Riabininohadros

Discovery and history

The taxon was first described as Orthomerus weberi by Anatoly Nikolaevich Riabinin in 1945, based on hindlimb elements collected in 1934 near Bakhchysarai in Crimea by Gertruda Weber. The find was reported in the scientific literature in 1937. The material was later considered a nomen dubium within Hadrosauridae, and the informal name "Riabininohadros" was proposed by Roman Ulansky, but that publication did not meet the requirements of the International Code of Zoological Nomenclature.

The name was formally published as new by Lopatin and Averianov in 2020. The holotype, ZGTM 5751, was found alongside remains of an unnamed styracosternan in a Maastrichtian marine deposit at Mt. Besh-Kosh, within a Belemnella casimirovensis ammonoid zone. It consists largely of a left hindlimb, including the thighbone, shinbone, calf bone, astragalus, calcaneus, second and third metatarsals, and the first phalanx of the second toe. In 2018 it was recognized that Riabinin had misidentified several elements, and part of the right calf bone was also present.

A juvenile postcranial skeleton excavated at Aleshino in 1965 was considered to represent a different hadrosaurid species.

Classification

Orthomerus has typically been placed within or near Hadrosauridae, and Riabininohadros weberae was similarly classified in reviews of that family. However, Lopatin and Averianov in 2020 noted that the femur is very distinctive and has no morphological equivalents within Iguanodontia. Based on its basal astragalus, they classified Riabininohadros as a primitive member of Ankylopollexia, specifically Styracosterna, well outside Hadrosauroidea, with uncertain relationships to other taxa in the clade.