Orthomerus

Extinct genus of dinosaurs

Orthomerus (meaning "straight femur") is a genus of dubious hadrosaurid dinosaur from the Late Cretaceous of the Netherlands. It is today an obscure genus, but in the past was conflated with the much better known Telmatosaurus.

The type species Orthomerus dolloi was in 1883 named by the well known British paleontologist Harry Govier Seeley. The genus name is derived from the Greek ὀρθός (orthos), "straight", and μηρός (meros), "thigh". The specific name honours the French/Belgian paleontologist Louis Dollo, who had identified the bones in August 1882, during a visit to London. The type specimen, formed by the syntypes BMNH 42954 57, was probably found in the chalkstone quarries of the Sint Pietersberg near the city of Maastricht, The Netherlands. It mainly consists of partial juvenile skeletal elements. These remains are from the Maastricht Formation of the late Maastrichtian stage of the Late Cretaceous, about 66 million years old. The type bones include the straight left and right femora that moved Seeley to give it its name. A left tibia and a metatarsal also discovered in the collection of Jacob Gijsbertus Samuël van Breda acquired by the British Museum of Natural History in 1871, were referred by him to the species. The leg bones are only about half the size of those belonging to the then largely unknown North American and Asian duckbills, with the femur long. A second species, Orthomerus weberi, was first described by Anatoly Nikolaevich Riabinin in 1945 for hindlimb elements from an unnamed Maastrichtian age formation in the Crimea of what is now Ukraine (then a part of the Soviet Union). These were found by Gertruda Weber who is honoured in the specific name. As Weber was female Lev Nesov in 1995 emended the name to Orthomerus weberae. In 2015 it was made the type species of the new genus Riabininohadros, which was not formally named until 2020. What is sometimes listed as a third species, O. transsylvanicus, is actually the type species of Telmatosaurus, which Franz Nopcsa in 1915 referred to Orthomerus, an assignment still accepted by Alfred Sherwood Romer in his review of reptiles. In recent publications Telmatosaurus is seen as a separate genus, though. If Orthomerus would be identical to Telmatosaurus the latter would be its junior synonym. Forgetting this, in 1984 Dutch geologist Eric Mulder renamed O. dolloi into Telmatosaurus dolloi. A fourth species name is Orthomerus hillii, a renaming in 1915 by Nopcsa of Iguanodon hillii Newton 1892, based on a tooth fragment. It is today seen as a nomen dubium. In 2019, a study designated the right thighbone as the lectotype. The left thighbone would be of a smaller individual.

The right thighbone has a length of about fifty centimetres. In 2019, no autapomorphies nor a unique combination of traits could be established. That would make Orthomerus a nomen dubium.

Usually, Orthomerus was considered a member of the Hadrosauridae. However, the 2019 study included a cladistic analysis indicating that Orthomerus, though a possible hadrosaurid, was more likely placed more basal in the Hadrosauroidea, close to Gilmoreosaurus and Bactrosaurus in the evolutionary tree, outside of the Hadrosauridae.

As a hadrosaurid, Orthomerus would have been a bipedal/quadrupedal herbivore, eating plants with a set of ever replacing teeth placed in jaw bones with limited mobility that provided grinding action.