Hadrosauridae

Hadrosaurids

Hadrosaurids, also called hadrosaurs or duck-billed dinosaurs, were a group of plant-eating dinosaurs. They had flat, duck-like bones on their snouts. They belonged to a family of dinosaurs called ornithopods and lived during the Late Cretaceous Period. Some well-known types include Edmontosaurus and Parasaurolophus.

These dinosaurs were very common in Asia and North America. They had special teeth in complex structures called dental batteries , which helped them chew tough plants. Hadrosaurids looked similar to earlier dinosaurs called iguanodontians.

Hadrosaurids could walk on two legs or four, depending on their age. Younger ones often moved on two legs, while adults usually walked on four. The family Hadrosauridae has two main groups: lambeosaurines, which had hollow crests on their heads, and saurolophines, which did not have hollow crests but sometimes had solid ones instead.

History of discovery

Illustration of Trachodon mirabilis teeth

Ferdinand Vandeveer Hayden found the first dinosaur fossils in North America between 1854 and 1856. Joseph Leidy studied these fossils and named two species: Trachodon mirabilis from the Judith River Formation and Thespesius occidentalis from the "Great Lignite Formation". These discoveries helped scientists learn about early hadrosaurs.

Later, in 1858, William Parker Foulke studied fossils found by a farmer and named the species Hadrosaurus foulkii. This gave a better picture of what hadrosaurs looked like. In 1908, Charles Hazelius Sternberg and his sons found a very complete fossil in Converse County, Wyoming, which was described by Henry Osborn in 1912. This fossil was called the "Dinosaur mummy" because it had well-preserved skin.

From the mid 19th century through much of the 20th century, hadrosaurs were considered aquatic animals which subsisted on soft water plants

In 1942, Richard Swann Lull and Nelda Wright published a major study about North American hadrosaurs. In 1964, John H. Ostrom suggested hadrosaurs lived on land and ate plants like conifers.

In 2011, the Royal Ontario Museum and the Royal Tyrrell Museum held an International Hadrosaur Symposium. Many presentations were made, and some were published in a book in 2015. The twenty-first century has been a busy time for hadrosaur research, with many new discoveries.

Distribution

Map of various hadrosaur taxa across North America

Hadrosaurids probably began in North America and then spread to Asia. During a time called the late Campanian to Maastrichtian, a type of hadrosaurid called a saurolophine moved from North America to South America. This led to a group called Austrokritosauria. Later, some hadrosaurids from Asia moved to an island in Europe that is now part of France and Spain. One of these groups then traveled from Europe to North Africa, as shown by a dinosaur called Ajnabia.

Classification

The family Hadrosauridae was named by Edward Drinker Cope in 1869. It first included just one dinosaur called Hadrosaurus. Over time, scientists found two main groups in this family: the hollow-crested group called Lambeosaurinae and the other group called Saurolophinae, often known as Hadrosaurinae. Both groups are accepted today.

Scientists use special methods to learn how these dinosaurs are related. They found smaller groups inside these main groups. For example, lambeosaurines can be split into groups like Parasaurolophini and Lambeosaurini. Newer studies added more groups, such as Tsintaosaurini and Aralosaurini.

Premaxilla of Eotrachodon, the taxon named by Prieto-Marquez et al. 2016

Some scientists have debated the name Hadrosaurinae because of the rules for naming animals. One scientist suggested using Saurolophinae instead for the group that includes most crestless hadrosaurids.

Phylogeny

Hadrosauridae is defined as a group that includes both Lambeosaurinae and Saurolophinae, along with their most recent common ancestor. Different scientists define the family in slightly different ways, but they all agree it includes well-known dinosaurs like Saurolophus, Parasaurolophus, and Hadrosaurus.

Skull of Lambeosaurus, the type taxon of Lambeosaurinae

Recent studies have helped scientists understand the family tree of hadrosaurids better. These studies look at many features of the dinosaurs' bodies to see how they are related. The family Hadrosauridae is now officially defined as the smallest group that includes Hadrosaurus foulkii, Lambeosaurus lambei, and Saurolophus osborni.

The two main groups, Lambeosaurinae and Saurolophinae, are part of a larger group called Euhadrosauria. This group includes Lambeosaurus lambei and Saurolophus osborni, but not Hadrosaurus foulkii.

Skull of Saurolophus, the type taxon of Saurolophinae

Below is a family tree from a study in 2016. This tree includes 61 different hadrosauroid species and looks at 273 features of their bodies. The tree was created using a method called Maximum-Parsimony.

The following family tree is from a study in 2022 and includes many recently named species.

Anatomy

Hadrosaurs, also called duck-billed dinosaurs, are known for their flat, duck-like snouts. Some hadrosaurs had large crests on their heads, which they may have used to show off or make sounds. Their front teeth formed a beak perfect for nibbling leaves and twigs from forests in Asia, Europe, and North America. The back of their mouths had many teeth for grinding food.

We know from fossils that hadrosaurs had scaly skin, not feathers like some other dinosaurs. They walked on their front legs with padded hands. Hadrosaurids are split into two groups: Lambeosaurinae, with hollow crests, and Saurolophinae (or Hadrosaurinae), with solid or no crests. The hollow crests of Lambeosaurinae might have helped them make special sounds and look more impressive.

Paleobiology

Diet

Main article: Hadrosaur diet

Hadrosaurs likely ate plants close to the ground, such as horsetails. We know this by looking at the scratches on their teeth. Some fossils suggest they might sometimes eat taller plants, but this is still being discussed.

Hadrosaurs could reach plants up to 2 meters high when on all fours and up to 5 meters when on two legs. They also ate rotting wood, which had nutrients. Shellfish were part of their diet too.

Neurology

Hadrosaurs had some of the most complex brains among plant-eating dinosaurs. Studies show they might have been quite intelligent for dinosaurs. Modern technology like CT scans has helped scientists learn more about their brains.

Reproduction

Scientists have found fossils of very young hadrosaurs and their eggs. These fossils show that hadrosaurs nested in different areas, possibly choosing spots based on behavior, food, soil, or other dinosaurs. Fossils of eggs are rare, but fossils of young hadrosaurs are more common.

Growth and development

Young hadrosaurs looked similar to adults, but their joints showed signs of wear. Some hadrosaurs, like Maiasaura, started on two legs and later used four legs. Other species, like Edmontosaurus, could use all four legs from a very young age.

Daily activity patterns

Studies of hadrosaur eyes suggest they were active throughout the day in short bursts.

Pathology

Scientists have found evidence of diseases and injuries in hadrosaurs, including healed wounds and some tumors. One hadrosaur showed signs of a spine condition.