Open Paleo

Stygivenator

Olshevsky et al., 1995 Disputed

From Styx, the river of the Greek underworld, and Latin 'venator' ('hunter'), alluding to the Hell Creek Formation.

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

Stygivenator, also known as the "Jordan theropod", is a dubious genus of tyrannosauroid dinosaur from the Hell Creek Formation; the type species is Stygivenator molnari. Originally considered a species of Aublysodon, another dubious genus, and then a juvenile of Tyrannosaurus, its specific identity cannot presently be determined because its poor preservation makes it impossible to rule out an identity as either a specimen of Tyrannosaurus or Nanotyrannus.

Under dispute

Molnar (1978) described LACM 28471 as a new theropod, Paul (1988) named it Aublysodon molnaris, and Olshevsky, Ford and Yamamoto (1995) moved it to a genus of its own. Against that, Brochu (2003) regarded the specimen as an immature Tyrannosaurus rex, holding that the features distinguishing it are the same ones claimed for Nanotyrannus; Currie and colleagues (2003) reached the same conclusion, Carr and colleagues (2005) listed the genus among invalid taxa, and Carr (2020) treats the specimen as a two-year-old juvenile referable to T. rex. More recent work has reopened the question rather than closing it. Longrich and Saitta (2024) find the type differs from Nanotyrannus lancensis in the antorbital fossa, the maxilla and the tip of the mandible, raising the possibility of two distinct lineages while stating that further work and more fossils are needed. Paul (2025) gives the genus a systematic entry among the small gracile tyrannosaurs but calls its diagnosis provisional and the data insufficient. Zanno and Napoli (2025) instead regard it as a nomen dubium, impossible to assign to either Tyrannosaurus or Nanotyrannus. Placed at Eutyrannosauria incertae sedis, the least inclusive clade common to these readings.

Species

Stygivenator molnari (Paul, 1988) Disputed

Type species

First named as Aublysodon molnaris

Honors Ralph E. Molnar, who first described the specimen.

Holotype
LACM 28471 , Natural History Museum of Los Angeles County
Material
Both maxillae with teeth, nasals, frontals and anteromedial fragments of the parietal, the anterior dentaries, a fragment of the surangular, and a partial endocranial mold taken from the frontals and parietals.
Notes
Collected in 1966 by Harley Garbani. Described by Molnar (1978) without a name, given the epithet molnaris by Paul (1988) under Aublysodon, and made the type of this genus by Olshevsky, Ford and Yamamoto (1995), who spell the epithet molnari.
Horizon
Hell Creek Formation, Maastrichtian, Late Cretaceous , 72.2–66.0 million years ago
Locality
F. S. McKeever Ranch, Montana, United States The specimen's collecting site is recorded under two different ranch names. Molnar (1978), who described it, gives the F. S. McKeever Ranch with a legal land description and a museum locality number; Olshevsky (1995) instead places it on the Engdahl ranch, the property that produced the Tyrannosaurus rex holotype, and Paul (2025) follows Olshevsky in tying it to that same ranch.

Synonyms

  • Aublysodon molnaris reassigned

    The original combination, published by Paul (1988) for the specimen Molnar had described in 1978, with the epithet spelled molnaris. Olshevsky, Ford and Yamamoto (1995) moved the species to a genus of its own.

References

  1. Olshevsky, G.; Ford, T. L.; Yamamoto, S. (1995). The origin and evolution of the tyrannosaurids (Part 1). Kyoryugaku Saizensen 9: 92-119.

    Erects the genus for Aublysodon molnaris, spelling the epithet molnari.

    olshevsky1995a
  2. Molnar, R. E. (1978). A new theropod dinosaur from the Upper Cretaceous of central Montana. Journal of Paleontology 52(1): 73-82. SEPM Society for Sedimentary Geology.

    Original description of the type specimen, then unnamed, from the Hell Creek Formation of Garfield County, Montana.

    molnar1978a
  3. Paul, G. S. (1988). Predatory Dinosaurs of the World. Simon & Schuster, New York 1-464.

    Names the species Aublysodon molnaris for the specimen Molnar had described.

    paul1988a
  4. Brochu, C. R. (2003). Osteology of Tyrannosaurus rex: insights from a nearly complete skeleton and high-resolution computed tomographic analysis of the skull. Society of Vertebrate Paleontology Memoirs 7: 1-138. doi:10.2307/3889334

    Regards the specimen as an immature Tyrannosaurus rex, the features distinguishing it being those claimed for Nanotyrannus.

    brochu2003a
  5. Currie, P. J.; Hurum, J. H.; Sabath, K. (2003). Skull structure and evolution in tyrannosaurid dinosaurs. Acta Palaeontologica Polonica 48(2): 227-234. https://app.pan.pl/article/item/app48-227.html

    Treats the material as probably a juvenile Tyrannosaurus rex.

    currie2003a
  6. Carr, T. D.; Williamson, T. E.; Schwimmer, D. R. (2005). A new genus and species of tyrannosauroid from the Late Cretaceous (Middle Campanian) Demopolis Formation of Alabama. Journal of Vertebrate Paleontology 25(1): 119-143. Informa UK Limited. doi:10.1671/0272-4634(2005)025[0119:angaso]2.0.co;2

    Lists the genus among taxa the authors consider invalid.

    carr2005a
  7. Carr, T. D. (2020). A high-resolution growth series of Tyrannosaurus rex obtained from multiple lines of evidence. PeerJ 8: e9192. PeerJ. doi:10.7717/peerj.9192

    Scores the type as a two-year-old juvenile referable to Tyrannosaurus rex in a growth series.

    carr2020a
  8. Longrich, N. R.; Saitta, E. T. (2024). Taxonomic Status of Nanotyrannus lancensis (Dinosauria: Tyrannosauroidea) — A Distinct Taxon of Small-Bodied Tyrannosaur. Fossil Studies 2(1): 1-65. doi:10.3390/fossils2010001

    Finds the type differs from Nanotyrannus lancensis in several cranial characters, raising the possibility of a distinct lineage.

    longrich2024d
  9. Zanno, L. E.; Napoli, J. G. (2025). Nanotyrannus and Tyrannosaurus coexisted at the close of the Cretaceous. Nature 648: 357-367. doi:10.1038/s41586-025-09801-6

    Regards the taxon as a nomen dubium that cannot be assigned to either Tyrannosaurus or Nanotyrannus.

    zanno2025a
  10. Paul, G. S. (2025). A presentation of the current data on the exceptionally diverse non-tyrannosaurid eutyrannosaur and Tyrannosaurini genera and species of western North America during the end Cretaceous North American interchange. Mesozoic 2(2): 85-138. doi:10.11646/mesozoic.2.2.1

    Gives the genus a systematic entry among small gracile tyrannosaurs while calling its diagnosis provisional.

    paul2025b
  11. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Theropods. Princeton University Press.

    Lists the genus as a subjective synonym of Tyrannosaurus rex.

    jones2026c