Arenysaurus (ah-REHN-ee-SAWR-us; “Arén lizard”) is a genus of hadrosaurid dinosaur, also known as a “duck-billed” dinosaur, that lived during the very end of the Late Cretaceous period, about 66 million years ago. Its fossils were discovered in the Arén region of Huesca, Spain, which is why it was given its name, meaning “Arén lizard.” This dinosaur was one of the last non-avian dinosaurs to live in Europe before the major extinction event that ended the Cretaceous period. Arenysaurus was first scientifically described in 2009 by a team of paleontologists based on a partial skull and other bones.

Description and Classification

Arenysaurus was a plant-eating dinosaur that walked on two legs but could also move on all fours. It is estimated to have been about 5 to 6 meters (16 to 20 feet) long, making it a medium-sized hadrosaur. Like other hadrosaurs, it had a distinctive beak-like mouth suited for cropping vegetation. As a member of the Lambeosaurinae subfamily of hadrosaurs, Arenysaurus likely possessed a crest on its head. However, this crest is thought to have been relatively small and possibly solid, unlike the large, hollow crests seen in well-known relatives like Parasaurolophus or Corythosaurus.

Arenysaurus belongs to the group Dinosauria, specifically within Ornithischia, the “bird-hipped” dinosaurs. Its more precise classification places it in the family Hadrosauridae, and further into the subfamily Lambeosaurinae. Paleontologists consider it to be a somewhat primitive or “basal” member of the lambeosaurines. Finding Arenysaurus in Europe is important because it shows that these types of advanced hadrosaurs were present there right up until the end of the age of dinosaurs.

Distinguishing Features

Arenysaurus had several features that help scientists tell it apart from other hadrosaurs. These include:

• A unique arrangement of the bones in its skull, particularly around the eye socket and the top of its head, with the frontal bone set back from the eye.
• It likely had a fairly small, solid bony crest on its snout, mainly formed by the nasal bones. This type of crest is different from the large, hollow crests of many other lambeosaurine hadrosaurs.
• The shape of its quadrate bone (a bone at the back of the skull connecting to the lower jaw) was straight when viewed from the side.
• Specific details in its jaw bones, such as a maxilla (upper jaw bone) with a short but tall shelf for muscle attachment, differed from those of its relatives.

Paleoenvironment and Diet

Arenysaurus lived in what is now the Pyrenees mountain range in northeastern Spain. During the Late Cretaceous, this area was part of an island landmass called the Ibero-Armorican Island. The environment was likely a coastal plain, with rivers, estuaries, and possibly swampy areas close to the sea. The climate would have been warm and humid. Arenysaurus shared its habitat with other dinosaurs, including other hadrosaurs (like Pararhabdodon), titanosaurian sauropods (like Lirainosaurus), and predatory theropods, as well as crocodiles and turtles.

As a hadrosaur, Arenysaurus was a herbivore. Its “duck-bill” was effective for snipping off leaves and twigs. Inside its mouth, it had hundreds of closely packed teeth forming “dental batteries.” These teeth were constantly being replaced and were excellent for grinding tough plant material. Arenysaurus likely fed on a variety of plants available at the time, such as conifers, ferns, and early flowering plants, probably browsing on vegetation at low to medium heights.

Significance and Ongoing Research

The discovery of Arenysaurus is significant because it is one of the most complete hadrosaur fossils found in Europe from the very end of the Cretaceous period. It provides valuable information about the diversity and evolution of dinosaurs in Europe just before the Cretaceous-Paleogene extinction event. Its presence helps scientists understand how European dinosaur faunas were changing and how they might have been connected to dinosaur populations in other parts of the world, like North America or Asia, potentially through island-hopping routes.

Ongoing research on Arenysaurus may focus on more detailed studies of its anatomy using the available fossil material to better understand its growth stages and to refine its exact relationship to other lambeosaurine hadrosaurs. Paleontologists are also interested in learning more about the ecosystem it inhabited and how the dinosaurs of the Ibero-Armorican Island adapted to their specific island environment. Any new fossil discoveries in the Arén region could further clarify the features, lifestyle, and biodiversity of Arenysaurus and other dinosaurs from this critical time in Earth’s history.