Bonapartenykus

Bonapartenykus (meaning "José F. Bonaparte's claw") is an extinct genus of alvarezsauroid dinosaur that lived during the Late Cretaceous (Campanian–Maastrichtian) of what is now the Allen Formation of the Río Negro Province, Argentina. The genus contains a single species, Bonapartenykus ultimus, known from a partially articulated, incomplete skeleton that was found in close association to two incomplete eggs and several clusters of eggshells belonging to the oogenus Arriagadoolithus. Bonapartenykus was named in 2012. It has an estimated length of 2.5–3.3 m (8.2–10.8 ft) and weight of 43–72 kg (95–159 lb), making it the largest member of the clade Alvarezsauroidea. Additional skeletons referrable to this genus have subsequently been described.

A partial skeleton of a theropod with eggs was collected in a surface of approximately 30 m² in fluvial sandstones of the upper Allen Formation in northwestern Patagonia, Argentina. The locality has also produced specimens of hadrosaurids, ankylosaurs, several titanosaur sauropods, several abelisaurids, indeterminate tetanurans, an incomplete large alvarezsaurid and a large unenlagiid. The skeleton was originally reported and described by Salgado et al. (2009) as an indeterminate alvarezsaurid. The specimen was later redescribed and named in 2012 by Federico L. Agnolin, Jaime E. Powell, Fernando E. Novas and Martin Kundrát. The holotype specimen, MPCA, 1290, consists of a mid-dorsal vertebra, both scapulocoracoids, left tibia and femur, left pubis articulated with the pubic peduncle of the ilium, the anterior blade of the left ilium, and two partially preserved eggs that were separated from the holotype by less than 20 cm (7.9 inches). Two specimens were referred to Bonapartenykus: MGPIFD-GR 166 and MGPIFD-GR 184, a blade of the left scapula, a left coracoid, a distal right pubis, four cervical vertebrae and a single caudal vertebra which all belong to the same individual.

The eggs of Bonapartenykus were considered unique enough for them to be given a new parataxonomic name, Arriagadoolithus, which was classified in a new oofamily, the Arriagadoolithidae, so named for the owner of the site where the discovery was made.

The generic name, Bonapartenykus, is derived from the Argentinean paleontologist José F. Bonaparte and the Latin word "onykus" (claw). The specific name, ultimus, is derived from the Latin word "ultimus" (latest), as it is one of the geologically youngest alvarezsaurids from South America.

In 2025, Meso and colleagues described various alvarezsaurid material first reported in the late 2000s from the same formation, specifically within the Salitral Ojo de Agua locality, as cf. Bonapartenykus ultimus.

Bonapartenykus was a large alvarezsaur, measuring 2.5–3.3 m (8.2–10.8 ft) long and weighing 43–72 kg (95–159 lb).

Agnolin et al. (2012) diagnosed Bonapartenykus based on the presence of spinopostzygapophyseal laminae that end abruptly above the postzygapophyses of the mid-dorsal vertebrae; the underside portion of the coracoid being strongly deflected towards the middle and decorated with delicate but abundant grooves as in Xixianykus; fused scapulocoracoids as in Ceratonykus; scapula with a very wide notch on the posterior margin of the bone; and a fused ilium and pubis.

The mid-dorsal vertebrae has diapophyses and parapophyses that are worn, with the former lacking its distal ends. As in other alvarezsaurids, the centrum of the mid-dordsal vertebrae lack pleurocoels. Unlike Haplocheirus and Patagonykus, the centrum has a cranial articular surface that is deeply concave with a sharp bony margin that surrounds it, which might be procoelous. The neural spine is proportionally taller than in Mononykini, but also resembles the condition seen in basal alvarezsauroids such as Haplocheirus. As in Mononykus and Patagonykus, both the prespinal and postspinal fossae are very deep. A unique feature of Bonapartenykus is the spinopostzygapophyseal laminae ending abruptly above the rear margin of the postzygapophyses. The mid-dorsal vertebrae has neural canals that are wide and prezygapophyses that shows a subcircular contour and a shallow concavity at its caudal margin. As in Patagonykus, the parapophyses connect to the margins of the cranial articular surface through a small ridge and the diapophyses connect to the parapophyses by a deep ridge. The centrodiapophyseal lamina was craniocaudally wider than in other alvarezsaurids, based on the preserved portions. The postzygapophyses are ellipsoidal in contour in dorsal view and show a notched caudal margin. The postzygapophyses are transversely wide and craniocaudally shortened, but lacks a lateroventral projection.