Statistical anatomy of tyrannosaurs

Abstract

Since dinosaur fossils are not numerous enough for statistics, the idea of studying dinosaurs at all has been questioned. Teeth of Judith River tyrannosaurs (late Cretaceous of south-central Alberta, Canada) are sufficiently numerous and rich in anatomical structures for statistical analysis: 1. No difference was found between widths of serrations of anterior and posterior rows (Mann-Whitney U test, two-tailed; n = 24 tooth fragments: p> .05; anterior and posterior x = 0.4 mm): 2. Reversals in uniform trends in serration width within each serration row were more numerous for the anterior (reversals = 51; non-reversals = 74) than for the posterior (reversals = 45; non-reversals = 135) serration rows (χ2 (2×2) = 7.82; d.f. = 1; p< .01); 3. Serrations of the anterior row are more steeply angled toward the point of the tooth than are those of the posterior row (Mann-Whitney U test, two-tailed, n = 21 tooth fragments; p< .01; anterior x̄ = 8.9°; posterior x̄ = 5.0°): 4. Small teeth (mean length = 9.8mm) show less tendency to be scratched than large ones (mean length = 25.0mm) (t(d.f. = 234) = 4.64; p< .01); 5. Inter-serrational slots of posterior row require less force (x̄ = 0.14 newton) to cut standard nylon filament (diameter = 0.04mm; tensile strength = 0.42 newton) than did slots of the anterior row (x̄ = 0.24 newton; Mann-Whitney U test, two-tailed; p< .05; n = 17 fragments); 6. There is a significant difference between edge radii r (expressed in microns μ) of serrations of three classes of tyrannosaur teeth: unerupted/partially erupted (x̄ = 78.2μ), vs. fully erupted but still attached to bone (x̄ = 94.7μ), vs. shed (x̄ = 139.3μ) teeth (two factor analysis of variance, F(2,173) = 10.86; p< .05). Teeth of Judith River tyrannosaurs possess numerous structures, both interior and exterior (see figure) whose geometry differs markedly from that of, e.g., troodon teeth (which may possess a radix, but no other differentiated internal structures), or thecodont teeth (which possess a peak of interior enamel intruding into each denticle and tilted toward the point of the tooth). Statistical anatomy of teeth may offer a rich source of information for studying taxonomy, evolution, and migration of tyrannosaurs.