Xibalbaonyx is an extinct genus of megalonychid ground sloth known from the Late Pleistocene of Mexico. Three species are known: X. oviceps and X. exiniferis from the Yucatan peninsula and X. microcaninus from Jalisco. The genus is named after Xibalba, the underworld in Maya mythology.

Quick Facts Scientific classification, Type species ...

Xibalbaonyx Temporal range: Late Pleistocene (Rancholabrean) ~0.016–0.010 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Pilosa
Family:	†Megalonychidae
Subfamily:	†Megalonychinae
Genus:	†Xibalbaonyx Stinnesbeck et al. (2017)
Type species
†Xibalbaonyx oviceps Stinnesbeck et al 2017
Other species
†Xibalbaonyx microcaninus Stinnesbeck et al 2018 †Xibalbaonyx exinferis Stinnesbeck et al 2020

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The holotype of X. oviceps is a mostly complete skeleton discovered in an underwater cave system,^[1] while X. microcaninus being known from a complete skull and mandible from the sediments of the former paleolake of lake Jalisco.^[2] The overlapping remains of the skull and mandibles have notable differences between them, enough to call them distinct species. A third species X. exiniferis was described in 2020, also from an underwater cave in the Yucatan Peninsula, it is known from a "fragmentary left mandibular ramus, an atlas, and a left humerus".^[3] In 2020, a description of the postcranial remains of the holotype X. oviceps was published^[4] In 2021, remains of a form closely related to Xibalbaonyx was reported from the Pleistocene of Cueva de Iglesitas near Caracas, Venezuela.^[5]

Xibalbaonyx oviceps was about 2 metres long and weighed about 200 kilograms,^[4] with a similar weight suggested for X. exiniferis.^[3] The forearms of X. oviceps are well built and likely had well developed muscles, with the range of mobility of the limbs being high. These are suggested to be adaptations for climbing, as is done by similarly sized black bears.^[4]