, cycads, and mosses. This niche diversification may have been a way to reduce competition among groups of animals sharing the same habitat or using the same limited food resources.

To understand dinosaur ecology and behavior, it is often necessary for paleontologists to discuss correlations between extant and extinct species. This is especially the case with ornithischians due to their ornate physical features such as frills, horns, armor, and dermal plates. Paleontologists believe that such structures were likely related to defense and combat and/or display and mate attraction, or possibly, in the case of dermal plates, thermoregulatory structures used to help control body temperature. Hypotheses related to defense or reproductive attraction find support in extant animal analogs, and, in turn, support the idea that ornithischians required such structures because they lived in or occasionally formed large mixed-sex herds. This idea is also supported by fossilized trackways and bone beds, which provide suggestive evidence that species such as the iguanodons and stegosaurs formed large herds. Bone beds and nesting sites also provide evidence of oviparous behavior, with research showing that dinosaur species laid eggs. Communal nesting sites and post-hatching care are relatively well documented in a number of ornithischian species, suggesting that at least some such species exhibited advanced parental care.

Ornithischian intelligence, as measured by body-to-brain ratio (encephalization quotient), fell approximately in the middle. Though the ankylosaurs and stegosaurs were less intelligent than the ceratopsians and ornithopods, all were more intelligent than the sauropods. Topping the scale of dinosaur intelligence, however, were the predominantly carnivorous theropods.

Paleontology News

Despite early attempts and descriptions of related fossil material as far back as 1820 and the naming of iguanodons in 1825, ornithischians were not defined as a dinosaur group until almost seventy years later. Ornithischia, meaning “bird-hipped,” was first introduced by paleontologist Harry Seeley in 1888. Although Seeley's ideas related to the paraphyletic Dinosauria have largely been disproven, his classification of dinosaurs based on hip orientation, which is the fundamental basis for the division of Saurischia and Ornithischia, remains to this day.

Since the first scientific description of dinosaurs, a large number of ornithischian genera have been named, some 180 at last count. Despite this, however, classification remains problematic due to the incomplete fossil record and few definitive ornithischian phylogenetic analyses. This is particularly true for the most ancient early ornithischian dinosaurs. For example, while some recent cladistic studies have designated Pisanosaurus mertii as a basal ornithischian, other researchers remain unconvinced about its classification and that of other early ornithischians. Many paleontologists state that the instability of clades such as the Ornithopoda demonstrates that ornithischian phylogeny remains unsettled.

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