Rose diagrams depicting the orientation of microwear scratches along the tooth row of <i>Centrosaurus apertus</i> (ROM 767).

<p>Where possible, scratch orientations have been standardized to correspond to teeth from the right dentary (0° =  mesial, 90° =  apical).</p

Jaw mechanics and evolutionary paleoecology of the megaherbivorous dinosaurs from the Dinosaur Park Formation (upper Campanian) of Alberta, Canada

<div><p>ABSTRACT</p><p>The question of what role differential jaw mechanics may have played in facilitating dietary niche partitioning among Late Cretaceous megaherbivorous dinosaurs from Laramidia is examined, using the fossil assemblage of the Dinosaur Park Formation as a test case. We use phylogenetic inference to reconstruct the mandibular adductor musculature of these animals, which we then apply to the construction of biomechanical lever models of the mandible to estimate relative bite forces. Our findings reveal predictably weak bite forces in ankylosaurs, and comparatively high bite forces in ceratopsids and hadrosaurids, both of which possessed a mechanical advantage that produced bite forces 2–3 times higher than those forces exerted by the adductor musculature. The impressive jaw mechanism shared by the last two taxa evolved in a stepwise fashion, independently in each lineage. There is tentative evidence to suggest that nodosaurids had more powerful bites than ankylosaurids, but the overall mechanical diversity among megaherbivores from the Dinosaur Park Formation is low, suggesting that differential jaw mechanics could have played only a subsidiary role in niche partitioning. Such mechanical conservatism may have may have been selected for, or it may simply reflect the limits imposed by evolutionary constraints. Regardless, mechanical disparity patterns remained stable throughout the ∼1.5 Ma evolution of the Dinosaur Park Formation megaherbivore chronofauna.</p><p>SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at <a href="http://www.tandfonline.com/UJVP" target="_blank">www.tandfonline.com/UJVP</a></p></div

Rose diagrams depicting the orientation of pooled microwear scratches in hadrosaurids.

<p>Scratches are typically oriented dorsodistally-ventromesially; however, a second common mode comprises mesiodistally oriented scratches. Where possible, scratch orientations have been standardized to correspond to teeth from the right dentary (0° =  mesial, 90° =  apical).</p

Statistical test results for the MAZ-2, family level (Ceratopsidae/Hadrosauridae) pairwise comparisons.

<p>Ankylosaurs are not included due to insufficient sample size. Significant results reported in bold.</p

Overview of ankylosaur teeth.

<p>A, unworn ankylosaurid tooth (TMP 1992.036.1178) in lingual view; B, unworn nodosaurid tooth (TMP 2000.012.0024) in lingual view; C, partial left maxillary tooth row of <i>Panoplosaurus mirus</i> (ROM 1215) in lingual view, exemplifying the distal shift in both tooth size and wear facet orientation (tooth positions numbered); D, isolated ankylosaurid teeth exhibiting various states of wear (facets shown with dashed outline). Clockwise from top left: TMP 1997.016.0106, TMP 1991.036.0734; TMP 1997.012.0042, TMP 1989.050.0026; E, isolated nodosaurid teeth exhibiting various states of wear (facets shown with dashed outline). Clockwise from top left: TMP 2000.012.0027, TMP 1997.012.0005 (this tooth exhibits paired, mesiodistally arranged wear facets indicative of interlocking tooth occlusion), TMP 1994.094.0014, TMP 1992.036.0101; F, microwear from an isolated ankylosaurid tooth (TMP 1991.050.0014) exhibiting many mesiodistally oriented scratches; G, pitted and scratched LM 12 microwear of the nodosaurid <i>P. mirus</i> (ROM 1215).</p

Linear measurements used in this study (compare with Table 1).

<p>A, ankylosaur skull in left lateral (left) and caudal (right) views; B, ceratopsid skull in left lateral (left) and caudal (right) views; C, hadrosaurid skull in left lateral (left) and caudal (right) views.</p

Mann-Whitney U test results for the time-averaged, family/subfamily level pairwise comparisons.

<p>Bonferroni corrected <i>p</i>-values shown in lower left triangle; uncorrected <i>p</i>-values shown in upper right triangle. Significant results reported in bold.</p

Skull Ecomorphology of Megaherbivorous Dinosaurs from the Dinosaur Park Formation (Upper Campanian) of Alberta, Canada

<div><p>Megaherbivorous dinosaur coexistence on the Late Cretaceous island continent of Laramidia has long puzzled researchers, owing to the mystery of how so many large herbivores (6–8 sympatric species, in many instances) could coexist on such a small (4–7 million km²) landmass. Various explanations have been put forth, one of which–dietary niche partitioning–forms the focus of this study. Here, we apply traditional morphometric methods to the skulls of megaherbivorous dinosaurs from the Dinosaur Park Formation (upper Campanian) of Alberta to infer the ecomorphology of these animals and to test the niche partitioning hypothesis. We find evidence for niche partitioning not only among contemporaneous ankylosaurs, ceratopsids, and hadrosaurids, but also within these clades at the family and subfamily levels. Consubfamilial ceratopsids and hadrosaurids differ insignificantly in their inferred ecomorphologies, which may explain why they rarely overlap stratigraphically: interspecific competition prevented their coexistence.</p></div

NPMANOVA test results for the MAZ-2, subfamily level pairwise comparisons.

<p>Bonferroni corrected <i>p</i>-values shown in lower left triangle; uncorrected <i>p</i>-values shown in upper right triangle. Significant results reported in bold.</p

Rose diagrams depicting the orientation of microwear scratches in ceratopsids.

<p>Most scratches are oriented dorsodistally-ventromesially; however, there appears to be a second common mode whereby the scratches are oriented mesiodistally. Where possible, scratch orientations have been standardized to correspond to teeth from the right dentary (0° =  mesial, 90° =  apical).</p