The internal crest anatomy of Lambeosaurini (Hadrosauridae: Lambeosaurinae).

The supracranial crests of lambeosaurine hadrosaurids have long been a focus of study due primarily to their extreme morphology. The external anatomy of lambeosaurine crests is understood to be highly variable between species, but variation in their internal anatomy is less well understood. The poor understanding of taxonomic variation in internal anatomy is due in large part to data limitations, where comparisons were historically made using fragmentary specimens, or more recently using CT imaging of primarily immature specimens. Here, we provide a detailed comparison of the internal anatomy of mature individuals of the lambeosaurines Corythosaurus casuarius, Corythosaurus intermedius, and Lambeosaurus lambei, and contrast their morphology with previously described juvenile congeners and mature Hypacrosaurus altispinus. The lateral diverticula of Corythosaurus spp. and L. lambei are large and lobate, with short anterior projections and well-developed posterior projections, lacking the elongated anterior expansions to the diverticula that are unique to H. altispinus. The s-loop of the nasal vestibule is highly developed in L. lambei, forming exaggerated loops anterior to the orbit. The s-loops of Corythosaurus spp. and L. lambei curve from lateral to medial within the premaxilla, differing from the direction of curvature of the s-loops in H. altispinus that curve from medial to lateral. Very few differences occur between C. casuarius and C. intermedius, suggesting that there is minimal interspecific variation between these congeneric species. Our results demonstrate that lambeosaurine nasal passages are highly variable between genera, supporting strong internal differences in crest morphology, even in closely related genera within the same tribe.