Open Paleo

Imperobator

Ely & Case, 2019 im-PERR-oh-BAY-tor

From Latin 'imperobator' meaning 'powerful warrior'. The species name refers to Antarctica, the continent where it was found.

72.2–66.0 million years ago Maastrichtian, Late Cretaceous
Late Cretaceous stages

Imperobator is a genus of probable unenlagiid paravian theropod dinosaurs, that lived during the Maastrichtian age of the Late Cretaceous in what is now James Ross Island in Antarctica. Imperobator is one of only two non-avian theropods known from Antarctica, crossing over to the landmass when it was part of Gondwana. The only described specimen was found in 2003 by an expedition launched by the University of California Museum of Paleontology and initially described as a dromaeosaur in 2007. The fossil was formally described as a new genus in 2019, and later searches reported more fossils from the site including teeth and skull bones.

Under dispute

Imperobator antarcticus is known from a fragmentary pes and cannot be confidently referred to any of the major paravian subclades. Its describers explicitly rejected a dromaeosaurid referral because it lacks the enlarged sickle claw and the ginglymoid distal metatarsal II characteristic of dromaeosaurids, recovering it only as Paraves incertae sedis (Ely and Case, 2019); a later study argued for unenlagiine affinities (Motta and colleagues, 2025). It is placed conservatively at Paraves incertae sedis.

Diagnosis

  1. Fusion of fibula and calcaneum
  2. Parallelogram-outline of metatarsal II cross section
  3. Metatarsal II medial slanting where diaphysis terminates
  4. Distal articular surface of metatarsal II non-ginglymoid
  5. Second pedal ungual small and lacking strong curvature

Species

Imperobator antarcticus Ely & Case, 2019

Type species

From Latin 'antarcticus' meaning 'of Antarctica', referring to the continent where the specimen was discovered.

Holotype
UCMP 276000 , University of California Museum of Paleontology
Material
Incomplete left pes and fragments of right pes: distal tibia, incomplete astragalus, calcaneum-fibula fusion, fragments of metatarsals I–IV, and pedal phalanges of all digits including the second pedal ungual
Horizon
Cape Lamb Member, Snow Hill Island Formation, Maastrichtian, Late Cretaceous , 72.2–66.0 million years ago
Locality
UCMP Locality RV-9502, The Naze, Antarctica

References

  1. Ely, R. C.; Case, J. A. (2019). Phylogeny of a new gigantic paravian (Theropoda; Coelurosauria; Maniraptora) from the Upper Cretaceous of James Ross Island, Antarctica. Cretaceous Research 101: 1-16. Elsevier BV. doi:10.1016/j.cretres.2019.04.003 ely2019a
  2. Motta, M. J.; Agnolín, F. L.; Brissón Egli, F.; Novas, F. E. (2025). Unenlagiid affinities for Imperobator antarcticus (Paraves: Theropoda): paleobiogeographical implications. Ameghiniana 62(1): 69-79. doi:10.5710/AMGH.13.11.2024.3604

    Proposed unenlagiid affinities for Imperobator based on phylogenetic analysis

    motta2025a