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| juvenile Pachycephalosauria indet. (CMNFV 22039) Life reconstruction in an environment typical of the upper Maastrichtian Frenchman Formation, Saskatchewan. in Moore, Evans, Ryan, Patterson et Mallon, 2026. Illustration by Kaitlin Lindblad. |
ABSTRACT
Pachycephalosaurian dinosaurs are represented in the fossil record primarily by their taphonomically resistant frontoparietal domes that developed fully only at maturity. Consequently, the postcranial record of Pachycephalosauria is poor, particularly for immature forms. Here, we describe the ontogenetically youngest known, and Canada’s second-most complete, pachycephalosaur postcranium, collected from the uppermost Maastrichtian Frenchman Formation, Saskatchewan. Despite its small size (estimated total length ∼90 cm), the skeleton shows several characters diagnostic of Pachycephalosauria, including a double ridge-and-groove articulation on the pre- and postzygapophyses of the dorsal neural arches, a medial flange on the postacetabular process of the ilium, and a highly reduced pubis that contributes only minimally to the acetabular margin. Histological sectioning of the crural bones shows an immature woven bone texture. The absence of cyclical or annual growth indicators suggests that this specimen was less than a year old at the time of death. Phylogenetic analysis recovers the specimen as a member of Pachycephalosaurinae, perhaps related to Prenocephale, but the lack of cranial data for our specimen and the complementary lack of postcranial data for other pachycephalosaurs make this determination tenuous. Spatiotemporal considerations suggest a possible but currently untestable affiliation with Sphaerotholus buchholtzae. The relatively long hindlimbs of the juvenile compared with those of adult pachycephalosaurs indicate probable negative ontogenetic allometry in the hindlimbs.
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| Preserved elements (in red) of juvenile Pachycephalosauria indet. (CMNFV 22039). The preserved left ischium does not show in this view. Diagram illustrates preserved elements only and does not accurately reflect the proportions of CMNFV 22039. Skeletal drawing modified after original illustrations of Pachycephalosaurus (postcranium) and “Dracorex” (cranium) by Gregory Paul. Used with permission. |
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Life reconstruction of CMNFV 22039 in an environment typical of the upper Maastrichtian Frenchman Formation. Illustration by Kaitlin Lindblad. Used with permission. |
Bryan R. S. Moore, David C. Evans, Michael J. Ryan, R. Timothy Patterson and Jordan C. Mallon. 2026. The ontogenetically youngest known pachycephalosaur (Dinosauria: Ornithischia) postcranium. Journal of Vertebrate Paleontology. e2616325. DOI: 10.1080/02724634.2026.2616325 [26 Feb 2026]
