Hadrosauridae

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

These dinosaurs were very common in places like Asia and North America during their time. They had special teeth arranged in complex structures called dental batteries , which helped them grind tough plants. Hadrosaurids had bodies similar to earlier dinosaurs called iguanodontians.

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

History of discovery

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, called the "Dinosaur mummy," had well-preserved skin.

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

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

Distribution

Hadrosaurids probably started 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, leading to a group called Austrokritosauria. Later, during the early Maastrichtian, some other 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 first named by Edward Drinker Cope in 1869. It originally included only one dinosaur called Hadrosaurus. Over time, scientists noticed two main groups within this family: the hollow-crested group called Lambeosaurinae and the other group called Saurolophinae, which were often called Hadrosaurinae. Both groups are widely accepted today.

Scientists use special methods to understand how these dinosaurs are related. They have found that within these groups, there are smaller groups called tribes. For example, lambeosaurines can be split into groups like Parasaurolophini and Lambeosaurini. More recent studies have added even more groups, such as Tsintaosaurini and Aralosaurini.

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

Phylogeny

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

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.

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 known as duck-billed dinosaurs, are famous 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, rather than higher-growing leaves. This idea comes from studying the scratches on their teeth, which showed they chewed the same way repeatedly. However, some fossils of their stomach contents showed they might sometimes eat higher plants, but this is still debated.

Hadrosaurs could reach plants up to 2 meters high when standing on all fours and up to 5 meters when standing on just two legs. Some hadrosaurs also ate rotting wood, which provided nutrients from fungi and small creatures living in the wood. Shellfish were also part of their diet.

Neurology

Hadrosaurs had some of the most complex brains among plant-eating dinosaurs. Studies of their brains show they might have been quite intelligent for dinosaurs, possibly needing sharp senses and complex behaviors because they didn’t have strong defenses. Modern technology like CT scans has helped scientists learn more about their brain structure and how it compared to other dinosaurs.

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 competition with other dinosaurs. While fossils of eggs are rare, fossils of young hadrosaurs are more common, showing they were buried close to where they lived.

Growth and development

Young hadrosaurs looked similar to adults, but their joints showed signs of wear from cartilage. Some hadrosaurs, like Maiasaura, started life walking on two legs but shifted to four legs as they grew older. Other species, like Edmontosaurus, could walk on 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 from predators and various tumors. One hadrosaur even showed signs of a spine condition.