Open Paleo

Xenotarsosaurus

Martínez et al., 1986 zen-o-TAR-so-SAWR-us

From Greek xenos ('strange'), tarsos ('tarsus' or 'ankle'), and sauros ('lizard'), meaning 'strange ankle lizard', referring to the exceptional build of the ankle

100.5–89.8 million years ago Cenomanian–Turonian, Late Cretaceous
Late Cretaceous stages

Xenotarsosaurus is a genus of abelisaurid theropod dinosaur that lived during the Late Cretaceous of Argentina.

Under dispute

Xenotarsosaurus is consistently an abelisaurid, but its finer position is unresolved: it is recovered as a basal abelisaurid outside the derived subclades (Tortosa and colleagues, 2014) or as the basalmost brachyrostran (Filippi and colleagues, 2016), and it behaves as an unstable taxon in recent analyses.

Diagnosis

  1. Anterior dorsal vertebrae with centrum wider than tall and anterior face concave, neural arch more elevated with deeper fossae above neural canal
  2. Astragalus and calcaneus fused, undifferentiated, with no visible boundary with tibia in posterior view
  3. Fibula with diaphysis highly compressed lateromedially
  4. Femur gracile with head projected antero-medially at ~45°, greater trochanter continuing with femoral head, fourth trochanter projecting postero-medially
  5. Tibia with relatively delicate proximal end and cavities at base of cnemial crest

Species

Xenotarsosaurus bonapartei Martínez et al., 1986

Type species

Named for José Fernando Bonaparte, the Argentine paleontologist who recovered additional holotype material and made major contributions to the study of South American dinosaurs.

Holotype
UNPSJB-PV 184, UNPSJB-PV 612 , Universidad Nacional de la Patagonia San Juan Bosco
Material
Two incomplete cervical-dorsal vertebrae, femur, tibia, fibula, complete astragalocalcaneum of the right side
Horizon
lower Bajo Barreal Formation, Cenomanian–Turonian, Late Cretaceous , 100.5–89.8 million years ago
Locality
6 km north of the main buildings of Estancia Ocho Hermanos, Chubut, Argentina

References

  1. Martínez, R. D.; Giménez, O.; Rodríguez, J.; Bochatey, G. (1986). Xenotarsosaurus bonapartei nov. gen. et sp. (Carnosauria, Abelisauridae), un nuevo Theropoda de la Formacion Bajo Barreal, Chubut, Argentina [Xenotarsosaurus bonapartei nov. gen. et sp. (Carnosauria, Abelisauridae), a new Theropoda from the Bajo Barreal Formation, Chubut, Argentina]. IV Congreso Argentino de Paleontologia y Bioestratigrafia 1: 23-31.

    Gives the horizon as the upper part of the lower member of the Bajo Barreal Formation, "pre-Maastrichtian Senonian".

    martínez1986a
  2. Martínez, R. D.; Giménez, O.; Rodríguez, J.; Luna, M.; Lamanna, M. C. (2004). An articulated specimen of the basal titanosaurian (Dinosauria: Sauropoda) Epachthosaurus sciuttoi from the early Late Cretaceous Bajo Barreal Formation of Chubut Province, Argentina. Journal of Vertebrate Paleontology 24(1): 107-120. doi:10.1671/9.1

    Dates the same horizon at Ocho Hermanos late Cenomanian to early Turonian, listing Xenotarsosaurus in its tetrapod assemblage.

    martínez2004a
  3. Tortosa, T.; Buffetaut, E.; Vialle, N.; Dutour, Y.; Turini, E.; Cheylan, G. (2014). A new abelisaurid dinosaur from the Late Cretaceous of southern France: Palaeobiogeographical implications. Annales de Paléontologie 100(1): 63-86. Elsevier BV. doi:10.1016/j.annpal.2013.10.003

    Recovered Xenotarsosaurus as a basal abelisaurid, outside the derived subclades.

    tortosa2014a
  4. Filippi, L. S.; Méndez, A. H.; Juárez Valieri, R. D.; Garrido, A. C. (2016). A new brachyrostran with hypertrophied axial structures reveals an unexpected radiation of latest Cretaceous abelisaurids. Cretaceous Research 61: 209-219. Elsevier BV. doi:10.1016/j.cretres.2015.12.018

    Recovered Xenotarsosaurus as the basalmost brachyrostran (weakly supported).

    filippi2016a
  5. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Theropods. Princeton University Press.

    Source of the hip height estimate, and the appearance features.

    jones2026c