Open Paleo

Acristavus

Gates et al., 2011 AK-riss-TAH-vuss

From Latin 'a-' meaning 'without', 'crista' meaning 'crest', and 'avus' meaning 'grandfather' or 'ancestor'; meaning 'non-crested grandfather'

83.6–72.2 million years ago Campanian, Late Cretaceous
Late Cretaceous stages

Acristavus is a genus of saurolophine dinosaur. Fossils have been found from the Campanian Two Medicine Formation in Montana and Wahweap Formation in Utah, United States. The type species A. gagslarsoni was named in 2011. Unlike nearly all hadrosaurids except Edmontosaurus, Acristavus lacked ornamentation on its skull. The discovery of Acristavus is paleontologically significant because it supports the position that the ancestor of all hadrosaurids did not possess cranial ornamentation, and that ornamentation was an adaptation that later arose interdependently in the subfamilies Saurolophinae and Lambeosaurinae. It is closely related to Brachylophosaurus and Maiasaura, and was assigned to a new clade called Brachylophosaurini.

Diagnosis

  1. Enlarged posterodorsal brow of postorbital and basioccipital extending posteriorly well beyond the dorsal border of the foramen magnum to a greater extent than in other hadrosaurine species.
  2. Deep laterally squared frontonasal suture, with posterolateral corners at nearly 90-degree angles, not seen in other hadrosaurids.
  3. Swelling of the interdigitate suture between the prefrontal and frontal, not seen in other hadrosaurids.
  4. Deep depression on the lateral surface of the postorbital jugal process, absent in all other known hadrosaurids.
  5. Dentary with nearly flat (straight) ventral margin, differing from the downturned anterior dentary of all other hadrosaurids.

Species

Acristavus gagslarsoni Gates et al., 2011

Type species

Named for 'Gags' Larson, nickname of landowner Russell Ellsworth Larson, on whose property the holotype was discovered.

Holotype
MOR 1155 , Museum of the Rockies
Material
Nearly complete partially articulated skull and associated articulated postcrania, including 11 cervical vertebrae, three fragmentary dorsal vertebrae, one anterior caudal vertebra, dorsal ribs, left humerus, left ulna, right sternal, left pubis, left femur, left tibia, left fibula, left metatarsals II and III, and several pedal phalanges.
Horizon
lower Two Medicine Formation, Campanian, Late Cretaceous , 83.6–72.2 million years ago
Locality
Museum of the Rockies locality TM-281, Montana, United States

References

  1. Gates, T. A.; Horner, J. R.; Hanna, R. R.; Nelson, C. R. (2011). New unadorned hadrosaurine hadrosaurid (Dinosauria, Ornithopoda) from the Campanian of North America. Journal of Vertebrate Paleontology 31(4): 798-811. Informa UK Limited. doi:10.1080/02724634.2011.577854 gates2011a
  2. Paul, G. S. (2024). The Princeton Field Guide to Dinosaurs (3rd ed.). Princeton University Press.

    Source of the body length and mass estimates.

    paul2024a
  3. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Ornithischians. Princeton University Press.

    Source of the hip height estimate.

    jones2026a