A JUVENILE CORYTHOSAUR (LAMBEOSAURINAE) FROM THE WAPITI FORMATION OF WESTERN ALBERTA, CANADA

The Wapiti Formation (WF) spans some 12 million years (~79–67 Ma), equivalent to the famed dinosaur-bearing beds of the Belly River and Edmonton groups in southern Alberta. Unlike these units, the WF preserves a continuous terrestrial record filling major gaps left by marine transgressions (e.g., Bearpaw Formation). The WF, therefore, provides important insights into macroevolutionary dynamics during the Late Cretaceous and, given the inferred high paleolatitudes for this region, faunal interchange with its southern counterparts. Despite these obvious incentives, collecting and prospecting in the WF are hampered by the restriction of surface outcrops to river valleys and it is only recently that we are beginning to uncover the faunal diversity of these deposits. Here we present the first lambeosaurine from the WF, found in the Bearpaw-equivalent Unit 3. Discovered in 2017 along the northern bank of the Wapiti River, the skeleton preserves a number of features that permit taxonomic evaluation, despite its juvenile state. The cranial anatomy, specifically the crest, reveals a bifurcated nasal, typical of both Corythosaurus and Hypacrosaurus but not Lambeosaurus. Diagnostic characters of the former genera are difficult to assess in juveniles, but the anteriorly straight to convex lacrimal and the <140° crest angle are more typical of Corythosaurus. Assessment of growth in the crest angle among contemporaneous lambeosaurines (N=52) places the WF specimen outside the range of variation of Hypacrosaurus and within the trajectories of Corythosaurus and Lambeosaurus. A broader multivariate analysis based on a principal component analysis of 12 cranial measurements reveals that crest angle is a major driver of size-independent variation among these taxa and similarly places the WF specimen among Corythosaurus variants. Corythosaurus is well-known from the lower, sandy strata of the Dinosaur Park Formation (DPF). Its discovery in Bearpaw-equivalent deposits extends its temporal range to ~74– 73.4 Ma and suggests that taxa along the Corythosaurus lineage may have dispersed north, likely tracking environmental changes resulting from the major transgression of the Western Interior Seaway that covered southeastern Alberta during this time. Rates of lambeosaurine evolution, inferred from the DPF, and certain crest characters suggest it is unlikely that the WF Corythosaurus pertains to one of the known species. However, owing in large part to its juvenile state, the specific affinities of the WF Corythosaurus remain uncertain at this time.

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