Harry Govier Seeley named this genus in 1879 for a disarticulated partial postcranial skeleton that had been uncovered at Reach, Cambridgeshire, composed of a left dentary fragment, numerous vertebrae from the neck, back, and sacrum, parts of the pectoral girdle, humerus fragments, part of the left femur, left tibia, foot bones, ribs, and other fragments.
The holotype consists of a dentary, cervical and sacral vertebrae, one ungual and remains of the pelvic region, that were collected near Presidente Prudente city São Paulo state.
A pseudotooth bird's lower right dentary piece (specimen YPM 4617) from near Charleston, South Carolina (USA) – apparently dredged up from near the source of the Stono River – was provisionally assigned to P. longirostris as it closely matches the holotype in size and appearance.
Fossils of the early whale genus Saghacetus ("Sagha whale", originally named "Zeuglodon osiris") were first collected at Qasr al Sagha by German explorer Georg August Schweinfurth in January 1886 (a well-preserved dentary).
The type species, U. itemirensis, was named by Alexander Averianov and Hans-Dieter Sues in 2007 on the basis of a single left dentary with teeth from the Cenomanian Dzharakuduk Formation.