A morphologic criterion to distinguish bird tracks

Abstract

Few anatomical criteria have been proposed to distinguish bird tracks from dinosaur tracks. The most commonly used criterion is the angle of divarication between toes II and IV, which is usually greater in birds than dinosaurs. The angle of divarication as related to some skeletal feature in modern birds may provide additional information about the trackmaker and its evolutionary relationships. A total of 186 bird tarsometatarsi (11 orders, 21 families, 39 genera, 41 species) from 8 morphotypes—shorebirds, wading birds, perching birds, zygodactyl birds, birds of prey, ground foragers, webbed-footed birds, and syndactyl birds—were sampled to test the hypothesis that the morphology of the distal end of the tarsometatarsus is the primary influence on the angle of divarication of the toes. Skeletal limb anatomy was also studied for correlations with stride length in an effort to predict pivot point height in birds from their trackways. Analysis shows a pronounced correlation between a large trochlea arc angle, and a small angle of divarication. Distinct differences in the trochlear arc angle and the angle of divarication could be correlated to such avian morphotypes as shorebirds, waders, ground foragers, and perching birds. No correlations between limb length and stride length could be established, and the equations commonly used for most vertebrate trackways did not hold for birds, most likely due to their unique hindlimb anatomy.