Ornithopoda

Ornithopoda is a large group of herbivorous dinosaurs known as ornithopods. These animals were among the most successful plant-eaters during the Cretaceous period. Their name comes from the Greek words *ornithos* and *pous*, which mean "bird feet." This name refers to their characteristic birdlike feet.

Ornithopods lived across the entire world. They eventually disappeared during the Cretaceous–Paleogene extinction event. This event also wiped out all other non-avian dinosaurs.

One of the most important features of ornithopods was their chewing apparatus. Over time, they developed a highly sophisticated way to process food. This system was the most advanced ever seen in a non-avian dinosaur. It was even comparable to the teeth and chewing abilities of modern mammals, like the domestic cow.

Because they could chew so well, they could eat many types of plants. This ability helped them become ecologically dominant in many environments.

Ornithopods changed significantly as they evolved. The earliest members were small and only about 1 meter long. These small dinosaurs were likely very fast runners. They used a stiff tail to help them balance while running on their hind legs.

As the group evolved into the subgroup Iguanodontia, they became much larger. Many became facultative quadrupeds. This means they could walk on all four legs to graze, but they could still run on two legs. Their spines even curved to resemble modern ground-feeders like the bison.

Different types of ornithopods had different diets. Scientists study dental microwear, which is the tiny wear patterns on teeth, to understand this. Early ornithopods were likely browsers or frugivores. This means they ate leaves or fruit. Larger, more derived iguanodontians were bulk feeders. They ate tough, less nutritious vegetation. They could survive on this food because they had a long gut passage time. This allowed them to extract more energy from the plants.

Some ornithopods had unusual physical traits. Certain species had thin cartilaginous plates along the outside of their ribs. In some fossils, these plates mineralized and turned into bone. Scientists do not yet know the exact function of these intercostal plates.

While ornithopods grew very large, they never reached the massive sizes of the long-necked sauropods. However, some were still enormous. The *Shantungosaurus* could reach 15 meters in length. It could weigh up to 23 metric tons. This weight is similar to a medium-sized sauropod.

Research into these dinosaurs has a long history. In 1870, Thomas Henry Huxley identified several families of dinosaurs. He included *Iguanodon* and *Hypsilophodon* in his classification. In 1881, Othniel Charles Marsh created the term Ornithopoda. He used it while investigating how to classify all dinosaurs. Marsh divided the order into three families: Camptonotidae, Iguanodontidae, and Hadrosauridae. He also named several American taxa. One of his names, *Camptonotus*, had to be changed in 1885. This was because a cricket already used that name.

Modern scientists use complex methods to classify these animals. Ornithopoda is usually ranked as a suborder within the order Ornithischia. Some researchers, like Benton in 2004, have placed it as an infraorder. The classification of the group Iguanodontia has also changed. Paul Sereno defined it in 1998, and he later amended that definition in 2005. In 2021, Iguanodontia received a formal definition under the PhyloCode. This definition identifies the smallest group containing specific species like *Iguanodon bernissartensis*.

This precise grouping helps scientists understand the exact relationships between different dinosaur lineages.